2 - Major Parts of a LOINC term

This section last updated: 2023-08-15 (1 year ago)

A LOINC term is defined as the combination of the LOINC code and the Fully Specified Name (FSN).

The LOINC code is a unique, permanent identifier. The LOINC code has no intrinsic structure except that the last character in the code is a mod 10-check digit. The algorithm to calculate this check digit is given in Appendix C. All of the structure associated with a single LOINC entity is stored in other fields in the LOINC database.

The FSN is composed of five or six main Parts: the name of the Component or Analyte measured (e.g., glucose, propranolol), the Property observed (e.g., substance concentration, mass, volume), the Time Aspect of the measurement (e.g., is it over time or momentary), the type of System or sample (e.g., urine, serum), the Scale of measurement (e.g., qualitative vs. quantitative), and where relevant, the Method of the measurement (e.g., radioimmunoassay, immune blot).

The FSN is the combination of the main Parts and the colon character, ":", which acts as a separator:

<Analyte/component>:<kind of property of observation or measurement>:<time aspect>:<system (sample)>:<scale>:<method>

The first part of the name can be further divided up into three subparts, separated by carats (^). The first subpart can contain multiple levels of increasing taxonomic specification, separated by dots (.). The third and fourth parts of the name (Time Aspect and System) can also be modified by a second subpart, separated from the first by a carat. In the case of Time Aspect, the modifier can indicate that the observation is one selected on the basis of the named criterion (maximum, minimum, mean, etc.); in the case of System, the modifier identifies the origin of the specimen if not the patient (e.g., blood donor, fetus, and blood product unit). The hierarchical structure is outlined in Table 1, with references to the section numbers where each item is explained in detail.

Table 1: Hierarchical Structure of Fully Specified Analyte Name

We used Tietz,¹ Henry,² IUPAC,³ EUCLIDES,⁴ diagnostic microbiology textbooks, such as Mahon and Manuselis,⁵ the American Association of Blood Banking,⁶ and other sources as well as the expertise of the individuals or the committee to choose preferred names.

Here are some examples of fully specified LOINC names:

Sodium:SCnc:Pt:Ser/Plas:Qn

Sodium:SCnc:Pt:Urine:Qn

Sodium:SRat:24H:Urine:Qn

Creatinine renal clearance:VRat:24H:Ur+Ser/Plas:Qn

Glucose^2H post 100 g glucose PO:MCnc:Pt:Ser/Plas:Qn

Gentamicin^trough:MCnc:Pt:Ser/Plas:Qn

ABO group:Type:Pt:Bld^donor:Nom

Body temperature:Temp:8H^max:XXX:Qn

Chief complaint:Find:Pt:^Patient:Nar:Reported

Physical findings:Find:Pt:Abdomen:Nar:Observed

Binocular distance:Len:Pt:Head^fetus:Qn:US.measured

2.1 General naming conventions

2.1.1 Abbreviations in names of Component (Analyte)

In general, abbreviations are not used in the Component. For example, we use "total", "fraction", "oxygen", "Alpha", and "Beta" rather than "tot", "frac", "O2", "A-", and "B-". However, there are a few exceptions, such as the commonly used abbreviations shown in Table 2.

Table 2: Example Component Abbreviations

2.1.2 General naming rules for the Component (Analyte)

2.1.2.1 Place the identifier of the substance being measured first. This means “Hepatitis A antibodies (Ab)” not “Antibodies, Hepatitis A.”

2.1.2.2 Use the generic name of a drug, not the brand name, when referring to drug concentrations and antimicrobial susceptibilities, e.g., Propranolol, not Inderal. We will usually include the brand or trade names in the related names (synonyms) field.

2.1.2.3 Use full taxonomic name of an organism or virus name (not the disease) when describing a test that diagnoses that disease. Say “Rickettsia rickettsii Ab” not “Rocky Mountain spotted fever Ab”. Say “herpes simplex virus Ab” not “HSV Ab.” The disease name should be included as a synonym in the related name field.

2.1.2.4 Species and groups of species: SP identifies a single species whose identity is not known. SPP identifies the set of species beneath a genus. We have a third case, however. In some tests, antibodies apply to different strains of species. In rickettsial diseases, the antibodies are then against groups of species, e.g., the spotted fever group or the typhus group. In this case we use Rickettsia spotted fever group and Rickettsia typhus group.

2.1.2.5 When tests include the name of a bacterium (e.g., Neisseria gonorrhoeae DNA probe) for the formal LOINC name we use the full bacterial name from the International Journal of Systematic and Evolutionary Microbiology. When it includes the name of a virus (e.g., West Nile Virus IgM antibodies), we use the viral name as given by Index Virum.

2.1.2.6 When the test measures an antigen to a specific species of organism but cross-reactivity is such that other organisms are identified, the name should be the principal organism that is targeted by the test.

2.1.2.7 Avoid “direct” and “indirect” except as parts of synonym names. Avoid “conjugated” and “unconjugated” when a more precise term, such as “glucuronidated” or “albumin-bound” is available.

2.1.2.8 Use “platelets”, not “thrombocytes.”

2.1.2.9 Name vitamins by the chemical name. For example, use thiamine not Vitamin B1, The name containing “Vitamin” will be included as a synonym. This is the only reasonable approach because all vitamins have a chemical name but not all vitamins have a “numbered” vitamin name. One exception to this rule is that we will use the widely accepted vitamin name for nutrition intake terms.

2.1.2.10 Always specify whether serology tests measure the antigen or antibody, using the abbreviation “Ab” for antibody and “Ag” for antigen. Remove the “anti” from “ANTI X Ab.” It is redundant and obscures the most significant word in the name. Thus, “anti-smooth muscle Ab” becomes “Smooth muscle Ab.” Common abbreviations or shortened names, e.g., ANA for anti-nuclear antibody, will be found in the related names field.

2.1.2.11 VDRL will be named Reagin Ab because that is what it is. We will have to depend upon synonyms and aliases to equate our “standardized” names with the old names.

2.1.2.12 Use the noun form of the target of the antibody, e.g., Myocardium Ab, not Myocardial Ab.

2.1.2.13 Anion vs. acid: Always use the anionic name for chemicals, not the acid name, e.g., lactate, citrate, and urate, not lactic acid, citric acid, and uric acid. The acid form of the name will be included in the related names field of the database.

2.1.2.14 Alcohols: Always use the single-word names for alcohols: methanol, not methyl alcohol; ethanol, not ethyl alcohol, and so on.

2.1.2.15 Always spell out OH as Hydroxy, or as - ol, with no space or hyphen between Hydroxy and the next word.

2.1.2.16 Greek letters, alpha, beta, gamma, etc., are always spelled out (e.g., alpha tocopherol, not A-tocopherol), with a space between the spelled out Greek letter and the rest of the chemical name.

2.1.2.17 Use pH, not log (H+).

2.1.2.18 Whenever possible, the Component will contain the scientific names of allergens. Note: This is a convention implemented in January 2002.

2.1.2.19 Avoid use of the word “total” in laboratory test names, except when denoting the denominator of a fraction. Thus it is Alkaline phosphatase, not Alkaline phosphatase.total, but Alkaline phosphatase.bone/Alkaline phosphatase.total.

2.1.2.20 For drug metabolites, we will generally use the “nor” form rather than “desmethyl”, e.g., nordoxepin not desmethyldoxepin. When the distinction between specific forms of the compound is important, however, we will make an exception, e.g., O-desmethylvenlafaxine and O-desmethylencainide.

2.1.2.21 Abbreviate units of time, such as Y for year, D for day, and W for week. This only applies to time as a unit of measure, e.g., in the past 7D, and not when year or day are used to refer to a date or general time construct, e.g., next year.

