Anatomy of an HL7 v2 Message

A message is a stack of segments

An HL7 v2 message is plain text. Each line is a segment, identified by a three-letter code, and the segments appear in an order defined for that message type ¹. Here is a minimal admit message:

MSH|^~\&|ADT1|HOSP|LAB|HOSP|202406011230||ADT^A01|MSG00001|P|2.5
EVN|A01|202406011230
PID|1||100711^^^HOSP^MR||DOE^JANE^Q||19800101|F
PV1|1|I|2000^2012^01||||004777^SMITH^JOHN

Read it top to bottom: the message header (MSH), the event (EVN), the patient identification (PID), and the patient visit (PV1). Each line is one segment.

Fields, components, and the encoding characters

Within a segment, the vertical bar | separates fields. Inside a field, the caret ^ separates components. Two more characters complete the set: ~ repeats a field, and & separates subcomponents. Together these four — plus the field separator itself — are the encoding characters.

Crucially, a message declares its own delimiters. Look at the start of MSH:

MSH|^~\&|...

The character immediately after MSH (here |) is defined as the field separator, and the next four (^~\&) are the component, repetition, escape, and subcomponent characters. A parser reads these first and uses them to interpret everything that follows ¹.

So in DOE^JANE^Q, the PID name field carries three components — family name, given name, and middle initial — not three separate fields.

Trigger events: why this message exists

The MSH-9 field above holds ADT^A01. The ADT message type covers Admission, Discharge, and Transfer activity; A01 is the specific trigger event meaning “patient admit.” HL7 v2 is event-driven: a real-world occurrence in one system (an admission) generates a message that notifies the others ². The same ADT structure, with a different trigger code, carries a discharge (A03) or a transfer (A02).

This pairing — a reusable message structure selected by a specific trigger event — is the core mental model of v2, and the thing to carry forward: later standards change the syntax, but the idea that a clinical event drives a structured exchange never goes away.