Short answer: LOINC identifies the question: which test, measurement, vital sign, questionnaire item or document type (Hemoglobin A1c in blood, Systolic blood pressure, Discharge summary). SNOMED CT identifies the answer and the clinical world around it: findings, disorders, organisms, substances, procedures, body sites (Staphylococcus aureus, Type 2 diabetes mellitus, Left kidney). In FHIR, that becomes a rule: Observation.code is LOINC; a coded Observation.value, bodySite or specimen type is SNOMED CT. Both are needed in every laboratory and most clinical records.
Side by side
Side by side, LOINC is maintained by the Regenstrief Institute and identifies what was measured, observed or asked, with about 100,000 terms built from a six-part name. SNOMED CT is maintained by SNOMED International and identifies what was found, present or done, with about 360,000 concepts in a polyhierarchy. In FHIR, LOINC uses http://loinc.org and SNOMED CT uses http://snomed.info/sct.
| LOINC | SNOMED CT | |
|---|---|---|
| Owner | Regenstrief Institute (Indianapolis) | SNOMED International (London) |
| Answers | What was measured, observed or asked? | What was found, present, done, where, by what? |
| Size | ~100,000 terms | ~360,000 active concepts |
| Structure | Six-part fully specified name: component, property, timing, system, scale, method | Concepts with descriptions and defining relationships in a polyhierarchy |
| Identifier | Number with check digit, e.g. 4548-4 | Numeric concept ID, e.g. 44054006 |
| Also covers | Panels, document types, survey instruments (PHQ-9), radiology procedures, clinical attributes | Procedures, observables, specimens, pharmaceutical products, situations, events |
| Licence | Free, with attribution and terms of use | Free in member countries; affiliate licence elsewhere |
| In FHIR | http://loinc.org | http://snomed.info/sct |
For each on its own: What is a LOINC code? and What is SNOMED CT?.
The six axes of a LOINC term
The six axes of a LOINC term are component (what is measured), property (mass or substance concentration, for example), timing (point in time or over a period), system (the specimen), scale (quantitative, ordinal, nominal or narrative) and method, used only when it changes the meaning. Changing any axis yields a different LOINC code, which is why laboratories use LOINC.
LOINC's power is its fully specified name. 2345-7 is Glucose (component) : MCnc mass concentration (property) : Pt point in time (timing) : Ser/Plas serum or plasma (system) : Qn quantitative (scale). Change one axis and you have a different test: 14749-6 is the same glucose as substance concentration (mmol/L); 2339-0 is glucose in whole blood; 41653-3 is glucose in capillary blood by glucometer. SNOMED CT has no equivalent of this axis structure for tests, which is why laboratories code with LOINC.
Where they overlap
LOINC and SNOMED CT overlap where SNOMED CT contains procedure and observable entity concepts for laboratory tests, which historically produced two codes for one test. Since the 2013 cooperation agreement between Regenstrief and SNOMED International, and the LOINC Ontology published from 2023, the division is settled: test identifiers belong to LOINC, clinical content to SNOMED CT.
SNOMED CT also contains procedure concepts for tests (Glucose measurement, procedure) and an observable entity hierarchy. Historically, that produced two codes for the same test. In 2013 Regenstrief and SNOMED International signed a cooperation agreement, and since 2023 the LOINC Ontology (published as a SNOMED CT extension) gives LOINC terms SNOMED-style definitions, so a system can navigate LOINC through SNOMED's hierarchy without recoding. The practical division stands: order and result identifiers in LOINC; clinical content in SNOMED CT.
The rule in FHIR, with an example
The rule in FHIR is that Observation.code is a LOINC code naming the question, while a coded answer in Observation.valueCodeableConcept, the bodySite, the method and the Specimen.type are SNOMED CT concepts. In the blood culture example below, LOINC 600-7 identifies the test and SNOMED CT 3092008 identifies the organism found, Staphylococcus aureus.
