LOINC

In one line

LOINC (Logical Observation Identifiers Names and Codes) gives every laboratory test and clinical observation a universal code — so "serum glucose, fasting" is 1558-6 in every hospital on Earth, whatever the local lab calls it.

How it works

Each LOINC term is defined by six axes — component (what is measured), property (mass? concentration?), timing, system (specimen: serum, urine…), scale (quantitative, ordinal…), and method. That six-part precision is the point: "glucose" alone is ambiguous; the axes pin down exactly which question the test asks. Maintained free of charge by the Regenstrief Institute, with answer-list support for surveys and panels for groupings (a CBC is a panel of LOINC codes).

Where it shows up in digital health

Observation.code in FHIR binds to LOINC almost everywhere (US Core mandates it; NRCeS uses it for diagnostics); lab-interface mapping projects are largely local-code→LOINC exercises; and the classic division of labour is worth memorising: LOINC codes the question, SNOMED CT codes the answer — test 1558-6, finding Diabetes mellitus.