FHIR gets the attention, but a great deal of real-time clinical messaging in operation today still runs on HL7 version 2 — a pipe-and-hat format that predates the modern web and remains the workhorse of hospital interfaces. Anyone who works near healthcare integration will eventually open an ADT message and need to read it. This guide walks through one without assuming prior knowledge.
What ADT means
ADT stands for Admit, Discharge, Transfer — the family of messages that announce patient-movement and registration events. When a patient is registered, admitted, transferred between units, or has their demographics updated, an ADT message typically fires from the source system to every downstream system that needs to know. Common event types include A01 (admit), A04 (register), and A08 (update patient information).
The shape of a message
An HL7 v2 message is a stack of segments, one per line, each beginning with a three-letter code and divided into fields by the pipe character (|). A typical ADT message leads with these segments:
- MSH — Message Header. Always first. It identifies the sending and receiving applications, the message type, a timestamp, and the field-separator and encoding characters used by the rest of the message.
- EVN — Event Type. Communicates the trigger event — for example, that this is an A04 registration — and when it occurred.
- PID — Patient Identification. The demographic core: patient identifiers, name, date of birth, sex, and address. This is the segment most integration work touches.
- PV1 — Patient Visit. Visit-level context such as patient class, assigned location, and attending provider.
Why the order and structure matter
The fixed segment order and positional fields are what let a receiving system parse a message deterministically. Within a field, components are separated by the caret (^) — so a name field might read Doe^Jane^A. The encoding characters declared in MSH tell the parser exactly which delimiters to expect, which is why MSH must come first.
Where v2 fits today
HL7 v2 is not going away soon. It excels at high-volume, real-time, event-driven messaging inside and between institutions — exactly the registration and movement events ADT carries. FHIR is increasingly the choice for query-based access and application data, but the two coexist. A competent Arizona integration team reads both.
The practical encouragement: an ADT message looks cryptic only until you know the segment codes. Once MSH, EVN, PID, and PV1 are familiar, most messages become readable at a glance. For more on how these messages move across the state, see our Network work area, and for definitions, the glossary.