A blood culture: the question is a LOINC code, the answer is a SNOMED CT organism, the specimen type is SNOMED CT:
{
"resourceType": "Observation",
"status": "final",
"category": [{"coding": [{"system": "http://terminology.hl7.org/CodeSystem/observation-category", "code": "laboratory"}]}],
"code": {"coding": [{"system": "http://loinc.org", "code": "600-7", "display": "Bacteria identified in Blood by Culture"}]},
"subject": {"reference": "Patient/example"},
"effectiveDateTime": "2026-03-14T08:40:00+01:00",
"valueCodeableConcept": {"coding": [{"system": "http://snomed.info/sct", "code": "3092008", "display": "Staphylococcus aureus"}]},
"specimen": {"reference": "Specimen/blood-1"}
}
Other placements that follow the same logic:
| Element | Code system | Example |
|---|---|---|
| Observation.code | LOINC | 8480-6 Systolic blood pressure |
| Observation.valueCodeableConcept | SNOMED CT | 260385009 Negative; 3092008 Staphylococcus aureus |
| Observation.bodySite | SNOMED CT | 368209003 Right upper arm |
| Observation.method | SNOMED CT | 37931006 Auscultation |
| Specimen.type | SNOMED CT | 119297000 Blood specimen |
| ServiceRequest.code | LOINC (test ordered) or SNOMED CT procedure | 57021-8 CBC panel; 71388002 Procedure |
| DiagnosticReport.code | LOINC panel | 58410-2 CBC panel |
| DocumentReference.type | LOINC document ontology | 18842-5 Discharge summary |
| Condition.code | SNOMED CT (with ICD for reporting) | 44054006 Type 2 diabetes mellitus |
| Questionnaire item | LOINC (for standard instruments) | 44261-6 PHQ-9 total score |
When it is not clear
When the choice between LOINC and SNOMED CT is unclear, the FHIR profiles decide: vital signs use fixed LOINC codes, a reported symptom is a LOINC questionnaire item whose value is a SNOMED CT finding, AllergyIntolerance.code is SNOMED CT with no role for LOINC, and imaging procedures take LOINC radiology codes or SNOMED CT procedures as the national guide decides.
- Vital signs: LOINC codes, fixed by the FHIR vital signs profile (8867-4 heart rate, 29463-7 body weight).
- Symptoms a patient reports: the observation that a symptom was assessed can be LOINC (a questionnaire item); the symptom itself is a SNOMED CT finding in the value, or a Condition if a clinician asserts it.
- Allergies: AllergyIntolerance.code is SNOMED CT (substance or product); LOINC plays no role.
- Imaging procedures: LOINC has radiology codes (the LOINC/RSNA Radiology Playbook); SNOMED CT has procedure concepts; national guides pick one.
In Anpheros
Anpheros uses LOINC in Observation.code for every measurement and laboratory result, UCUM for units, and SNOMED CT wherever the source supplies a coded finding, body site or specimen. Its lab connector requires LOINC codes, and its symptom entries reference both SNOMED CT and ICD-10 so that clinical systems can read what a person logs.
The lab connector requires LOINC codes, the mapping guide explains how to get there, and the developer site documents the code systems in use.
Related pages
- What is a LOINC code?
- What is SNOMED CT?
- How to map local lab codes to LOINC
- FHIR Observation explained
- UCUM units in lab results
- FHIR code systems (developer site)
Sources
- LOINC: about LOINC
- LOINC and SNOMED International cooperation
- SNOMED International: LOINC Ontology
- HL7 FHIR R4: using LOINC with FHIR
- HL7 FHIR R4: Vital Signs profile
Frequently asked questions
Can I use SNOMED CT instead of LOINC for lab tests?
SNOMED CT has procedure and observable concepts for tests, but laboratories, national guides and the FHIR profiles for lab results use LOINC in Observation.code. Using SNOMED there makes your results unreadable to most systems.
Is LOINC free?
Yes. Regenstrief distributes LOINC free of charge under a licence that requires attribution and forbids altering the codes. The mapping tool RELMA is also free.
What is the LOINC Ontology?
A joint product of Regenstrief and SNOMED International, released from 2023, that expresses LOINC terms with SNOMED CT-style definitions in a SNOMED extension, so the two can be navigated together. The codes themselves do not change.
Which code system for a document type?
LOINC: its document ontology covers discharge summaries, consultation notes, imaging reports and more, and FHIR DocumentReference.type and CDA document codes both use it.
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