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US Core is the HL7 FHIR Implementation Guide that tells developers how to expose US patient data through FHIR R4 using a common set of profiles. This page is for clinical data and informatics staff who need to know what it requires, how its numbered versions line up with USCDI releases, and where to read the authoritative text. It is general information about a technical specification, not legal or certification advice.
What is the US Core?
The US Core Implementation Guide (US Core IG) is published by HL7 International. Per the guide’s home page at hl7.org/fhir/us/core (read 2026-11-07), it sets minimum constraints on FHIR resources to create the “US Core Profiles”: the required elements, extensions, value sets and minimum RESTful interactions for accessing patient data. It is built on FHIR R4 (4.0.1), is a trial-use specification, and is released annually to reflect changes to USCDI. The version published at that date was 9.0.0 (STU 9).
For background on the base standard, see the CASRAI guides to FHIR and HL7.
What does USCDI stand for and what is its purpose?
USCDI is the United States Core Data for Interoperability. According to the ONC-run USCDI page at isp.healthit.gov (read 2026-11-07), it is a standardized set of health data classes and data elements for nationwide, interoperable health information exchange. A data class is a group of elements organized by theme or use case; a data element is a single defined piece of data. The same page lists USCDI v6, published by ASTP/ONC on July 24, 2025, as the most recent published version, with v7 shown as a draft.
How USCDI and US Core divide the work
- USCDI defines what data is in scope (classes and elements), and is maintained by ASTP/ONC.
- US Core defines how that data is represented and accessed in FHIR R4, and is maintained by HL7.
How is FHIR used in healthcare?
FHIR is HL7’s standard for exchanging health data as discrete resources (Patient, Encounter, Observation and so on) over web-style APIs. On its own FHIR leaves many choices open; an implementation guide such as US Core narrows them for a given jurisdiction so that different systems can interpret the same resource the same way. HL7 v2 messaging and CDA documents remain in use alongside it.
US Core profiles: Encounter, MedicationRequest, Condition and others
The IG’s “Profiles and Extensions” page (read 2026-11-07, v9.0.0) lists one or more profiles per FHIR resource. Examples that match common searches:
- US Core Encounter Profile (Encounter)
- US Core MedicationRequest Profile (MedicationRequest)
- US Core Condition Encounter Diagnosis Profile and US Core Condition Problems and Health Concerns Profile (Condition), so a single resource type can have more than one US Core profile
- US Core DiagnosticReport Profile for Laboratory Results Reporting and US Core Laboratory Result Observation Profile for lab data
Lab observations are typically coded with LOINC and medications with RxNorm; check each profile page for the value sets it binds.
US Core versions and the USCDI each implements
The IG’s USCDI page (hl7.org/fhir/us/core/uscdi.html, read 2026-11-07) maps each US Core release to a USCDI version. This answers the common version searches (3.1.1, 5.0.1, 6.1.0) in one place.
| US Core version | USCDI version it maps to |
|---|---|
| 3.1.1 | USCDI v1 |
| 4.0.0 | USCDI v1 (improved Must Support guidance) |
| 5.0.1 | USCDI v2 |
| 6.1.0 | USCDI v3.1 |
| 7.0.0 | USCDI v4 |
| 8.0.1 | USCDI v5 |
| 9.0.0 | USCDI v6 |
The same page states the profiles were initially designed to meet the 2015 Edition certification criterion for Patient Selection and Application Access. It does not itself state which versions a certification program requires; the IG’s conformance pages and the relevant ONC program documents govern that, so confirm required versions there before committing a build.
Where to find and read the guide
- Open the canonical page at hl7.org/fhir/us/core for the current published version.
- Use the “Version History” link in the guide’s menu to reach earlier releases such as 3.1.1 or 6.1.0.
- Read the USCDI page to see which USCDI release your target version implements.
- Use “Profiles and Extensions” to find the profile for each resource.
- Read the Must Support and Conformance pages before implementing.
The HL7 build site also hosts ballot versions (for example a v10.0.0 ballot appeared in search results on 2026-11-07); ballot content is not the published release.
Strengths and limits
- Strength: one agreed set of profiles, value sets and minimum interactions for US patient data on FHIR R4, tied to USCDI by version.
- Limit: it is a trial-use specification (STU), so later releases can change requirements; the 9.0.0 page lists Maturity Level 3.
- Limit: the guide specifies data representation and access; it does not on its own tell you which version a program or contract mandates.
FAQ
Is US Core the same as USCDI?
No. USCDI is the data set definition from ASTP/ONC; US Core is the HL7 FHIR guide that implements it.
Which US Core version matches USCDI v3?
Per HL7’s mapping, US Core 6.1.0 maps to USCDI v3.1.
Which FHIR release does US Core use?
FHIR R4 (4.0.1), per the guide’s home page.
Sources
- HL7 International, US Core Implementation Guide, read 2026-11-07.
- HL7 International, US Core and USCDI, read 2026-11-07.
- HL7 International, Profiles and Extensions, read 2026-11-07.
- ASTP/ONC, United States Core Data for Interoperability (USCDI), read 2026-11-07.
