Skip to content

Reverse Chained Search (_has)

Overview

Reverse chained search (using _has) allows you to find resources based on other resources that reference them. This is the opposite of regular chained search - instead of "find A where A references B matching criteria", it's "find A where B references A and B matches criteria".

Syntax

{ResourceType}?_has:{ReferencingType}:{referenceParam}:{searchParam}={value}

Components: - {ResourceType} - The resource type to return (e.g., Patient) - {ReferencingType} - The type that references the target (e.g., Condition) - {referenceParam} - The reference parameter in ReferencingType (e.g., patient) - {searchParam} - Search parameter on ReferencingType (e.g., code) - {value} - The value to match

How It Works

  1. The server parses the _has parameter format
  2. Searches for all {ReferencingType} resources matching {searchParam}={value}
  3. Extracts the references from {referenceParam} field
  4. Returns only {ResourceType} resources that are referenced

Examples

Find Patients with Diabetes Diagnosis

curl "http://localhost:8000/Patient?_has:Condition:patient:code=diabetes"

Returns Patients who have a Condition with code containing "diabetes".

Find Patients with Recent Observations

curl "http://localhost:8000/Patient?_has:Observation:subject:code=8480-6"

Returns Patients who have an Observation with LOINC code 8480-6 (Systolic BP).

Find Practitioners with Active MedicationRequests

curl "http://localhost:8000/Practitioner?_has:MedicationRequest:requester:status=active"

Returns Practitioners who have written active medication prescriptions.

Find Organizations with Encounters

curl "http://localhost:8000/Organization?_has:Encounter:serviceProvider:status=finished"

Returns Organizations that have finished Encounters.

Find Patients with Specific Allergies

curl "http://localhost:8000/Patient?_has:AllergyIntolerance:patient:code=penicillin"

Returns Patients with penicillin allergies.

Supported Patterns

Target Type Referencing Type Reference Param Common Search Params
Patient Condition patient, subject code, clinical-status
Patient Observation subject code, status
Patient MedicationRequest subject status, medication
Patient Procedure subject code, status
Patient Encounter subject status, class
Patient DiagnosticReport subject code, status
Patient AllergyIntolerance patient code, clinical-status
Patient Immunization patient vaccine-code, status
Patient CarePlan subject status, category
Patient Goal subject lifecycle-status
Patient ServiceRequest subject status, code
Patient DocumentReference subject type, status
Patient MeasureReport subject status, measure
Practitioner MedicationRequest requester status
Practitioner Encounter participant status
Practitioner Procedure performer status
Organization Encounter serviceProvider status

Combining with Other Parameters

Reverse chained parameters can be combined with regular search and chained parameters:

# Find active Patients with diabetes
curl "http://localhost:8000/Patient?_has:Condition:patient:code=diabetes&active=true"

# Find Patients with diabetes AND hypertension
curl "http://localhost:8000/Patient?_has:Condition:patient:code=diabetes&_has:Condition:patient:code=hypertension"

# With pagination
curl "http://localhost:8000/Patient?_has:Condition:patient:code=diabetes&_count=10"

Use Cases

Clinical Decision Support

Find patients who need follow-up based on existing conditions or observations: 

# Patients with HbA1c > 7%
curl "http://localhost:8000/Patient?_has:Observation:subject:code=4548-4"

Population Health

Identify patient cohorts for quality measures: 

# Patients with diabetes for measure evaluation
curl "http://localhost:8000/Patient?_has:Condition:patient:code=44054006"

Care Coordination

Find patients with specific care needs: 

# Patients with active care plans
curl "http://localhost:8000/Patient?_has:CarePlan:subject:status=active"

Performance Considerations

  • _has searches require searching the referencing resource type first
  • Multiple _has parameters multiply the search effort
  • Consider caching strategies for frequently used queries
  • For large datasets, direct searches may be more efficient

Limitations

  • Only single-level reverse chaining is supported
  • The referencing resource type must be supported by the server
  • Complex logical combinations (OR between _has clauses) are not supported
  • Each _has parameter is evaluated independently (AND logic)

Error Handling

Invalid _has parameters are gracefully ignored: - Unknown resource types - Unknown reference parameters - Unknown search parameters

The server continues processing with valid parameters.

FHIR Specification

For more details on reverse chained parameters, see: - FHIR R4 Search - Reverse Chaining