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This section contains a high-level overview of LOINC with some additional information in areas of direct relevance to subsequent chapters of this guide. For a more complete introduction to LOINC and supporting resources see

Logical Observation Identifiers Names and Codes (LOINC®) is a terminology standard for identifying laboratory tests and other measurements. It specifies universal codes, names, and other attributes for laboratory results as well as clinical reports, physical exam findings, survey instruments and other observations. It was developed to enable the exchange and pooling of results from diverse sources in order to enhance clinical care, outcomes management and research.


LOINC codes include laboratory and other clinical observations. The laboratory portion of LOINC includes measurements made on specimens, such in chemistry, hematology, serology, microbiology (including parasitology and virology), toxicology, cell counts, antibiotic susceptibilities, and more. The clinical portion of LOINC includes codes for observations made on patients and populations. LOINC has codes for observations like vital signs and a wide range of other clinical observations. Vital signs and anthropomorphic measurement are included in the scope of the cooperation agreement. Other clinical domains are not currently included in the scope of the agreement with IHTSDO.

LOINC includes codes that identify test observations (e.g. blood culture, antibiotic sensitivity). Other code systems, including SNOMED CT, often provide values that can be applied to represent results (e.g. staphylococcus, amoxicillin). If we consider the observation as a question and the observation values as answers, LOINC provides codes for the questions and SNOMED CT provides codes for many of the non-numeric answers.

Maintenance, Governance and Licensing

LOINC is owned, maintained and licensed by the Regenstrief Institute, Inc. (RII). RII is a non-profit medical research organization associated with Indiana University School of Medicine. LOINC is available free of charge subject to the license conditions and terms of use Updated versions are released twice a year. The LOINC web search tool is available at The LOINC database and a free browsing and mapping program, the Regenstrief LOINC Mapping Assistant (RELMA®), can be downloaded from


LOINC is widely adopted, and the user community continues to grow rapidly. The worldwide LOINC community presently has more than 34,000 users in 163 countries (see

Within the USA, LOINC has been adopted by large reference laboratories, health information exchanges, healthcare organizations, insurance companies, research applications, and several national standards initiatives and programs. In particular, LOINC was adopted as the standard for laboratory orders and results as part of the Centers for Medicare and Medicaid Services Electronic Health Record (EHR) "Meaningful Use" incentive program as specified in the Standards and Certification Criteria.

Outside the USA, LOINC has also been adopted as a national standard in more than 25 countries. In addition, there are many large data exchanges using LOINC around the world.


Each test is represented by a formal six-part LOINC name and assigned a LOINC code, which is a number with a check digit (see Table 1). Each code is also assigned an observation class (e.g., chemistry, hematology, and radiology); related names (to assist searches of the database); and other attributes.

For most classes of laboratory observations, there is also a "short name" (less than 40 characters long), and a Long Common Name that is more clinician friendly.

LOINC Terms, Codes and Axes

LOINC fully-specified names (including laboratory test results, clinical measurements, and results of other diagnostic studies) are defined in terms of six major axes as described in Table 2: 1. Component name, 2. Property, 3. Time, 4. System, 5. Scale, and 6. Method. The fully-specified (formal) LOINC name must include entries for the first five major axes; the method axis is included only when the method distinction makes an important difference to the clinical interpretation of the result.

Four additional minor axes are challenge information; adjustments; supersystem, e.g., fetus, blood product; and time operators (maximum, minimum, last, first), which are only used when relevant. The challenge axis is the most complex of the minor axes and includes the amount, route, and timing (e.g., oral glucose tolerance test). The details about these other axes can be found in the LOINC Users 'Guide.

Examples of LOINC terms are shown in Table 3.1-1.

Table 3.1-1: Examples of laboratory LOINC codes and formal LOINC names


LOINC name (Componentname:Property:Time:Specimen:Scale:Method)


















Table 3.1-2: Formal model for constructing LOINC fully specified names

Axis Name


Component name

The analyte or attribute being measured or observed. E.g., sodium, body weight.

(Kind of) Property

Differentiates kinds of quantities relating to the same substance. E.g., mass concentration, catalytic activity.

Time (Aspect)

Identifies whether the measurement is made at a point in time or a time interval. E.g. 24H for a urine sodium concentration.


The specimen, body system, patient, or other object of the observation. E.g. cerebral spinal fluid, urine, radial artery.

(Type of) Scale

The scale or precision that differentiates among observations that are quantitative, ordinal (ranked choices), nominal (unranked choices), or narrative text.