2.1.3 Punctuation in Analyte names

A number of Analyte names include punctuation characters such as commas, for example, to identify the position of multiple alkyl groups in a carbon chain. We will avoid special characters, e.g., commas, dashes, and parentheses, except where they are included in the name specified by IUPAC, the Chemical Abstract Service (CAS) convention, or another international convention. So for example, commas will appear in multiple substitutions of alkyl chains per the CAS standard, dashes, asterisks and colons will appear in HLA allele names, colons will appear in the names of some microorganisms, and parentheses (i.e., round brackets) will appear in the names of red blood cell antigens.

2.1.4 Case insensitivity

All names are case insensitive. Prior to December 2006, we used upper case in the database and our examples, but we then changed to mixed case for easier readability. In electronic messages senders and receivers can use upper, lower or mixed case. However, the meanings should not be sensitive to case conversions to avoid any possibility of confusion when the information is sent over networks that may apply case conversion. To identify parts of the few names that by international convention are case sensitive, such as red blood cell antigens, we use the word “little” in front of the letter that is lower case. We use a similar convention to indicate superscripts with the word SUPER. See examples in Table 3.

Beginning in June 2019 with LOINC version 2.66, we adopted tall man lettering for drugs with similar names based on the list recommended by the Institute for Safe Medication Practices, which includes the list previously published by the U.S. Food and Drug Administration. Tall man lettering uses upper case letters to distinguish key parts of the drug name in order to help visually differentiate similar names, such as cycloSPORINE and cycloSERINE. This format is still case insensitive in that the meaning is the same regardless of the case; however, for senders and receivers that can support mixed case display, the tall man lettering helps draw attention to the discriminating parts of similar names.

Table 3: Example Case Specifying Conventions

2.1.5 Roman numerals vs. Arabic numerals

Whenever possible, numerals shall be represented in their Arabic form. However, when the conventional name uses Roman numerals, as is the case for clotting factors such as factor VIII, the LOINC primary name will use Roman numerals and we define a synonym containing Arabic numerals.

2.1.6 Exponents

We will use the phrase “exp” to indicate an exponent in the Component. According to the Unified Code for Units of Measure (UCUM), exponents are typically represented using an asterisk (*) or carat (^). However, asterisks and carats both already have defined meanings in LOINC. (As described later, asterisks are used in variables that report another specific part of the LOINC name, and carats are used as delimiters of subparts.) For this reason, we use “exp” to avoid ambiguity. For example, the concept “height raised to the power of 2.7” is represented as "height exp 2.7".

2.1.7 Use of the slash (/)

In the Component, a slash (/) is used to distinguish between a numerator and a denominator (a.k.a. Divisor) for terms that represent fractions or ratios. For example, Basophils/100 leukocytes represents the fraction of basophils out of the parent population of 100 leukocytes, and Lecithin/Sphingomyelin represents the ratio of lecithin to sphingomyelin. In the System, a slash is used as a conjunction and means "or". A common example is Ser/Plas, which means that either Serum or Plasma is a suitable specimen for measuring a particular analyte.

2.1.8 Use of curly braces {} in LOINC names

In Section 2.5 we describe the use of XXX in the System when the material is unknown or specified elsewhere in the HL7/ASTM message. In some domains, particularly clinical observations, we have adopted a newer style of notation with curly braces to indicate that the information is provided elsewhere.

For example, LOINC term 32491-3 represents a deep tendon reflex observation where the anatomic location is not specified in the term name:

Deep tendon reflex:Find:Pt:{Reflex location}:Ord:Observed

Such terms could be used in post-coordinated expressions where the value of “reflex location” is communicated elsewhere. The curly braces notation has the advantage over using XXX in some cases because it allows a more precise indication of the set of possible entities to expect for that piece of information. In the reflex example above, saying {reflex location} narrows the expected set of possible values down to those anatomic regions where a muscle tendon reflex can be observed.

We have made use of this notation primarily in the System and Method part of the name.

2.2 Component/Analyte (1st part)

The first main part consists of three subparts: (1) the principal name (e.g., the name of the Analyte or the measurement); (2) the Challenge or provocation, if relevant, including the time delay, substance of challenge, amount administered, and route of administration; and (3) any standardization or Adjustment.

The three subparts of the first part follow this syntax:

<[analyte].[subclass].[sub-subclass]> ^
<[time delay] post [amount] [substance] [route])> ^
<adjustment>

In the above syntax, the carat (^) is a required delimiter and the “dot” (.) separates the Analyte name from its subspecies.

This convention also implies that dots (.) and carats (^) cannot be a formal part of any of the words that are connected by these delimiters.

These subparts are described in greater detail below, Sections 2.2.1 through 2.2.3.

2.2.1 Analyte name (1st subpart)

The first subpart names the Analyte, including any relevant sub-classifications, separated from the main Analyte name by dots.

2.2.1.1 Analyte/subclass

The principal name (the first subpart) can be divided further by subclass (e.g., Calcium by itself is one Component, Calcium.ionized names another test that measures a subclass of calcium). Subclasses are separated by dots. Examples of common subclasses include: bound, free, and bioavailable; ionized and non-ionized; glycated; glucuronidated and non-glucuronidated; IgA, IgD, IgE, IgG, and IgM as modifiers indicating the subspecies of antibodies. Note that bioavailable is distinguished from free by including both free and partially bound moieties.

If the antibody is from a particular subclass of antibodies, specify the type of immunoglobulin (IgM, IgG, IgA, or IgD), e.g., Hepatitis A virus Ab.IgG, Hepatitis A virus Ab.IgM. If more than one subclass of immunoglobulin is included in the measurement, all are listed in the subclass, e.g., Mumps virus Ab.IgG+IgM with a plus sign (+) to separate the subspecies. There should be no spaces between the plus sign and the words it connects.

If two or more constituents are measured as one quantity, each constituent should be named in the Component separated by a plus sign, e.g., Cyclosporine+metabolites or Human papilloma virus 16+18+31+33+35+45+51+52+56 DNA.

If multiple Analytes are measured separately, the Analytes are separated by an ampersand (&) surrounded by spaces. Two cases that use the ampersand convention are the names of panel terms and impression terms.

The naming of panel terms is described more completely in Section 8, but here we describe its use of ampersand. The enumerated child elements of a panel are each measured individually, so we often use ampersand in the name of the parent term (the panel term), e.g., ABO & Rh group panel. This particular example of ABO & Rh group also illustrates how panel terms using ampersand are different than an observation term with ampersand. The ABO & Rh group panel is linked to two separate observation codes, one for ABO and another for Rh group, that each carries its own result. An ABO & Rh group observation term would carry a combined (but separately measured) result (e.g. A positive). Either term could be used in ordering, depending on the reporting approach.

Impression terms may also use ampersand, for example, Hepatitis A virus Ab.IgM & total impression. In the case of the Hepatitis antibody impression, both the IgM antibody and the total impression are described separately.

In some cases, panel or impression Components contain both a plus sign (+) and an ampersand (&), for example, Human papilloma virus 16 & 18 & 31+33+35+39+45+51+52+56+58+59+66+68 DNA impression. In this example, the HPV 16 and HPV 18 impressions are both described separately, as is the impression for the group HPV 31+33+35+39+45+51+52+56+58+59+66+68, which is measured as a single quantity.

2.2.1.2 Divisor

Some Analytes contain a Divisor, which represents the denominator of a result that is expressed as a fraction or ratio. The numerator does not have a separate name because by default the Analyte is the numerator for all terms. In Analytes that do have a Divisor, the numerator is separated from the Divisor by a slash (/). For example, in the Analyte Albumin/Protein.total, the numerator is Albumin and the Divisor is Protein.total, so that the overall Analyte represents albumin as a fraction of the total protein. Similarly, Albumin/Creatinine represents the ratio of albumin to creatinine. Note that in the former case, the Property would be a type of Fraction, since albumin is a type of protein (i.e., albumin represents a fraction of total protein), whereas in the latter, the Property would be a type of Ratio, because the amount of albumin is being compared to the amount of creatinine, but neither of them is a parent compound of the other.

