1 - Introduction

This section last updated: 2020-06-10 (5 years ago)

LOINC provides a set of universal names and ID codes for identifying laboratory and clinical test results.¹,² LOINC facilitates the exchange and pooling of results, such as blood hemoglobin, serum potassium, or vital signs, for clinical care, outcomes management, and research. LOINC's universal identifiers (names and codes) can be used in the context of order and observation exchanges between information systems that use syntax standards such as HL7³, CEN TC251, ISO TC215, ASTM⁴, and DICOM. Specifically, the identifier can be used as the coded value for an observation in any other standard that uses the observation/observation value paradigm, whether messages, documents, application programming interface (API), etc. For example, LOINC codes are used widely in the OBX segment “Observation Identifier” field (OBX-3) of an ORU HL7 (HL7 version 2.x or ASTM 1238-9410) message that may be sent between a Clinical Laboratory Information Management Systems (LIMS) and Electronic Health Record Systems (EHR).⁵, ⁶ In this way, LOINC codes provide “universal” identifiers that allow the exchange of clinical data between heterogeneous computing environments.

If data producers use only their internal (and idiosyncratic) codes to identify the variables, then receiving medical information systems cannot fully “understand” the results they receive unless they either adopt the producer's codes (which is impossible if information system receives results from multiple sources, or invest in the work to map each producer's coding system to their internal code system.⁷ If medical information producers who wish to communicate with each other adopt LOINC codes to identify their results in data transmissions, this problem would disappear. The receiving system with LOINC codes in its master vocabulary file would be able to understand and properly file HL7 results messages that identified clinical observations via LOINC codes. Similarly, if test and observation codes were reported with the LOINC codes, government agencies would be able to pool results for tests from many sites for research management and public health purpose. LOINC terms should be of interest to many in the healthcare ecosystem, including hospitals, clinical laboratories, outpatient services, state health departments, governmental health care providers, health IT vendors, diagnostic testing vendors, third-party payers, and organizations responsible for quality assurance and utilization review.

In the beginning, we used many sources as substrate that informed the creation of LOINC, including the Silver Book from the International Union of Pure and Applied Chemistry (IUPAC) and the International Federation of Clinical Chemistry (IFCC),⁸ textbooks of clinical pathology (e.g., Henry⁹ and Tietz¹⁰), the expertise and work of the LOINC members, and EUCLIDES. We also reviewed the master test files of eight sources (Indiana University/Regenstrief, University of Utah, Association of Regional and University Pathologists (ARUP), Mayo Medical Laboratories, LDS Hospital in Salt Lake City, the Department of Veterans Affairs, Quest Diagnostics, and University of Washington). From the start, this was an empirical effort with the goal to provide codes that correspond to the concepts in real world laboratories’ and clinical departments’ master files.

We aim to achieve a level of detail in the definition of a LOINC term that will map one-to-one to the separately reported observations on a clinical laboratory or other clinical system report. If a test has its own column on a clinical report, or has a reference range that is significantly different from other tests, or has a different clinical meaning than other related tests, it will usually be assigned a separate LOINC code and name. LOINC creates terms both for individual variables (reportable tests or clinical measurements) and collections of such variables. We create terms that represent two kinds of collections: Panels, where the child elements are enumerated, and Documents where the terms represent general information collections whose contents are not explicitly enumerated.

The Regenstrief Institute maintains LOINC and makes it available in a number of file formats. LOINC and the related files (such as this document) are copyrighted to assure that multiple variants of the standard do not emerge. Having many variants would defeat the purpose of a universal identifier for test results. LOINC is made available at no cost worldwide under the license at http://loinc.org/license. The LOINC Table, supporting documentation and supplemental files, and the RELMA® mapping program are all made available by the Regenstrief Institute on the LOINC website (http://loinc.org). LOINC content and associated documentation has been translated into many languages in further support of adoption globally.¹¹

1.1 Acknowledgments

Over the years, LOINC has been supported financially by many organizations and individuals. We are thankful for all of their generous support, which we acknowledge on the LOINC website.

This work was initiated by and is performed under the auspices of the Regenstrief Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of any of our individual or organizational supporters.

We remain grateful to the late Dr. Henrik Olesen for his very helpful comments and insights about laboratory test coding. Dr. Olesen served as chair of the Commission on Nomenclature for Properties and Units (1989) and member of the joint IUPAC-IFCC Commission on Nomenclature, Properties and Units (1988-1997).