(Type of) Method

An optional axis that identifies the way the observation was produced. It is used only when needed to distinguish observations that have clinically significant differences in interpretation if made by different methods.


LOINC creates only those combinations that have clinical relevance in laboratory medicine. Terms are not created by blind permutations. Regenstrief (with guidance from the LOINC committee) reviews new code requests carefully to make sure that only meaningful LOINC codes that can be pragmatically used by the LOINC community are added to the database.


The atomic elements that comprise a fully-specified LOINC name are called LOINC "Parts". Each fully-specified name will consist of 5 or 6 parts (depending on whether the Method is important for interpreting the result), each with a part type corresponding to one of the major axes described above. Each LOINC Part is also assigned an identifier (that begins with the prefix "LP"), and internally Regenstrief maintains links between the full LOINC term and the Parts that comprise it. Regenstrief uses LOINC Parts in many aspects of LOINC development, such as: adding synonymy, building hierarchies, creating alternate display names, linking descriptive text, and more.

The Parts and their linkages are not distributed as part of the main LOINC table, but they are part of the content used by the RELMA program.

LOINC "part" concepts (e.g. sodium) serve as building blocks for the description of tests and observations, in association with a set of semantic relations. For example, Sodium:SCnc:Pt:Ser/Plas:Qn, the laboratory test in which the molar concentration of sodium is measured in the plasma (or serum) is identified by 2951-2. The list of relations of this concept to other concepts ("parts") is shown in Table 3.1-3 and Table 3.1-4. For example, the "part" concept Sodium is linked to this test by the relationship component.

Table 3.1-3: Example of the relation of the LOINC code 2951-2 to LOINC Part codes






Sodium [Mass or Moles/volume] in Serum or Plasma

Part Type

Part No.

Part Name






SCnc [Substance Concentration]



Pt [Point in time (spot)]



Ser/P1as [Serum or Plasma]








Table 3.1-4: Example of the relation of the LOINC code 5778-6 to LOINC Part codes






Color of Urine

Part Type

Part No.

Part Name









Pt [Point in time (spot)]






Nom [Nominal]




The LOINC terminology does not use description logic. However, the formal definitions provided by LOINC all conform to the 6-axis template (described in Table 2) and make use of named semantic relations.

In addition to creating codes for single tests, measurements, or observations, LOINC also defines concepts to represent collections of discrete elements such as panels (batteries), forms, and data sets.

For example, a CBC/FBC test (complete/full blood count) is expected to deliver a set of results for different components including leukocytes, erythrocytes, hemoglobin, hematocrit, etc.

Hierarchy Tree Structure

Regenstrief creates hierarchies to organize LOINC terms based on a structured arrangement of LOINC elements (also known as parts).RELMA has 5 selectable hierarchy trees that are commonly used to narrow the search limits returned:

  • Class
  • Multiaxial (component/system)
  • System (specimen)
  • Component
  • Method

The LOINC hierarchy group LOINC concepts by specifying the parent-child relationship between the elements used in one (or more of the axes).

Most often, the hierarchies are used to restrict searches performed using RELMA.

The Multiaxial hierarchy organizes LOINC codes based on more than one of the LOINC name axes. For laboratory tests, it organizes first by the Component and then by the System. The Multiaxial Hierarchy is distributed as an accessory file that is part of the LOINC release.

Figure 3.1-1: Class hierarchy showing Class classification of laboratory tests

Figure 3.1-2: Multiaxial hierarchy of LOINC showing relations in Microbiology parts of component

Figure 3.1-3: Multiaxial hierarchy of LOINC showing relations in system ax: specimen


McDonald CJ, Huff SM, Suico JG, Hill G, Leavelle D, Aller R, Forrey A, Mercer K, DeMoor G, Hook J, Williams W, Case J, Maloney P. LOINC, a universal standard for identifying laboratory observations: a 5-year update. Clin Chem. 2003 Apr;49(4):624-33.

Vreeman DJ, Chiaravalloti MT, Hook J, McDonald CJ. Enabling international adoption of LOINC through translation. J Biomed Inform. 2012 Aug;45(4):667-73.

Bodenreider O. Issues in mapping LOINC laboratory tests to SNOMED CT. AMIA Annu Symp Proc. 2008 Nov 6:51-5.

Steindel S, Loonsk JW, Sim A, Doyle TJ, Chapman RS, Groseclose SL. Introduction of a hierarchy to LOINC to facilitate public health reporting. Proc AMIA Symp. 2002:737-41.

Adamusiak T, Bodenreider O. Quality assurance in LOINC using Description Logic. AMIA Annu Symp Proc. 2012;2012:1099-108.