2.2.2 Challenge test (2nd subpart)

The second subpart contains information necessary to interpret “challenge” (or loading or tolerance) tests. Variables that report the result of a measurement taken a certain amount of time post challenge (e.g., glucose after an oral glucose tolerance test) must be distinguished according to the challenge and the time post challenge. Thus, the Challenge subpart has a substructure that identifies the time interval or time difference and the challenge, using the following syntax, where the word “post” (or base line) is required.

<time delay> “post” <challenge>

where the challenge can be further characterized as

<amount given> <substance/treatment given> <route given>

An example of a Challenge that used all parts would be: Aldosterone^1H post 25 mg captopril PO

The time difference follows the syntax: n<S|M|H|D|W> where n is a number (possibly a decimal); S denotes seconds; M denotes minutes; H denotes hours; D denotes days; and W denotes weeks. The time delay can be preceded by a 'greater than' (>) sign, e.g., >4H. Table 4 lists some possible values for time difference, but any time specification that follows the above syntax would be legal.

In addition to specifying a time elapsed since challenge, the time delay slot can be used to name a clock time when the measurement was taken, e.g., Glucose^10 AM specimen, or to specify the ordering of specimens, e.g., ^1st specimen, ^2nd specimen. Use this syntax to indicate pre- and post-immunization specimens, acute and convalescent specimens, or a series of specimens for which no more detailed information is available.

Table 4a: Example Time Delay Post Challenge

Table 4b: N minutes/hours/days/weeks/months/etc. after challenge begun

The second subpart is also used to describe measurements taken at a specified point after the beginning of an ongoing treatment, such as peritoneal dialysis, e.g., Creatinine^12H post peritoneal dialysis. More generally, this syntax can be used to indicate that observations were recorded, e.g., ^post partum, ^postoperative, or ^post EDTA therapy.

The syntax of the second subpart can be specified in various ways to indicate Challenges of greater or lesser specificity, corresponding to the amount of detail that the laboratory knows about the challenge specimen.

Examples of the range of possibilities include:

Table 5: Example Challenge Subparts

We denote the route of the challenge by HL7 Version 2.3 “abbreviations for medication routes” (Table 6). An oral route of administration would be denoted by PO,⁷ an intravenous route by IV.

Table 6: Example Route Abbreviations for Challenge Part (from HL7 v.2.3, Chapter 4)

Examples:

Glucose^30M post 100 g glucose PO:MCnc:Pt:Ser/Plas:Qn

Gentamicin^trough:MCnc:Pt:Ser/Plas:Qn

For drug peak (obtained at a time presumed to reflect the highest concentration) and trough (obtained at a time presumed to reflect the lowest concentration) measures, the nature of the substance loaded is the same as the Analyte name, and need not be included.

2.2.2.1 Reporting the baseline measure as part of a Challenge test

Through LOINC release 2.52, we have defined one baseline term for different Challenge batteries when the Challenge is given by the same dose and route. For example, we define one baseline serum glucose for the 100 gm oral glucose tolerance test regardless of the number of separate measurements defined in the battery:

Glucose^pre 100 g glucose PO

A laboratory could use this same test identifier to identify the baseline result of a two-hour glucose tolerance and a three-hour glucose tolerance, for example.

However, the number and variety of challenge terms users have been requesting are growing rapidly. It now appears impractical to create a baseline term for every dose and route combination. Furthermore, baseline measurements are not affected by the subsequent challenge and could in principle be reported simply as the “unadorned” measurement with no named relation to a coming Challenge. Therefore, we have created generic baseline “pre challenge” terms, for example, Glucose^pre XXX challenge. For these terms, the specifics of the challenge would be reported elsewhere.

2.2.2.2 Physiologic challenges

Some challenges are defined in terms of a physiologic stress, not a dose of a chemical substance. The LOINC names currently cover calorie fasts (no calorie intake), exercise, and fluid restrictions. These Challenges are denoted by codes given in Table 7.

In the case of such Challenges, the syntax also includes the duration of the Challenge. For example:

post <duration><physiologic challenge>
Triglyceride^post 12H CFst

Table 7: Example Nature of Challenge

The naming structure is an exact analogous structure to that of chemical challenges. A test for glucose after 12 hours of an energy fast would be represented as:

Glucose^post 12H CFst:MCnc:Pt:Ser/Plas:Qn

A test for osmolality after a 12-hour fluid restriction would be:

Osmolality^post 12H FFst:Osmol:Pt:Urine:Qn

A test for triglyceride after 12-hour energy fast would be:

Triglyceride^post 12H CFst:MCnc:Pt:Ser/Plas:Qn

Two durations can appear in one specification, for example:

Cortisol^1.5H post 0.05-0.15 U insulin/kg IV post 12H CFst:MCnc:Pt:Ser/Plas:Qn

Our rules for naming challenge tests work well only when there is a single intervention followed by a test for one or more Components over time. Complex challenge tests involving more than one intervention or complicated sampling techniques need a unique name, but the name may not provide a complete description of all of the test parameters.

2.2.2.3 Reporting characteristics of challenge as separate observations

Because we cannot anticipate every type of challenge and route of administration, and because some challenge tests have no usual dose, some challenge tests will not contain a dose. Challenge observations that do not include a specific dose in the name have the word “dose” where a numeric dose would otherwise appear. The general form is:

<analyte>^<time> post dose <route>

Examples:

Glucose^1H post dose insulin IV:MCnc:Pt:Ser/Plas:Qn

The actual dose might then be sent as a comment or as a separate “test” that carries the dose as its value. To accommodate laboratories that wish to transmit the relevant challenge dose as a separate observation, we also define separate test names (and codes) for reporting such doses. This dose could then be sent by the reporting service as a separate result in a separate OBX segment.

The name of the observation that identifies the value of the dose would have the form:

<drug or challenge substance>: <time> post dose <challenge substance>

Examples:

Glucose.PO:Mass:Pt:Dose:Qn

Gentamicin:Mass:Pt:Dose:Qn

Thus we distinguish a drug concentration from the drug dose by means of the System (sample), 4th part, of the test name (see Section 2.5). You can find the observations that carry the dose of drugs or challenges grouped in the class DRUGDOSE in the LOINC database. This approach has the advantages of parsimony and practicality. It also provides an observation ID for the piece of information that must be transmitted along with the request for the observation.

Another example would be:

Oxygen:PPres:Pt:BldA:Qn

Oxygen inhaled:VRat:Pt:Inhl gas:Qn

Example Units: liters/minute or milliliters/second

Oxygen inhaled mechanism:Type:Pt:Dose:Nom

Comment: to report kind of delivery mechanism, e.g. nasal cannula

An analogous approach is used for reporting many kinds of associated variables when the variables are not conventionally embedded in the name. We take this approach in part because there are too many levels of the variables and it is not feasible to represent them all.

2.2.2.4 Generic Challenge specifications

We allow for a range of specificity regarding Challenges from fully specified to very generic.

Some Challenges will be specified fully as described above, e.g., ^30M post 100 g glucose PO. We will also include: challenges without the amount specified, e.g., ^30M post dose glucose; those that specify a time elapsed but not a particular challenge, e.g., ^1H post XXX challenge; those that do not specify the exact time but provide ordering information, e.g., ^2nd specimen post XXX challenge; or even more generic, ^post XXX challenge. These latter variants are needed to accommodate challenges that do not fit any common protocol, or referrals to reference laboratories where the study protocol is not reported.

2.2.2.5 Acute and convalescent, pre and post immunization

To assess the efficacy of immunizations, antibody levels are measured before and after the immunization; similarly, evidence for acute infection is obtained by assessing acute and convalescent screens. Both of these cases are reported with the 1st specimen, 2nd specimen syntax, for example:

Acute specimen, 1st specimen, pre-immunization specimen:

Streptococcus pneumoniae Ab.IgG^1st specimen:ACnc:Pt:Ser:Qn

Convalescent specimen, 2nd specimen, post-immunization specimen:

Streptococcus pneumoniae Ab.IgG^2nd specimen:ACnc:Pt:Ser:Qn

2.2.3 Adjustments/corrections (3rd subpart)

The third subpart of the data element contains calculations that adjust or correct some measured value. We use this subpart to distinguish corrected or adjusted values from the uncorrected measurement, e.g., corrected cell counts from the raw cell counts. Since these attributes are unique to each measurement, they will be short phrases of text rather than a controlled vocabulary to define the content of the third subpart. However when defined, such a test will have a unique LOINC code and the meaning will be fixed by the text in the third part.

