What is FHIR?
FHIR (Fast Healthcare Interoperability Resources, pronounced "fire") is a standard for exchanging healthcare information electronically, developed by HL7 International. FHIR combines the best features of HL7's earlier standards (v2, v3, CDA) with modern web technologies to create a developer-friendly, API-based approach to healthcare interoperability.
Core Design Principles
- RESTful API — Uses standard HTTP verbs (GET, POST, PUT, DELETE) over HTTPS
- Resource-based — Data is organized into discrete, reusable "resources" (Patient, Observation, Claim, etc.)
- JSON and XML — Supports both formats natively; JSON is most commonly used
- Extensible — Base resources can be profiled and extended without modifying the core specification
- Open — The specification is freely available at hl7.org/fhir
Why FHIR Replaced Earlier Standards
| Standard | Era | Limitations |
|---|---|---|
| HL7 v2 | 1987+ | Pipe-delimited messages; inconsistent implementations; no formal data model |
| HL7 v3 / CDA | 2005+ | XML-based; overly complex; high implementation barrier |
| FHIR R4 | 2019+ | RESTful, JSON-native, developer-friendly, implementation guide ecosystem |
FHIR Version History
- DSTU 1 (2014) — First draft standard for trial use
- DSTU 2 (2015) — Significant resource additions, API refinements
- STU 3 (2017) — Matured many resources to trial-use level
- R4 (2019) — First normative release; ~60 resources reached normative maturity. This is the CMS-mandated version.
- R5 (2023) — Incremental improvements; R4 remains the implementation baseline for U.S. regulatory use
SMART on FHIR
SMART on FHIR (Substitutable Medical Applications, Reusable Technologies) is an open standard for app authorization and launch on top of FHIR:
- Uses OAuth 2.0 for authorization
- Defines an app launch framework (EHR launch, standalone launch)
- Enables third-party apps to be embedded within EHR workflows
- Required for CMS Patient Access API implementations
CMS Mandate for FHIR
CMS has mandated FHIR adoption for payer-facing APIs:
- CMS-9115-F (2020) — Required Patient Access API using FHIR
- CMS-0057-F (2024) — Expanded to include Provider Access, Payer-to-Payer, and Prior Authorization APIs
- All mandated APIs must use FHIR R4 with appropriate implementation guides (US Core, CARIN, Da Vinci)
