HL7: Health Level Seven
HL7 (Health Level Seven) is the international standards development organisation responsible for the most widely used healthcare data exchange frameworks, including HL7 V2 messaging, Clinical Document Architecture (CDA), and FHIR.
What is HL7?
Health Level Seven International (HL7) is a not-for-profit, ANSI-accredited standards developing organisation established in 1987. HL7 develops and maintains a comprehensive framework of standards for the exchange, integration, sharing, and retrieval of electronic health information. The "Level Seven" refers to the seventh and highest layer of the ISO Open Systems Interconnection (OSI) communications model — the application layer — reflecting HL7's focus on data content and semantics rather than physical communication mechanics.
HL7 V2: The Legacy Standard Still Dominant
HL7 Version 2 (V2) is the most widely implemented healthcare data standard in the world, used by an estimated 95% of US hospitals and the majority of EHR interfaces. V2 messages use a pipe-delimited ASCII format — segments separated by carriage returns, fields separated by the "|" character. Despite its age (V2.1 was released in 1987), it remains the dominant integration mechanism for real-time clinical data exchange.
Common HL7 V2 message types include:
- ADT (Admit, Discharge, Transfer): A01 (Admission), A03 (Discharge), A08 (Update patient information). The most commonly used HL7 message for hospital event notification.
- ORU (Observation Result): Lab results, radiology reports, and other clinical observations
- ORM (Order Message): Physician orders for laboratory, radiology, and other ancillary services
- MDM (Medical Document Message): Clinical documentation — discharge summaries, operative notes, transcribed notes
HL7 V3 and CDA
HL7 V3 (released 2005) used an XML-based approach and a formal information model (Reference Information Model — RIM). It was clinically richer than V2 but vastly more complex to implement. Adoption was limited except in specific use cases. The Clinical Document Architecture (CDA) — most commonly the Consolidated CDA (C-CDA) — is the most successfully adopted V3 artefact, used for structured clinical documents like discharge summaries, transition of care summaries, and continuity of care documents.
FHIR as HL7's Next Generation
FHIR (Fast Healthcare Interoperability Resources) is HL7's next-generation interoperability standard, released as a normative standard in 2019 (R4). FHIR adopts modern web technologies (REST APIs, JSON, OAuth 2.0) and a modular resource-based architecture. It is mandated by CMS and ONC for specific interoperability use cases, though V2 and C-CDA remain the dominant operational standards in most US health systems.