1.2 What is not part of the name

LOINC terms are not intended to transmit all possible information about a test or observation. They are only intended to identify the test result or clinical observation. Other fields in the message can transmit the identity of the source laboratory and special details about the sample. For instance, the result code may identify a blood culture, but the message source code can be more specific and identify the sample as pump blood.

The level of detail in the LOINC Fully Specified Name was intended to distinguish tests that are usually distinguished as separate test results within the master file of existing laboratory systems. Indeed, at the outset, we used the master files from seven U.S. laboratories to shape this effort, and requests from commercial labs and hospitals continue to shape the content of the LOINC effort. Therefore, certain parameters and descriptions pertaining to test performance are specifically excluded from the fully specified test name. These parameters will typically be reported in separate fields (attributes) of a test/observation report message, not as part of the observation name. Attributes that we explicitly exclude from the fully specified name are:

• the instrument used in testing
• fine details about the sample or the site of collection, such as “right antecubital fossa”
• the priority of the testing, e.g., whether stat or routine
• who verified the result
• the size of the sample collected
• the place of testing (e.g., home, bedside, clinical lab)

In the case of laboratory tests, the name does include information that identifies the type of sample (or specimen). However, the “sample” part of the name is not meant to carry all possible information about the sample, but only enough to indicate significant differences in the result and to reflect current usage in test names. For example, laboratories usually define urine sodium, sweat sodium, and serum sodium as different tests because each of these has a different normal range. But laboratories do not define different tests to distinguish the concentration of arterial serum sodium from venous serum sodium, though the lab may report that the sample was venous or arterial in another part of the report. We are guided by the pragmatics of conventional usage. If laboratories define separate tests for the same measurements done on different specimens (this usually implies a well-defined normal range difference), we will define different “result-able” tests in our dictionary. If they do not, we will not.

The extent to which we include methods as part of the name is also guided by pragmatics. We distinguish tests/observations by the type of method used to produce the results only if a given type of method has an important effect on the interpretation of the result. This is a complex subject and it is difficult to fully describe our rationale in this guide. Where laboratories do not tend to include the method in the name (e.g., most of chemistry) we do not include the method in the name. Where they tend to (e.g., in immunochemistry) we do. For some tests, this can be justified by the standardization of methods to produce “equivalent” results, and sometimes by the many variables (method, reagent) that one could never hope to represent fully in a single name. However, even when we do distinguish these cases, we distinguish by type of method, not the most detailed possible method distinction. (See Section 2.7 - Type of Method, for more details.)

The College of American Pathologists produces statistical summaries of the results for measurements of standard samples broken down by laboratory and by instrument or procedure. (These are called CAP surveys.) We considered using this CAP survey data to decide empirically when test names should be distinguished by method, but decided this was not feasible because many of the apparent differences in method obtained with the standard samples were artifacts of the sample matrix and did not apply to serum specimens. In addition, the variation among laboratories was often of the same magnitude as the variation among methods within laboratories for the same method.

We do not mean to underrate the importance of method differences. The result message will still include information about the normal range for that particular test, the source laboratory and, if the laboratory wishes, specific information about the method. However, such information is reported in separate fields in the HL7 message (e.g., OBX-17 can carry very detailed method information). It is not embedded in the name of the test.

1.3 Scope of LOINC

The current scope of the existing laboratory portion of the LOINC database includes all observations reported by clinical laboratories, including the specialty areas: chemistry, including therapeutic drug monitoring and toxicology; hematology; serology; blood bank; microbiology; cytology; surgical pathology; and fertility. A large number of terms used in veterinary medicine have also been included. In addition, the scope includes those non-test measurements that are commonly required to interpret test results and are usually included as part of the report with the laboratory observations. Examples include:

• for cervical pap smears, the phase of menstrual cycle or use of estrogens
• for arterial blood gases, inspired oxygen
• for drug concentrations used in pharmacokinetics, the dose
• for a blood bank, the number of units dispensed

The June 2000 release contained our first foray into order sets/batteries. Existing LOINC codes could always be used to order the specific tests observation, but prior to 2000 there was no mechanism to use LOINC codes to order a set of observations.

The clinical portion of the LOINC database covers tests, measures, and other observations about a patient that can be made without removing a specimen from them. LOINC has codes for observations like vital signs, hemodynamics, intake/output, EKG, obstetric ultrasound, cardiac echo, urologic imaging, gastroendoscopic procedures, pulmonary ventilator management, radiology studies, clinical documents, selected survey instruments (e.g. Glasgow Coma Score, PHQ-9 depression scale, CMS-required patient assessment instruments), and other clinical observations.