Group — Identified collection of entities

Patient cohort, herd, device batch, medication basket: Group bundles entities under a shared identity, either enumerated or rule-defined.

Purpose

Group identifies a coherent set of entities. Typical uses:

• Research cohorts or population follow-up groups.
• FHIR bulk export: export the Patient subset of a Group via GET /Group/[id]/$export.
• Care programs: list of patients enrolled.
• Veterinary: cattle herd.
• Pharmacy: medication batch of a delivery.

Two modes:

• membership=enumerated — members are listed explicitly in member[].
• membership=definitional — members are defined by criteria in characteristic[], no enumeration.

Key fields

JSON example

Cohort of 3 type-2 diabetic patients enrolled in a 2026 follow-up program, age criterion 40–80:

{
  "resourceType": "Group",
  "id": "diabetes-cohort-2026",
  "identifier": [{
    "system": "http://acme.com/cohorts",
    "value": "DM2-2026"
  }],
  "active": true,
  "type": "person",
  "membership": "enumerated",
  "code": {
    "coding": [{
      "system": "http://snomed.info/sct",
      "code": "44054006",
      "display": "Diabetes mellitus type 2 (disorder)"
    }]
  },
  "name": "Type 2 Diabetes Cohort — 2026 Follow-up Program",
  "description": "Patients enrolled in the type 2 diabetes follow-up program for 2026.",
  "quantity": 3,
  "managingEntity": { "reference": "Organization/hospital-1" },
  "characteristic": [{
    "code": {
      "coding": [{ "system": "http://loinc.org", "code": "30525-0", "display": "Age" }]
    },
    "valueRange": {
      "low": { "value": 40, "unit": "years" },
      "high": { "value": 80, "unit": "years" }
    },
    "exclude": false
  }],
  "member": [
    { "entity": { "reference": "Patient/example" }, "period": { "start": "2026-01-15" } },
    { "entity": { "reference": "Patient/patient-2" }, "period": { "start": "2026-02-01" } },
    { "entity": { "reference": "Patient/patient-3" }, "period": { "start": "2026-03-10" } }
  ]
}

Common pitfalls

• Enumerated Group with constraining characteristic — inconsistent: when enumerated, characteristic is informational only.
• Bulk export on definitional Group — not supported: $export requires enumeration.
• Inconsistent quantity vs member.length — always sync.
• Confusion Group / List — List is generic (heterogeneous), Group targets a single entity type (type).
• Bulk export Group of millions of Patients without _typeFilter — volumetric explosion server-side.

Related resources

• Patient, Practitioner, Device — typical member types.
• List — alternative for heterogeneous lists.
• CarePlan, CareTeam — associated care programs.
• ResearchStudy — clinical studies.