"FHIR format" is often invoked as a magic phrase. Understanding what it actually is (and isn't) prevents integration confusion.
What FHIR is
1. A specification. HL7 FHIR is a healthcare data exchange specification. 2. A resource model. ~140 resources representing clinical, administrative, financial healthcare concepts. 3. REST API definition. Standard HTTP verbs on resources. 4. Multiple serialization formats. JSON (default), NDJSON (bulk), XML (legacy), RDF (semantic). 5. A conformance framework. Profiles narrow the base spec for specific use cases (e.g., US Core for US healthcare).
What FHIR isn't
1. A database format. FHIR resources are exchanged; storage format is implementation choice. 2. A specific server product. Multiple servers implement FHIR (HAPI, Aidbox, Medplum, etc.). 3. A replacement for HL7v2. FHIR complements v2; most upstream data is still v2. 4. A single format. JSON is default, but NDJSON for bulk, XML for legacy, RDF for semantic. 5. A billing/reimbursement standard. X12 handles claims; FHIR captures clinical.
Serialization decisions
| Use case | Format |
|---|---|
| Individual resource CRUD | JSON |
| Bulk export | NDJSON |
| Legacy XML clients | XML |
| Semantic integrations | RDF/Turtle |
| Bundle transactions | JSON |
FHIR versions in production (mid-2026)
| Version | Status | Where used |
|---|---|---|
| R4 | Normative | Most production deployments |
| R4B | Normative | SubscriptionTopic support |
| R5 | Normative | Newer deployments; not required by US Core |
Common FHIR misunderstandings
1. "FHIR is our EHR." No — FHIR is the exchange format. 2. "FHIR handles auth." SMART on FHIR handles auth, separate spec. 3. "FHIR is one format." Multiple serializations exist. 4. "US Core is all US healthcare." It's a baseline; specialty IGs extend.
FHIR is a specification with a resource model, REST API, and serialization options. Understanding what it is clarifies what to expect from FHIR-based systems.
