LOINC uses three broad categories that beginners often blur together: lab tests, panels, and observations. The distinctions are small in the vocabulary and large in the terminology tool. Getting them right the first time saves the "why does my Observation say CBC" conversation later.
Naming the categories is the discipline. For related walkthroughs, the FHIR knowledge collection collects the surrounding material.
Lab Tests
A LOINC lab test is a single measurement code. Hemoglobin, sodium, creatinine, glucose. Each has one component, one specimen, one property, one method. The output is a single value.
Lab tests are the simplest category to search. Clinicians name the analyte; developers look up the code. A pass through the site's LOINC picker covers the ~90 most common lab codes for quick reference.
Panels
A LOINC panel is a collection of related lab codes bundled under a parent code. A CBC panel has hemoglobin, hematocrit, white blood cell count, platelet count, and more as components. A basic metabolic panel has sodium, potassium, chloride, glucose, and more.
Panels have their own LOINC codes. In FHIR, a panel result typically appears as an Observation with Observation.code set to the panel code and Observation.hasMember referencing individual result Observations. For the wider terminology framing, starting a LOINC search when the requester speaks clinical, not codes covers the entry pathway.
Observations (Beyond Labs)
Observation LOINC codes cover measurements beyond lab tests: vital signs (blood pressure, heart rate), assessments (Braden scale, PHQ-9), and social-determinant surveys. These have the same LOINC structure but come from a broader clinical context.
Every observation in FHIR uses a LOINC code where one exists. Deployments that mix LOINC observations with locally-coded observations end up with a terminology audit trail that varies by category.
The FHIR Interaction
A FHIR Observation carries Observation.code as the primary identifier. For lab tests, use the individual lab code. For panels, use the panel code and reference component Observations. For non-lab observations, use the observation-specific LOINC code.
Getting the level right matters for FHIR interoperability. Downstream systems read the code and dispatch based on the category; wrong codes produce wrong dispatch.
The Parts Model as a Diagnostic
The LOINC parts model reveals which category a code belongs to by inspecting the property axis. Mass concentration and count properties usually indicate labs. Rating scales indicate assessment observations. Structured survey properties indicate SDOH tools.
For the full parts framing, the LOINC parts model: property, system, and method at a glance covers the axes.
When SNOMED Fits Better
Some measurements straddle LOINC and SNOMED CT. A clinical finding might be better expressed as a SNOMED code than as a LOINC observation. For the specific decision, LOINC vs SNOMED for a specific observation — how to decide covers the split.
The rule of thumb: LOINC for measurements, SNOMED for findings and conclusions. The two vocabularies complement rather than compete.
The Documentation Habit
Every LOINC integration benefits from a short internal document mapping the categories to the FHIR resources the deployment uses. Lab tests → Observation with single value. Panels → Observation with hasMember. Assessments → Observation with component measures.
The map is short and durable. Deployments that skip it discover the shape by trial and error. Naming the distinctions early is the shortcut past that trial.
Sources
- LOINC canonical users-guide chapter defining panels - LOINC canonical users-guide chapter defining panels, tests, and observation grouping
