HL7 v2 Segment Reference
Field-by-field documentation for the most common HL7 v2.5.1 segments
HL7 v2 messages are built from segments — three-letter blocks like MSH, PID, and OBX, each carrying a specific slice of clinical or administrative data. This reference documents every field of the segments you meet most often in real-world interface traffic, with data types, requirement levels, and realistic examples.
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MSH
Message Header
Defines the intent, source, destination, and framing of the message. Required as the first segment of every HL7 message.
PID
Patient Identification
Contains permanent patient demographic and identification information. Required in most patient-related messages.
PV1
Patient Visit
Contains visit-specific information: admission, location, attending provider. Required for most inpatient/outpatient visit messages.
EVN
Event Type
Describes the trigger event for ADT messages. Identifies when and who recorded the event.
NK1
Next of Kin / Associated Parties
Information about the patient’s family members, contacts, or associated parties. Repeats for each contact.
ORC
Common Order
Shared order control information. Appears once per order and is followed by order-specific segments (OBR for labs, RXO for meds).
OBR
Observation Request
Describes a requested observation (lab order, imaging, etc). Each OBR can be followed by multiple OBX result segments.
OBX
Observation / Result
A single result value. Multiple OBX segments follow each OBR to report individual result components.