Group — detail: cohorts & populations

Patient cohorts, Specimen lots, livestock herds: Group is the generic container of homogeneous items.

Enumerated vs descriptive

R5 introduces the membership field:

• definitional — descriptive Group. Members are not enumerated; defined by characteristic (eligibility criteria).
• conceptual — abstract, e.g. "pregnant women in France."
• enumerated — enumerated Group, with member[].entity pointing to each entity.

Key fields

Bulk Data $export by Group

The FHIR Bulk Data $export operation at Group level exports all resources related to the group's members: GET /Group/cohort-diabetes-2026/$export. Standard used for clinical research, quality reporting, regulatory submissions.

Example: diabetes cohort

{
  "resourceType": "Group",
  "id": "cohort-diabetes-2026",
  "identifier": [{ "system": "http://example.org/cohorts", "value": "DIA-2026" }],
  "type": "person",
  "membership": "enumerated",
  "code": {
    "coding": [{
      "system": "http://snomed.info/sct",
      "code": "44054006",
      "display": "Diabetes mellitus type 2"
    }]
  },
  "name": "Diabetes type 2 cohort — retrospective study 2026",
  "quantity": 1247,
  "managingEntity": { "reference": "Organization/research-team" },
  "characteristic": [{
    "code": { "text": "age >= 18 and <= 75 at index date" },
    "valueRange": { "low": { "value": 18, "unit": "a" }, "high": { "value": 75, "unit": "a" } },
    "exclude": false
  }, {
    "code": { "text": "HbA1c >= 6.5% in last 12 months" },
    "valueQuantity": { "value": 6.5, "comparator": ">=", "unit": "%" },
    "exclude": false
  }]
}

Common pitfalls

• Inconsistent type — a Group type=person with member[].entity pointing to a Device is invalid.
• characteristic + enumerated — using characteristic on an enumerated Group is redundant; pick one.
• quantity does not match member — on an enumerated Group, quantity must match the cardinality of member[].

Related resources

• Group — overview
• Patient
• List
• ResearchStudy