Observation — Atomic clinical measurement

The elementary unit of clinical data in FHIR. Every vital sign, lab result, anthropometry, social-history item, and clinical score flows through Observation.

Purpose

Observation represents an atomic clinical measurement or assertion made about a patient, a specimen, a device or a group. Typical usages:

• vital signs (blood pressure, heart rate, temperature, SpO₂, weight…);
• lab results (CBC, biochemistry, serology);
• quantitative imaging (lesion measurement);
• scored clinical assessments (Glasgow, ECOG, NIHSS);
• social determinants of health (smoking status, education);
• patient-reported outcomes (PROMs).

An Observation can be atomic (one value) or composite via component (blood pressure, cardiac cycles, differential counts).

Key fields

JSON example

A blood pressure measurement, 122/78 mmHg, taken on the right arm, status final:

{
  "resourceType": "Observation",
  "id": "blood-pressure",
  "status": "final",
  "category": [{
    "coding": [{
      "system": "http://terminology.hl7.org/CodeSystem/observation-category",
      "code": "vital-signs",
      "display": "Vital Signs"
    }]
  }],
  "code": {
    "coding": [{
      "system": "http://loinc.org",
      "code": "85354-9",
      "display": "Blood pressure panel with all children optional"
    }],
    "text": "Blood pressure"
  },
  "subject": { "reference": "Patient/example" },
  "encounter": { "reference": "Encounter/example" },
  "effectiveDateTime": "2026-05-14T10:30:00+01:00",
  "performer": [{ "reference": "Practitioner/example" }],
  "bodySite": {
    "coding": [{
      "system": "http://snomed.info/sct",
      "code": "368209003",
      "display": "Right arm"
    }]
  },
  "component": [{
    "code": {
      "coding": [{
        "system": "http://loinc.org",
        "code": "8480-6",
        "display": "Systolic blood pressure"
      }]
    },
    "valueQuantity": {
      "value": 122,
      "unit": "mmHg",
      "system": "http://unitsofmeasure.org",
      "code": "mm[Hg]"
    },
    "interpretation": [{
      "coding": [{
        "system": "http://terminology.hl7.org/CodeSystem/v3-ObservationInterpretation",
        "code": "N",
        "display": "Normal"
      }]
    }]
  }, {
    "code": {
      "coding": [{
        "system": "http://loinc.org",
        "code": "8462-4",
        "display": "Diastolic blood pressure"
      }]
    },
    "valueQuantity": {
      "value": 78,
      "unit": "mmHg",
      "system": "http://unitsofmeasure.org",
      "code": "mm[Hg]"
    }
  }]
}

• code = LOINC 85354-9 (Blood pressure panel).
• Two component entries: systolic (LOINC 8480-6) and diastolic (LOINC 8462-4), each with valueQuantity and UCUM unit mm[Hg].
• bodySite = SNOMED CT 368209003 (Right arm).

REST API

• GET /Observation/blood-pressure — fetch by id.
• GET /Observation?patient=example&code=loinc|85354-9 — all BPs for a patient.
• GET /Observation?patient=example&category=vital-signs&date=ge2026-05-01 — recent vital signs.
• GET /Observation?patient=example&code=loinc|2339-0&value-quantity=gt7.0|http://unitsofmeasure.org|mmol/L — glycaemias > 7 mmol/L.
• POST /Observation — creation.
• $lastn — SMART operation that returns the last N observations grouped by code.

Profiles

Common pitfalls

• value[x] + dataAbsentReason: invariant obs-6 forbids having both at once. Pick one.
• Display unit vs UCUM: valueQuantity.unit is free-text but valueQuantity.code must be a strict UCUM code. mmHg is the display unit, mm[Hg] is the official UCUM code.
• component vs hasMember: use component for intrinsically-linked measurements (BP syst/diast); use hasMember for a panel where each member is itself a stand-alone Observation (CBC).
• Wrong category: for US Core Lab, missing category=laboratory drops the resource out of profile. Query layers stop finding it.
• Status final for unvalidated data: instrument-raw values should be preliminary until biologist validation, then promoted to final. Posting everything as final bypasses validation workflow.
• Absolute vs relative reference: "reference": "Patient/example" inside a relative Bundle vs "reference": "https://example.com/fhir/Patient/example" in direct REST. Mixing both breaks strict servers.

Related resources

• DiagnosticReport — wrapper for a complete report, references one or more Observations plus any images.
• Patient — observation subject.
• Encounter — care context.
• Practitioner — who performed the measurement.
• Specimen — analysed specimen for biology observations.
• Device — measurement device (analyser, monitor).
• HL7 v2 equivalent — ORU^R01 segment OBX.