FAQ: Structure of LOINC Codes and Names

What is the LOINC code structure?

LOINC codes have a fixed length field of 7 characters within the LOINC database. Current codes are from 3-7 characters long. If you are currently building a system to include LOINC codes, we advise allowing up to 10 characters (or at least 8 characters) to allow for further expansion. When the LOINC database contains 100,000 different LOINC records, the fixed length field will need to expand to 8 characters. The last digit of the LOINC code is a check digit and is always preceded by a hyphen(dash). The hyphen, as well as all the numbers, is part of the LOINC and must be included. The check digit is always a number form 0-9 and serves to help avoid errors in transcription of the code. Current licensing agreement for use of LOINC codes requires you to send the complete LOINC code.

What is the fully specified LOINC name structure?

A fully specified name in LOINC includes the following attributes:

<component/analyte>:<kind of property>:<time aspect>:<system type>:<scale>:<method>

Each of these attributes, or "parts" is described in detail in the LOINC Users' Guide and briefly in this FAQ. You can also watch our video Understanding the structure of LOINC Names.

What are the "parts" of a LOINC code?

A fully specified name of a test result or clinical observation has 5-6 main parts including: the name of the component or analyte measured, its property (substance concentration, mass, volume), the timing of the measurement, the type of sample (serum, urine, etc.), the scale of measurement (qualitative vs. quantitative, etc.), and the method (if it is relevant-RIA, Immune Blot, etc.).

Explain the component/analyte "part".

The component/analyte consists of three main subparts:

  1. The principal name (e.g., the name of the analyte or the measurement);
  2. challenge of provocation, if relevant, including time delay, substance of challenge, amount administered, and route of administration; and
  3. any standardization or adjustment.

For more information, refer to Component/Analyte (1st Part) section of the LOINC Users' Guide.

Explain the property "part".

The kind of property distinguishes between different kinds of quantities of the same substance. Analytes are often measured using different types of units. Kinds of properties include: Mass, Substance, Catalytic Activity, Arbitrary, and Number. Pharmaceutical industry terms for tests include properties, such as Mass Substance Concentration (MSCnc) or Mass Substance Rate (MSRat). Definitions for the main property categories may be found in LOINC Users' Guide.

Each of the properties also has subtypes including: concentration (amount divided by a volume like mg/dl or gm/L), contents (amount divided by a mass like umol/ g creatinine), ratios (one measure divided by another taken from the same system), fractions (ratios of a part over a whole, usually reported as a percent), and rates [a measure taken over time like mg/day (a mass rate=MRat) or a clearance volume rate (expressed as a clearance=VRAT)]. For more information, refer to the Kind of Property (2nd Part) section of the LOINC Users' Guide.

Explain the time aspect "part".

A measurement may be taken at a moment in time or measured over a specified time interval. For more information, refer to Time Aspect (Point or moment in time vs. time interval) (3rd part) section of the LOINC Users' Guide.

Explain the system (sample) type "part".

A system may consist of up to two parts.

  1. The first part names the system, and
  2. the second part names a subpart of the sample that is not the patients (e.g., fetus, donor, blood product unit, etc.).

Chemistry tests are usually run on serum, urine, blood and/or cerebrospinal fluid (CSF). The code "XXX" is used to identify a material that is not specified in another part of the HL7 message (e.g., OBX specimen segment). When testing on serum or plasma is clinically equivalent, the system "Ser/Plas" should be used.

If the test is run on a combination of types of systems (such as the ration of substance found in CSF and plasma) the codes are joined with a "+" such as PLAS+CSF.

In cases where super sytems apply, the subpart will be delimited with a "^" such as A AG:ACNC:PT:RBC:^BPU:ORD which specifies the A Antigen reported on a blood product pack assigned to that patient. Refer to System (Sample) Type (4th part) section of the LOINC Users' Guide.

Explain the scale type "part".

Type of scale specifies the scale of the measure. The following scale types are defined: Quantitative(Qn), Ordinal(Ord), Nominal(Nom), Narrative(Nar). For more information, refer to Type of Scale (5th part) section of the LOINC Users' Guide.

What is the method type?

Type of method specifies the method used to perform the test. The method axis is an optional component of a fully-specified LOINC name. It is only used when the information in the other axes does not sufficiently distinguish clinical measurements that have very different reference ranges, sensitivities, or specificities. In general, a method is specified when clinicians would want to see observations made by one particular method separated from measurements made by a different methodology when displayed in a clinical report. For the purpose of data exchange and further specificity, the reference range and method can also be sent in other fields of ASTM, HL7, and CEN TC251 result messages. For more information, refer to Type of Method (6th part) section of the LOINC Users' Guide.