Examples:

Calcium.ionized^^adjusted to pH 7.4:SCnc:Pt:Ser/Plas:Qn

Leukocytes^^corrected for nucleated erythrocytes:NCnc:Pt:Bld:Qn

2.2.4 Distinguishing multiple values for any test via the test name (4th subpart)

HL7 messaging allows for multiple results for one observation. However, some systems cannot distinguish separate answers per observation, so they made the test names like organism 1, organism 2 or substance 1, substance 2 to report multiple organisms or substances identified in samples. We do not encourage this type of reporting because that distinction can more clearly be accomplished by using one test name (e.g., organism identified) and the HL7 sub ID to distinguish the multiple organisms/substances. However, we have created a few terms to accommodate systems that bind the distinction into their test names. The fourth subpart of the Component name will allow reporting of repeat observations taken at the same time and/or on the same specimen.

Example:

Bacteria identified^^^2:Prid:Pt:Stool:Nom:Culture

2.3 Kind of Property (also called kind of quantity) (2nd part)

The second part of the fully specified name distinguishes between different kinds of quantities relating to the same substance, e.g., the mass concentration versus the substance (molar) concentration of sodium in a urine sample, or the absolute eosinophil count versus the percent of the total white count that is made up of eosinophils. The type of Property (kind of quantity) is an IUPAC concept described in the Silver Book.⁸ We include most of the relevant IUPAC quantitative Property types in LOINC. We also have a set of Properties, such as PrThr (Presence or Threshold), Type, Seq (nucleotide sequence), and Loc (geographic location), that are used in qualitative terms. More examples of Properties are given in Table 8. The complete set of active LOINC Properties is available in the LOINC Part File and can be identified because they contain a PartTypeName of PROPERTY.

In Version 2.75, August 2023, the semi-quantitative (SemiQn) scale was re-introduced for non-continuous measurement of numeric values. This new scale option has an impact on categorizing PROPERTY. Mass and substances concentration properties need scrutiny to determine whether the measurement was on a continuous linear calibration or if ranges/buckets of non-continuous, non-overlapping values were made and being reported. For this reason, properties such as Mass and Substance concentrations are represented under BOTH quantitative and semi-quantitative sections.

2.3.1 Quantitative Properties

2.3.1.1 Main quantitative Property categories

Mass: Observations reported with mass (milligrams, grams, etc.) in the numerator of their units of measure have Properties that begin with the word mass: mass content, mass concentration, etc.

Substance: Observations reported with moles or mill equivalents in the numerator of their units of measure have Properties that begin with the word substance.

Catalytic activity: Observations that report enzymatic activity have Properties that begin with catalytic, e.g., catalytic concentration, catalytic content.

Arbitrary: Results that report arbitrary units in the numerator of their units of measure have a Property that begins with arbitrary.

Number: Counts are associated with Properties that begin with "number", e.g., a white blood cell count reported as a number of WBCs divided by volume of blood, would have a Property of number concentration.

2.3.1.2 Quantitative Property category subtypes

Each of the above major Property categories has number of derivatives: concentration, content, ratio, fraction, and rate (see LOINC properties table (Table 8)).

Concentrations: An amount divided by a volume. These have units such as mg/dL, or gm/L.

Contents: An amount divided by a mass. These have units such as mg/gm sample or mg/total protein.

Ratios: When a result is reported as one measure divided by another taken from the same System, the Property is a ratio. The ratio of the mass concentration of substance A divided by the mass concentration of creatinine in a urine sample, for instance, is a mass ratio (MRto). The numerator and denominator of a ratio must come from the same System. If the measures come from different specimens, e.g., PT patient/PT control or creatinine serum vs. creatinine urine, it is a relative ratio (RelRto). The ratio of times coming from an actual and normal control (as in some coagulation tests) will be relative time (RelTime), a ratio of mass concentrations coming from two different specimens will be relative mass concentration (RelMCnc), and a ratio of catalytic concentrations from different specimens will have the property of relative catalytic concentration (RelCCnc).

Fractions: Fractions are ratios of a part over a whole: Creatine kinase.MB/Creatine kinase.total, if measured in grams, is a mass fraction (MFr). Fractions are usually reported as percent. In Canada and other countries, fractions are measured as pure decimal fraction. For instance, a 95% O2 saturation would be resulted as 0.95. For some Analytes, both styles of reporting are used in Canada, so we were asked to distinguish decimal fractions from pure fractions. Beginning with the June 2012 release, we now include terms that have .DF appended to the existing fraction properties. For example, some terms now have properties of MFr.DF, SFr.DF, VFr.DF, etc. Because of the confusion that occurs in countries that report results as both decimal fraction and percent, these properties were created to represent decimal fractions that are reported without units.

Rates: A rate is a measure per a time period, e.g., mg/day would be a mass rate (MRat). Clearances have the Property of volume rate, but “Clearance” will be included in Analyte name to clarify meaning, for example:

Sodium renal clearance:VRat:24H:Urine:Qn

Some measures do not fit the above schema. For instance, IUPAC describes an entitic quantity. This refers to measure per entity (e.g., cells, receptors, and molecules). Entitic quantities usually have units that include the name of some entity, e.g., red blood cells (“per 106 RBCs”).

One must be careful when mapping measures of constituents of red blood cells to LOINC code because they can be expressed many ways, e.g., as an amount “per mass of hemoglobin”, “per liter of blood” or “per red blood cell”. The first is a mass content, the second a mass concentration, and the last is an entitic mass (mass per entity) — all different properties.

The pharmaceutical industry has the need for laboratory terms that are not specific as to whether the test measures a substance (substance concentration or substance rate) or mass (mass concentration or mass rate). We have created terms with the properties of MSCnc or MSRat to represent these more general test observations. By default, both RELMA and the online search application (http://search.loinc.org) hide these terms from the search results, but they can be displayed by adjusting the search limit settings in these programs.

Some tests report the name of an organism (or initially report the presence of any organism, and later identify the particular strain), toxic substance, antibody or antigen, as a test result. Use Prid (presence or identity) as the type of Property field for results of this sort.

Examples:

Bacteria identified:Prid:Pt:Isolate:Nom:Bacterial subtyping

Barbiturates positive:Prid:Pt:Urine:Nom:Confirm

2.3.1.2.1 Arbitrary Concentration (ACnc)

ACnc means the number of arbitrary units in a volume (arbitrary concentration). We originally used ACnc as a “temporary” place-holder for observations with ordinal answers. We then transitioned to replacing ACnc with either Pr (for results simply based on whether the Analyte is present or not without being determined by a cut off value) or Threshold (for observations reported as “positive” or “negative” based on an internal threshold or cut off). As we updated existing terms with the new model, in many cases it was difficult to definitively know how results were determined. Therefore, as of release 2.56, we are using a single Property of PrThr to represent results based on either the presence or absence of an Analyte regardless of whether or not it is based on an internal cut off. This change was approved by the Laboratory LOINC Committee in June 2016.The display name will continue to say Presence. All of the existing terms with a Property of ACnc, Pr or Threshold and a Scale of Ord were updated to have the Property PrThr for the 2.56 release. A single term with Property ACnt and Scale Ord was also updated to have the Property PrThr.

Examples:

Clonazepam:PrThr:Pt:Ser/Plas:Ord

Hepatitis B virus surface Ag:PrThr:Pt:Ser/Plas:Ord:Confirm

Burkholderia mallei:PrThr:Pt:XXX:Ord:Organism specific culture

VKORC1 gene.c.1173C>T:PrThr:Pt:Bld/Tiss:Ord:Molgen

The Property of Prid will continue to be used when the result is selected from a list of organisms, as described above.

