Health Level Seven (HL7)

Key facts

Organization

HL7 International, a nonprofit, ANSI-accredited standards developer

Founded

1987

Name origin

Layer seven, the application layer of the OSI networking model

Major standards

HL7 Version 2, Version 3, CDA, C-CDA and FHIR

Cost

Core standards free to use under HL7 license terms since 2013

What is HL7?

HL7 International was founded in 1987 to create standards for moving data between hospital information systems. The "Level Seven" in its name refers to the seventh, application layer of the Open Systems Interconnection (OSI) model, the layer concerned with the content of data rather than how it travels over a network.

In everyday use, "HL7" often means HL7 Version 2 messages specifically, as in "an HL7 feed" from a lab or registration system. The organization publishes much more than that, including the document standards CDA and C-CDA and the API-based standard FHIR (Fast Healthcare Interoperability Resources).

Main HL7 standards

The HL7 family spans several generations:

Common Version 2 message types include ADT (admission, discharge and transfer), ORM (orders), ORU (observation results, such as lab results) and SIU (scheduling).

• Version 2 (v2): the most widely implemented standard for messaging inside hospitals. Messages are text made of segments, such as PID for patient identification and OBX for an observation result, with fields separated by pipe characters.
• Version 3 and the Reference Information Model (RIM): a more formal, model-driven standard that saw limited adoption for messaging but underpins CDA.
• Clinical Document Architecture (CDA): an XML standard for clinical documents such as discharge summaries, with C-CDA as its main U.S. implementation.
• FHIR: a newer standard that exposes data as resources through web APIs, now referenced in several U.S. federal rules.

Why HL7 matters

Knowing which HL7 standard a system supports determines how its data can be reached:

• Most hospital lab, radiology, registration and billing systems still exchange data internally through HL7 v2 interfaces, so integration projects usually start there.
• Federal health IT certification and CMS interoperability rules reference HL7 standards, especially C-CDA and FHIR, which shapes what data providers and payers must make available.
• Extraction options range from real-time v2 event feeds to C-CDA documents to on-demand FHIR API queries, each with different latency and completeness.
• Public health reporting, such as immunization and electronic lab reports, commonly uses HL7 v2 implementation guides.