LOINC
LOINC (Logical Observation Identifiers Names and Codes) gives a universal identifier to the question being asked in a laboratory test, a clinical measurement or a standardized questionnaire.
Local laboratory codes are meaningless outside the laboratory that assigned
them. HGB, HB, HEMO and 001234 might all mean haemoglobin in blood — and
only LOINC lets a receiving system know that.
The six axes
Every LOINC code is defined by six parts, which is what makes it precise:
| Axis | Question | Example |
|---|---|---|
| Component | What is measured? | Hemoglobin |
| Property | What kind of quantity? | MCnc (mass concentration) |
| Time | Point in time or interval? | Pt |
| System | What specimen or body site? | Bld (blood) |
| Scale | Quantitative, ordinal, nominal, narrative? | Qn |
| Method | How, if it matters? | (often unspecified) |
718-7 = Hemoglobin : MCnc : Pt : Bld : Qn
Codes are numeric with a check digit; they carry no meaning in themselves, which is deliberate.
What LOINC covers
- Laboratory tests — chemistry, haematology, microbiology, serology
- Clinical measurements — vital signs, anthropometry, scores
- Survey instruments — standardized assessment questions and answers
- Document types — the kind of clinical note or report
LOINC identifies the observation; the result is carried separately, with units (UCUM) and, where the result is itself coded, a terminology such as SNOMED CT.
LOINC in FHIR
{
"resourceType": "Observation",
"status": "final",
"code": {
"coding": [{
"system": "http://loinc.org",
"code": "718-7",
"display": "Hemoglobin [Mass/volume] in Blood"
}]
},
"valueQuantity": {
"value": 11.2,
"unit": "g/dL",
"system": "http://unitsofmeasure.org",
"code": "g/dL"
}
}
LOINC is the expected code system for Observation.code in most
FHIR implementation guides, and it appears in OBX-3 of
HL7 v2 result messages.
Mapping a laboratory catalogue
- Export the local test catalogue with names, specimens, units and methods.
- Map the highest-volume tests first — a small number of tests usually cover most of the traffic.
- Use RELMA or the LOINC search tools; match on all six axes, not on the name.
- Have a laboratory professional review the mappings; specimen and property mistakes are easy to make and hard to detect later.
- Version the mapping table and treat it as maintained metadata, not a one-time migration artefact.
Common mistakes
- Mapping on component alone. Glucose in serum and glucose in urine are different codes.
- Ignoring property. Mass concentration and substance concentration are not interchangeable, and neither are their units.
- Mapping panels to member codes. Panels have their own codes.
- Skipping units. A LOINC code without UCUM units is only half an answer.
References
- LOINC — https://loinc.org/
- UCUM units of measure — https://ucum.org/
- SNOMED CT for coded result values