2.3.2 Semi-quantitative Properties

Mass: Observations reported with mass (milligrams, grams, etc.) in the numerator of their units of measure have Properties that begin with the word mass: mass content, mass concentration, etc. with a method of Test Strip. Assays measuring in mass concentration on a non-continuous numeric Examples include mass or molar concentrations detected by chromogenic changes on a test strip. The changes in color indicate intervals of concentration, such as 2, 4, 8, and 12 mg/dL of urobilinogen.

Substance: Observations reported with moles or mill equivalents in the numerator of their units of measure have Properties that begin with the word substance with a method of Test Strip Assays measuring in substance concentration on a non-continuous numeric Examples include mass or molar concentrations detected by chromogenic changes on a test strip. The changes in color indicate intervals of concentration, such as 2, 4, 8, and 12 mg/dL of urobilinogen.

Titer: Titers are another example of discrete values reported serially eg 1:8, 1:16 but not all values sequentially are reported. The true concentration may have been 1:9, but the measurement of 1:8 implies a true measure between 1:8 and 1:16.

Percentile: Percentiles are varying thresholds distributing values into buckets under 100. The difference between the 20th and 25th percentiles are only the values within the 21st, 22nd, 23rd, 24th and 25th brackets. Examples include types of lipid distributions amongst a general population’s findings.

NCNCRange: Created in 2019 for molecular level quantitation of infectious microorganisms, this property is used for very large ranges of numeric formats. Examples include measurement of microorganism DNA reported in ranges such as < 10^3 copies/mL; 10^3 – 10^4.5 copies/mL; 10^4.5 – 10^5 copies/mL.

ScoreRange: With repeated measurements of a patient, a confidence interval is an example of ScoreRange property.

Arbitrary Concentration: When used to report test strip values, such as Ehrlich units that are segregated into buckets, ACNC can be used as a semi-quantitative property.

Numeric Concentration: When used to report test strip values, such as Leukocytes and Erythrocytes, NCNC can be used as a semi-quantitative property.

Relative Density: When used to report test strip values, such as Specific Gravity, RelDen can be used as a semi-quantitative property.

2.3.3 Qualitative Properties

Qualitative Properties describe what is being measured for non-quantitative laboratory and clinical concepts. Such Properties describe a wide variety of results, such as whether or not an analyte is present, the specific type of analyte detected, a person's address, telephone number, or e-mail address, or a clinical finding. Three of the most common qualitative Properties in LOINC are PrThr, Prid, and Type. These specific Properties are often confusing to LOINC users, so they are described in more detail in this section.

2.3.3.1 PrThr

The Property PrThr stands for “Presence or Threshold”, meaning either a) the actual presence or absence of an analyte, or b) that the amount of analyte detected is over some predetermined threshold.

2.3.3.2 Prid

Prid, which stands for “Presence or identity”, is used for assays that detect whether or not an analyte of a particular kind is present, and if it is, to identify the specific analyte. One example of an assay that would be represented by a LOINC term with Property Prid is a bacterial culture. When a culture is performed, it is not known in advance whether or not any bacteria will be present. Therefore, one possible result for a bacterial culture is “No growth”. But, if one or more types of bacteria are determined to be present, then the specific identity (e.g. Staphylococcus aureus) is determined and reported.

2.3.3.3 Type

The Property Type is used for assays that identify the specific analyte in cases when the baseline presence of the analyte is known. For example, if an isolate known to contain Streptococcus pneumoniae is further analyzed to determine the specific serotype, the serotype assay would have a Property of Type, not Prid, because the presence of the Streptococcus is already known.

2.3.4 More examples of LOINC Properties

Correct assignment of Property tends to be the most difficult task for new users of LOINC. Section 4 describes more information about Properties for clinical terms, and Appendix E provides more explanation and many detailed examples.

Table 8: Example LOINC Properties

* = from IUPAC Silver Book

2.4 Time Aspect (point or moment in time vs. time interval) (3rd part)

One can either measure a Property at a moment (point) in time or measure it over a time interval and integrate, in the mathematical sense, over time. In the latter case, we aggregate a “series” of physiologic states into a single scalar value that reflects some “average” Property measured over the specified time interval. Intervals also have relevance for rate measurements such as excretion (substance rate or mass rate) or clearances (volume rates). The amount over an interval is often expressed as a mass rate (MRat, e.g., g/24h) or a substance rate (SRat, e.g., mol/24h). Interval measurements often apply to urine and stool (e.g., collection over 24 hours and calculation of a concentration, total amount, or clearance). They also apply to clinical measurements, such as urine outputs where we have shift totals and 24-hour totals. Event counts on physiologic monitors, such as the number of premature ventricular contractions (PVCs) over 24 hours on a Holter monitor, are also of this type, as are look back periods for survey instruments.

The allowed values for non-point Time Aspect are defined as a syntax exactly like the syntax for the times in challenge tests, for example:

<numeric value><S|M|H|W>

The most common one is 24H. Table 9 gives some other examples. The complete set of active LOINC Time Aspects is available in the LOINC Part File and can be identified because they contain a PartTypeName of TIME.

For urine collection, 24H is the “standard” integrated measure and these are almost always reported as mass rates (MRat), substance rates (SRat), or catalytic (CRat) rates. These would contrast with spot or random urine tests that are represented as point (Pt) measures in our nomenclature and usually reported as concentrations – MCnc, CCnc, or SCnc for mass, catalytic, and substance concentrations respectively. However, we can also report the average concentration on a 24-hour specimen – in this case the Time Aspect value would be 24H but the Property would be MCnc/SCnc/CCnc instead of MRat/SRat/CRat.

The designation of 24H collection is maintained for tests that traditionally have reference ranges based on amount of substance of a Component cleared or excreted in 24 hours. However, a given specimen could have a 23-hour collection time and would still be called a 24H study. Depending upon the policies and procedures of the lab, they might extrapolate the reported value to what it would have been if the collection continued for the full 24 hours and report it as moles per day.

We also allow indirect specifications of a time window. Stdy identifies the duration of the study (without specifying an exact time); Enctr identifies the Encounter (ER visit, hospital stay, etc.).

Sample volumes reported for timed measurements are carried in other fields or as separate “test” results in other OBX segments.

Table 9: Example Duration Categories

2.4.1 Time Aspect Modifier

The second and optional subpart of the Time component allows an indication of some sub-selection or integration of the measures taken over the defined period of time: 8H^max heart rate would be the highest heart rate observed over 8H (Shift). Min, max, first, last, mean are the other possible values for this subpart. When nothing is stored in this subpart, we assume a mean value over the time period in questions. Valid values for this subpart are listed in table below.

Table 10: Time Aspect Modifier Codes

2.5 System (sample) type (4th part)

System (sample) type is the fourth part of the fully specified test name. It consists of two subparts; the first part names the System, the optional second part, delimited with a “^”, indicates the Super System source of the sample if it is not the patient, e.g., fetus, blood product unit, donor, etc.

We define different tests for the combination of Component (Analyte) and type of System (sample) that are commonly reported. In practice, laboratories include a relatively small range of sample types in their test names. Chemical tests commonly distinguish between serum, urine, blood, and cerebrospinal fluid. Microbiology cultures tend to distinguish between greater numbers of sources.

Example Systems are listed in Table 11. The complete set of active LOINC Systems is available in the LOINC Part File and can be identified because they contain a PartTypeName of SYSTEM.

Laboratories commonly build separate assays in their laboratory information systems (LIS's) for serum, plasma, blood, urine, and cerebrospinal fluid. In addition, they may have a single test that includes all other body fluid specimens. The LOINC System Body fld is intended to reflect this distinction and represents body fluids other than serum, plasma, blood, urine, and cerebrospinal fluid. When LOINC terms with the System Body Fld are used, information about the specific specimen source or body fluid type should also be provided. Keep in mind that in many cases for tests that are run on pleural, peritoneal, synovial, and other types of fluid, LOINC has specific terms to represent these tests, and we encourage you to use the specific terms rather than the Body fld terms when possible.

