Short answer: HL7 FHIR R4 (4.0.1, 2019) was the first release with normative content, such as the core of Patient and Observation, which means those parts will not change in incompatible ways. R5 (5.0.0, March 2023) adds new resources and redesigns some areas, notably subscriptions. Most production systems, national programmes and implementation guides, including US Core and the International Patient Summary, are still built on R4, so R4 remains the safe default for most projects in 2026.
What changed in R5
- Subscriptions were redesigned around SubscriptionTopic (topic-based), an approach also back-ported to R4 through the Subscriptions R5 Backport IG.
- New resources and reorganised workflow and medication-definition areas; R4B (2022) was an interim release mainly for medication regulatory resources.
- Many smaller changes to elements and terminologies, which mean R4 and R5 resources are not interchangeable without conversion.
How to choose
Build on the version your partners and the implementation guides you must follow use. For most clinical integrations that is R4. Use R5 when a specific guide or ecosystem requires it, and plan for conversion at the boundaries. R6 is being developed with the aim of more normative content.
In Anpheros
Anpheros Platform implements FHIR R4 (4.0.1), the version used by the International Patient Summary and most national programmes. Developers: the FHIR guide and versioning.
Related pages
Sources
Frequently asked questions
Which FHIR version is most used?
R4. Most production systems and implementation guides use it.
Is R5 backward compatible with R4?
Not fully. Normative parts are stable, but many resources changed, so conversion is needed at the boundaries.
What is FHIR R4B?
An interim 2022 release, mainly for medication definition and regulatory use cases.
Should a new project start on R5?
Only if a guide or partner requires it; otherwise R4 keeps you compatible with the largest ecosystem.
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