We use XXX in the System to identify a material that is unknown or not specified — it could be solid or fluid, for example. XXX is also used in the System when the specimen or material is known but recorded elsewhere in the HL7/ASTM message. Yet, using XXX as a System can be problematic (see also Section 2.5.1 - Special issues related to XXX as a System below). XXX and Body fld are different in that Body fld represents a biologic fluid (other than serum, plasma, blood, urine, and cerebrospinal fluid, as stated above), whereas XXX can represent any material, biologic or nonbiologic, solid or fluid.

For many types of tests, the distinction between plasma and serum is irrelevant. When testing on serum or plasma is clinically equivalent, the System should be recorded as Ser/Plas, meaning “either Serum or Plasma”. Note that the use of a LOINC code with Ser/Plas as the System to report the result of a specific manufacturer's assay does not imply that the given assay is approved for use with both serum and plasma specimens; it simply means that the results obtained from testing serum and plasma are clinically equivalent, independent of which assay was used. Sometimes the test can only be run on either plasma or serum; the Component will then be associated with either Ser or Plas in one observation. If the test can be run on either but the results are clinically different and standardized (a very rare circumstance), two separate tests will be defined in our file, one with a System Plas and one with a System Ser. The current LOINC database includes some Ser tests and some Plas tests that should really be Ser/Plas. As we determine that a Ser or Plas test really should have been designated Ser/Plas, we will change the designation.

As with Ser and Plas, when testing on various specimens is clinically equivalent and the system for the existing test(s) includes only one or a few of those systems, we will review such tests upon request and update them as appropriate to include the additional system(s). One such example is toxicology testing, in which at least a subset of tests (e.g., confirmatory) can be run on instruments that accept either Ser, Plas or Bld as the specimen, so we may update existing Ser/Plas or Bld terms to have the System Ser/Plas/Bld as appropriate.

If the test is run on a combination of specimens (such as a ratio of substance found in CSF and plasma), the sample types are joined with a “+”, for example: Plas+CSF, Ser+CSF, or Isolate+Ser.

Details about the exact source and collection method (e.g., blood drawn from the right arm and maintained on ice) are not a proper part of the test name and are reported in other parts of the message. For example, the SPM specimen segment of an HL7 message can be used to convey detailed information regarding the type of specimen, where and how it was collected, who collected it, and some basic characteristics of the specimen. Earlier versions of HL7 used OBR-17 as a field to report specimen source.

Table 11: Example Laboratory System/sample Types

These abbreviations are used in the laboratory LOINC codes. Systems in clinical LOINC terms are spelled out in full and should be easily understood.

2.5.1 Special issues related to XXX as a System

Increasingly we get requests, often from the public health context, for tests with XXX as the specimen. XXX is a valid LOINC specimen, and often needed in many cases. However, LOINC typically needs a term with a more specific specimen instead of, or in addition to the XXX. Recall that XXX can be used to represent any kind of specimen (insects, road kill, and ingested food) when the material is unknown or specified elsewhere in the HL7/ASTM message. LOINC already has some general specimen terms that are broad but more constrained than XXX, for example, Bld/Tiss and Body fld.

Use of XXX for the specimen can create problems. Firstly, the reference ranges for measures on a continuous scale change with specimen differences, and most systems depend upon the test code to get to the reference ranges. So using the same code for very different specimens can break the range checking in many systems. Of course the XXX specimen type is most commonly requested for microbiology tests used to identify micro-organisms by culture, DNA/RNA or specific antigens. In this case the nature of the specimen might not change the clinical implication for some organisms; we suspect that anthrax found anywhere in the body has the same implication. But it does make a big difference for some organisms that are natural inhabitants of the skin or digestive system, but not of the blood.

Thirdly, if there is no hint of the specimen in the test name, clinicians will have a more difficult time ordering common microbiology tests. They won’t be able to order urine culture and blood culture each with a single click, and laboratories will have a more difficult time managing these orders. This is especially true given the current state of many HL7 messages that only rarely use codes in the specimen field. (Granted, there is hope for the future.) Fourthly, and most importantly, there is the issue of approved use. Package inserts are very specific and limiting about the specimens for which the test kit is approved. We have had a recent request for a test with specimen of XXX whose package insert allowed its use only on Saliva. What do we do? Referral laboratories tend to define tests for class of specimens approved for use on the instruments/test kits they employ. We have received requests for terms with XXX, but the test is really only used for specimens from body parts that can “catch” STD.

So, the bottom line is that we won’t accept requests for tests with XXX specimens without a narrative description of the most common specimen and some sense of the range of specimens for which it is used. We will likely argue for either adding a distinct code for the dominantly common specimen (e.g. Stool for Entamoeba histolytica) or the specimen prescribed by the package insert, or for entertaining a new specimen code broader than those now available but narrower than XXX to accommodate a particular need, e.g. sterile body fluid, or Genital/anal/throat (for STD specimens). This will, of course, take time and discussion.

2.5.2 Super System (2nd subpart)

The second subpart of the System identifies a “super-system” when the source of the specimen is someone or something other than the patient and the result needs to be distinguished from the patient's results because they will be stored together in the patient's record. Examples of Super Systems include a blood product unit (BPU), a bone marrow donor, or a fetus. The Super System is also used when the clinical finding (history, physical) is about someone other than the patient and needs to be distinguished from the patient's findings, for example, a history of diseases in a family member recorded in the patient's chart. When the Super System is not included in a name, “patient” is the assumed default value.

For instance, an example of representing a coagulation study that uses measures on both patient and a control might be:

Coagulation reptilase induced:Time:Pt:PPP:Qn:Coag

Coagulation reptilase induced:Time:Pt:PPP^control:Qn:Coag

Blood banks often report red blood cell antigens for the patient and for each blood product pack assigned to that patient. So we have:

A Ag:PrThr:Pt:RBC:Ord

A Ag:PrThr:Pt:RBC^BPU:Ord

2.6 Type of Scale (5th part)

The fifth data part of the test name specifies the Scale of the measure, and is a required part. The abbreviation of the type of Scale (previously called precision), given in Table 12, should be used in the fully specified name. Note that with the release of Version 1.0K, May 1998, we changed the codes for these from SQ to ORD and from QL to NOM to more accurately identify the meaning.

In Version 2.75, August 2023, the semi-quantitative (SemiQn) scale was re-introduced for non-continuous measurement of numeric values. See the Technical Brief Non-linear Numerical Values “Binned” to Ordinal or Range for further, extensive discussion.

Table 12: Type of Scale

2.6.1 Quantitative (Qn)

Identifies Scales that can be tied to some physical quantity through a linear equation. This means that if we have two reports for the same quantity one with a value of 5 and the other a value of 10 we know that the two are related in amount through the linear equation Y = aX +b. When the intercept, b, is non-zero, we have a difference scale. (Fahrenheit temperature is a difference scale.) When it is zero we have a ratio scale (Kelvin temperature is a ratio scale).⁹,¹⁰ A Qn value may be reported as a value for a “continuous” Scale, as is the case for serum sodium.

2.6.2 Semi-Quantitative (SemiQn)

The SCALE designation Semi-Quantitative provides a warning that this data should not be presumed to be true “linear” quantities. This SCALE represents ranges, buckets or binning of upper and lower thresholds due to the methods of detection. The SemiQn scale is associated with PROPERTIES such as Titers, NCNCRange, ScoreRange, and also MCNC and SCNC. The latter being invoked with METHODS of test strip detection or RAST Classes, amongst others. Please note urinalysis test strips provide two different SCALE options: the colored test pads may represent units of mg/dL (SemiQn) or 1+,2+,3+ (Ord).

2.6.3 Ordinal (Ord)

Some observations have values that are well ordered, e.g., “present, absent”, “1+, 2+, 3+”, or “negative, intermediate, positive”, but the values have no linear relationship to one another. We do not know that positive is two or three times as much as intermediate, we just know that positive is more than intermediate. Pain scales are an example of this arbitrary measurement. These kinds of observations have an ordinal Scale (Ord). Tests with “yes/no” answers are always ordinal (Ord). Tests reported as negative when less than the detection level but as quantified values otherwise should be regarded as quantitative (Qn).

2.6.4 Quantitative/Ordinal (OrdQn)

Rarely, a result can be reported in either an ordinal or quantitative Scale. The principal examples of this scale are microbiology susceptibilities: Agar diffusion (Kirby Bauer (KB)), Minimum Inhibitory Concentration (MIC) and others. A MIC, which can be reported as either resistant/intermediate/susceptible or by the MIC numeric value. The need for terms with OrdQn as Scale was further obviated by clarification from HL7 that results such as "POS" and "NEG" should go in the OBX-8 field for normalcy status. Thus, LOINC codes with Scale of Qn can be appropriately used in these cases even if the "values" coming back are coded interpretations of the true numeric result value.

2.6.5 Nominal (Nom)

Some observations take on values that have no relative order. Think of the numbers on football jerseys. These simply identify the players; they do not provide quantitative information or rank ordering of the players. We refer to these as nominal (Nom) in Scale. Blood culture results provide a good example. Possible values could be Escherichia coli (or a code for E. coli) or Staphylococcus aureus. Other examples are admission diagnoses and discharge diagnoses. Any test or measure that looks broadly at patient or specimen and reports the name of what it finds is a Nom Scale. The values of nominal scaled observations are assumed to be taken from a predefined list of codes or from a restricted vocabulary (e.g. a menu of choices). These observations would typically be sent in an HL7 message OBX segment with a Coded Element (CE) data type (in earlier HL7 versions) or its superseding Coded with No Exceptions (CNE) and Coded With Exceptions (CWE) variants (later HL7 versions). It is important to note that the CE and CWE data types allow values to be set as codes with their print text or just as their print text alone. These data types and the Nom Scale would not be used for running narrative.

2.6.6 Narrative (Nar)

Some observations are reported as free text narrative. The content is not drawn from a formal vocabulary or code system. A dictated present illness would be an example of a Scale of narrative (Nar). Many clinical LOINC codes will come in two versions: one for the nominal (coded) version and one for a narrative (free text) version.

2.6.7 Multi

We strongly encourage all reporting to be at the most granular level of detail. That is, if three numbers were reported, they would each be reported under a unique LOINC code and transmitted in a separate HL7 OBX segment. Occasionally reporting systems are not able to comply with this dictum. For example, some chromatography instruments can identify chemicals from the entire spectrum of known chemicals (CAS identifies more than 10 million distinct chemicals), and we may not have specific LOINC codes for reporting out these details. We have designated the Scale of Multi to identify results that include many separately structured results as one text “glob” with or without embedded (display formatting). Some laboratories report all of the details of many multiple measure tests under such globs with test names that correspond to their order name. We strongly discourage such reporting. It defeats the very purpose of individual codes to tag content.

2.6.8 Document (Doc)

The Scale of Doc represents a collection of information that is either structured or unstructured. Individual LOINC codes are assigned for different collections of information regardless of the format in which they are presented, meaning that the same LOINC code should be used to represent a given document type regardless of whether it is in PDF, text document, JPG, XML, or HTML formats. The difference between Doc and Nar is that Nar represents a single free text result, while Doc is used for collections of results reported together, which may include narrative results.

2.7 Type of Method (6th part)

The Method by which the test was performed is the sixth part of the test name. Methods need only be expressed as part of the name when they provide a distinction between tests that measure the same Component (Analyte) but which have different clinical significance or have a different clinical reference ranges. For instance, whole blood glucose tested with a test strip might be distinguished in the Method field.

A non-exhaustive list of example Methods are listed in Table 13. The complete set of active LOINC Methods is available in the LOINC Part File and can be identified because they contain a PartTypeName of METHOD. Further, many Methods have descriptions that can be found on the Details Page for that Part. For example, see the details page for LP70657-9 RPR.

Laboratories do not include the Method as part of the name for most common chemical and hematological tests. They often need the freedom to choose the instrument according to time of day, urgency of the request for service, availability of the instruments and so on, even though the instruments may employ different methods. The laboratories then adjust each of the “interchangeable” instruments to produce equivalent results even though the instruments may use different methods. Therefore, we do not want to distinguish too finely on the basis of methods. When a LOINC term does include a Method, it usually describes a type or class of method and does not make fine-grained distinctions except in special cases.

Though Method is rarely significant for many chemical and hematological tests, it is often important to immunochemical/serology testing, because the sensitivity and specificity of some tests varies greatly with the Method. For this reason, you will commonly see Methods included in microbiology tests and coagulation tests within the LOINC database.

This does not mean that detailed information about the Method is irrelevant, but that it is not always useful or practical to bind it to the test name. It is an essential element of the internal quality assurance of laboratories and there are fields for reporting other details about the Method in HL7 and CEN TC251 test result messages.

Table 13: Laboratory Method Type Abbreviations

2.7.1 Methods for identifying nucleic acids

We distinguish Methods that detect nucleic acid based on two aspects of the assay: whether or not the target nucleic acid is amplified (copied), and how it is detected. We are still working on determining what the most important distinctions are in the variety of nucleic acid detection methods that are used, and may update our Methods to reflect these distinctions in the future. As of the 2.63 release, however, we use the following Methods:

Probe-based detection without amplification (Probe)

Definition: A hybridization probe is a typically a short nucleic acid segment that binds to a complementary nucleic acid sequence that is specific to the target of interest. The probe is typically attached to signaling molecule in such a way that a signal is only generated when the probe binds to the target nucleic acid. In LOINC, the Probe method is used for assays that do not include either a nucleic acid amplification or a signal enhancement step.

Target amplification followed by probe-based detection (Probe.amp.tar)

Definition: The Probe.amp.tar Method in LOINC is used for assays that include a nucleic acid amplification step, in which many copies of the nucleic acid sequence(s) of interest are made, followed by detection of the target nucleic acid of interest using a hybridization probe. Nucleic acid amplification can be done using different techniques such as polymerase chain reaction (PCR). The primary difference between the Probe.amp.tar and Non-probe.amp.tar Methods is the technique used for target nucleic acid detection. Note that for historical reasons, this Method also includes older techniques for identifying PCR target amplification products, such as gel separation and staining to identify the fragments based on their expected sizes.

The Probe.amp.tar Method is represented as NAA+probe in the Short Name and NAA with probe detection in the Long Common Name.

Signal amplification followed by probe-based detection (Probe.amp.sig)

Definition: Probe with signal amplification is a lab method that uses a hybridization probe, which is a typically a short nucleic acid segment that binds to a complementary nucleic acid sequence that is specific to the target of interest, followed by a signal enhancement step, in which the signal that is generated when the probe binds to the target sequence is multiplied so that it is “brighter” and easier to detect. Signal amplification can be done using different techniques, including the branched-chain DNA (bDNA) method. In theory, signal amplification is more sensitive than a probe by itself because it generates a brighter signal per each copy of the target sequence that is present. In LOINC, the Probe.amp.sig method is assigned to codes for assays that do not include a nucleic acid amplification step.

The Probe.amp.sig Method is represented as Probe+sig amp in the Short Name and Probe with signal amplification in the Long Common Name.

Target amplification followed by non-probe based detection (Non-probe.amp.tar)

Definition: The LOINC Non-probe.amp.tar method is used for assays that include a nucleic acid amplification step, in which many copies of the nucleic acid sequence(s) of interest are made, followed by detection of the target nucleic acid of interest using a method other than a hybridization probe, such as melt curve analysis or turbidity measurement. The primary difference between the Probe.amp.tar and Non-probe.amp.tar Methods is the technique used for target nucleic acid detection.

The Non-probe.amp.tar Method is represented as NAA+non-probe in the Short Name and NAA with non-probe detection in the Long Common Name.

Historical information: In LOINC version 2.54, we added a new Method for target amplification with melt curve analysis (Melt.amp.tar) to distinguish assays that use melt curves rather than probes for analyte detection. For version 2.56, we replaced Melt.amp.tar with a more encompassing non-probe based Method (Non-probe.amp.tar) that is used for PCR assays with non-probe based detection technology, including melt curve analysis, and which still distinguishes from probe-based assays (Probe.amp.tar). The Method for the terms originally created with the Melt.amp.tar Method was updated to Non-probe.amp.tar for the 2.56 release, and Melt.amp.tar is no longer in use.

Examples of specific techniques and their corresponding Methods

The tables below describe specific techniques that are included in each of the four Methods described above.

Table 14a: Examples of methodologies that are included in the Probe Method

Table 14b: Examples of methodologies that are included in the Probe.amp.tar Method

Table 14c: Examples of methodologies that are included in the Probe.amp.sig Method

Table 14d: Examples of methodologies that are included in the Non-probe.amp.tar Method

2.7.2 Immune assays

Immune assays in the LOINC world include a large swath of tests. We have historically lumped together immune assays that detect the linking of antigens and antibodies via special signaling mechanisms, such as EIA, ELISA, chemiluminescence and other similar tests that produce one measure (quantitative or qualitative) of the analyte of interest. We thought of this class of tests as “EIA-like” and in the absence of an existing short acronym to describe their constituents, we borrowed EIA to provide this meaning in the Short Name and used Immunoassay in the Long Common Name to signal that this method type was not limited to pure EIA tests. In LOINC version 2.56, we changed the name of the EIA Method to IA (and EIA.rapid to IA.rapid), in order to mitigate some of the confusion regarding the EIA name. The Long Common Name will continue to use Immunoassay as it always has, and the Short Name will use IA rather than EIA.

Some immunologic tests have always had their own Method type and were never lumped in the category of EIA-Immune assays described above. Immune blot (IB) tests, immune diffusion and immune based flow cytometry (FC) tests have always had specific method names because they are different in that they usually yield multiple related observations.

Immune fluorescence tests (IFA, DFA and ACIF) also have had their own Method type, namely IF, because they can provide information about the presence or amount of a target analyte AND its location (or pattern) on a smear, tissue slice or cell. Immunofluorescent cell binding assays, which use cells that are engineered to express a single antigen of interest, use the Method CBA IFA. LOINC has also assigned distinct method types to some other immunologic tests including VDRL, Latex fixation, and other kinds of agglutination tests because these were their historic names or they had specialized uses or limits.

We classify peroxidase and all other non-IF immune stains of tissue under the Method category immune stain.

2.7.3 Immunoblot and Line blot Methods

LOINC has two different Methods for Immunoblot (IB) and Line blot. IB is used in cellular and molecular biology to identify a specific protein in a complex mixture extracted from cells. Gel electrophoresis separates polypeptide chains (subunits) into bands according to their molecular weight. The bands are transferred onto a membrane and incubated with a primary and secondary antibody to visualize the target subunits. The Line blot Method is similar in that it utilizes membrane strips that are pre-coated with a specific set of antigens in parallel lines that are incubated with antibodies in order to detect the targets of interest. However, LOINC distinguishes the two Methods based on when the electrophoretic/chromatographic separation occurs. In IB, the separation occurs in the testing laboratory, whereas in Line blot, it occurs during the manufacturing step and not in the testing laboratory.

2.7.4 Coagulation

We distinguish among three kinds of coagulation methods: those that measure the coagulation activity (Coag), those that measure the coagulation factor via enzyme rate (Chromo or enzymatic), and those that measure the amount of the coagulant protein, not its activity. The Method that measures the amount of protein was called Imm until the 2.58 release, for which it was renamed IA to be consistent across laboratory classes.

2.7.5 Stains

We provide very detailed distinctions among various tissue stains, naming them in full. Stain methods that are modifications of a basic method are named using a <basic>.<modification> syntax, e.g.:

Methenamine silver stain.Jones

2.7.6 Substrates used to measure enzyme activity

For tests that are measuring enzyme activity using various substrates, the name of the enzyme or enzyme group will be the Component and the substrate will be the Method:

Mitochondrial respiratory chain enzymes: CCnt: Pt: Fibroblasts: Qn: 1-14C-glutamate substrate

2.7.7 "Detection limit" Methods

In specific instances, including viral load testing, urine albumin measurement and certain hormones (e.g., thyrotropin), we have LOINC codes where the Method value is the detection limit for the test. These were primarily created based on industry requests to differentiate tests that have at least an order of magnitude (10x) difference in detection thresholds, because this detection limit information was deemed more important than the specific laboratory method that was used. For example, we have codes for urine albumin with the Method Detection limit <= 20 mg/L and Detection limit <= 1 mg/L, and we also have thyrotropin terms with Method Detection limit <= 0.05 mIU/L and Detection limit <= 0.005 mIU/L. In some cases, the detection limit values are not round numbers, e.g., Detection limit <= 3.47 pmol/L. This seemingly odd value of 3.47 pmol/L is the molar equivalent of 1.0 ng/dL.

Based on discussion at the June 2018 Laboratory LOINC Committee meeting, LOINC terms that specify a detection limit in the Method are no longer recommended for use, except in a select number of use cases such as thyrotropin and urine albumin. In general, we will not create new terms with such Methods because the detection limit is not actually a Method but rather a measure of the sensitivity of the test. Instead, we recommend using a more generic code, such as a term with Probe.amp.tar as the Method for a viral load, and reporting the detection limit separately using LOINC 87706-8 Laboratory device Detection limit.

2.7.8 "Confirm" Method outside of Drug/Tox

In a few special cases outside of Drug/Tox, we have codes with the Method Confirm. In each of these instances, the fact that the specific instance of the result is a confirmatory result is deemed more important than the specific laboratory method used to obtain the result. Two such cases include ABO & Rh group typing, and Hepatitis B virus surface antigen testing in pregnant women.

2.7.9 Special issues with Methods for clinical measures

2.7.9.1 Estimated versus measured values

For some kinds of clinical measures we distinguish the Method Reported from Estimated and Measured. For example, reported body weight is the stated weight from a patient or surrogate. Estimated body weight is estimated by an observer, and measured would be the body weight as measured on a scale.

2.7.9.2 Oximetry

Use of oximetry as a Method can cause confusion. For details see the Oxygen Saturation Technical Brief at the end of this guide, but briefly, oximetry has several specific types, including pulse oximetry, co-oximetry and heme-oximetry. Unfortunately, at one time the word “oximetry” was used interchangeably with “pulse oximetry,” which created confusion in LOINC because we have one Method called oximetry and another called pulse oximetry. To avoid ambiguity, as of version 2.54 we deprecated most of the terms with oximetry as the Method, and moving forward we will continue to review the ones that are remaining and create new terms with the Method pulse oximetry as needed.

2.7.10 Imaging studies

We distinguish among the major imaging modalities for most measures derived from such imaging studies (e.g., cardiac outputs from a MUGA scan, angiography, 2D Echo, Doppler, etc.).

2.7.10.1 Methods that include imaging modality and calculation

In some cases, the Method includes two concepts: the imaging modality and the method of calculation for the specific measure. Through LOINC version 2.50 we used the pattern <imaging modality>.<calculation>, for example, US.2D.Teichholz. This format, however, does not clearly differentiate between the two distinct concepts being represented. This was further complicated by our convention of using a dot (.) to specify the submodality (e.g. 2D). Beginning in LOINC version 2.52, we separated the two concepts with a plus (+) rather than a dot (.), so that US.2D.Teichholz became US.2D+Calculated by Teichholz method. We have now updated all existing terms, and are using this format going forward.