HL7 ADT Messages: Trigger Events and Message Types (A01-A13)

An HL7 ADT message is a Version 2 message from the Patient Administration chapter of the HL7 standard. It tells other systems that something happened to a patient’s registration, location or status, such as an admission, a transfer or a discharge. Each kind of event has its own trigger event code (A01, A02, A03 and so on), carried in the message type field, MSH-9. This guide lists the trigger events, shows how an ADT^A01 message is laid out, and sets out the usage rules the standard itself states. It was checked against HL7 v2.6 and v2.3.1 chapter 3 as published by HL7 Europe on 2026-11-08.

What an HL7 ADT message is

ADT stands for admit, discharge, transfer. In HL7 v2 these messages belong to chapter 3, Patient Administration, and keep receiving systems (laboratory, radiology, billing, public-health feeds) in step with the registration or patient-administration system. A message is identified by its message type and trigger event, for example ADT^A01^ADT_A01: the type (ADT), the trigger event (A01) and the message structure used for that event (ADT_A01). The trigger event describes the real-world event that caused the message to be sent.

Searchers use many word orders for the same thing (HL7 ADT messages, ADT message types in HL7, ADT HL7 message, HL7 message types ADT). They all refer to this one family of messages and the same list of trigger events below.

HL7 ADT message types and trigger events

The table paraphrases chapter 3 of HL7 v2.6 (HL7 Europe copy of the standard text). It covers the events most often asked about, not every ADT event in the standard, which runs well beyond A13.

Trigger event	What the standard says it is for
A01 Admit/visit notification	Intended to be used for admitted patients only.
A02 Transfer a patient	Issued as a result of the patient changing his or her assigned physical location.
A03 Discharge/end visit	Signals the end of a patient’s stay in a healthcare facility.
A04 Register a patient	Signals that the patient has arrived or checked in as a one-time or recurring outpatient.
A05 Pre-admit a patient	Sent when a patient undergoes the pre-admission process.
A06 Change an outpatient to an inpatient	Sent when a patient present for a non-admitted visit is being admitted.
A07 Change an inpatient to an outpatient	Sent when an admitted patient changes status to no longer admitted.
A08 Update patient information	Used when any patient information has changed but no other trigger event has occurred.
A11 Cancel admit/visit notification	Sent when an A01 event is cancelled.
A12 Cancel transfer	Cancels an A02 event.
A13 Cancel discharge/end visit	Cancels an A03 event.

Structure of an ADT^A01 message

The v2.6 definition of ADT^A01 (structure ADT_A01) lists these segments in order. Square brackets mark segments the standard lists as optional; MSH, EVN, PID and PV1 are the unbracketed core.

1. MSH message header
2. [SFT] software segment, [UAC] user authentication credential
3. EVN event type
4. PID patient identification, [PD1] additional demographics, [ARV] access restrictions, [ROL] role, [NK1] next of kin
5. PV1 patient visit, [PV2] additional visit information
6. [DB1] disability, [OBX] observation, [AL1] allergy, [DG1] diagnosis, [DRG] diagnosis related group
7. PR1 procedure group, [GT1] guarantor, IN1 insurance group
8. [ACC] accident, [UB1] and [UB2] UB82 and UB92 data, [PDA] patient death and autopsy

The v2.3.1 chapter defines the EVN fields as: EVN-1 event type code, EVN-2 recorded date/time, EVN-3 date/time planned event, EVN-4 event reason code, EVN-5 operator ID and EVN-6 event occurred. It also states that the patient class for any visit-related information must be specified in PV1-2, patient class.

Rules the standard gives for A02 and A08

When to send A02

The v2.3.1 text recommends that A02 be used only for a real change in the census bed in the Patient Administration system. A transfer message is about physical location, not any other change to the record.

A02 followed by A08

When a location change and a demographics change happen together, the v2.6 text recommends, but does not require, sending two messages: an A02 followed by an A08.

A08 as the catch-all update

A08 is used when any patient information has changed but no other trigger event has occurred. The standard strongly recommends using it to update fields that are not updated by any of the other trigger events, rather than as a substitute for them.

Cancelling and merging

A11, A12 and A13 undo an earlier A01, A02 or A03. For merging patient records, the v2.3.1 text says A40-type events should be used for a current episode, and prefers them to A18 wherever possible.

Strengths and limits

• Strength: the trigger-event model gives every receiver one agreed vocabulary for patient-flow events, so a laboratory system and a billing system can react to the same A01 or A03.
• Limit: the standard defines the events and their segments, but sites decide how to populate optional segments and fields, so two compliant feeds can look different. Check each partner’s interface specification, not just the standard.
• Limit: the event sequence is not strictly enforced by the standard; a patient is normally admitted before a transfer or discharge, but a receiver should not assume perfect ordering (editorial interpretation, consistent with the Ringholm HL7 ADT overview).

ADT events used in public-health feeds

Syndromic surveillance is the best-known public-health use of ADT. The CDC’s National Syndromic Surveillance Program message guide (PHIN Messaging Guide for Syndromic Surveillance, Release 2.0, April 2015) builds on HL7 v2.5.1, backward compatible with v2.3.1, and covers ADT events A01, A03, A04 and A08. That is a four-event subset of the larger list above. See syndromic surveillance reporting for hospitals for how those feeds are used, and what HL7 is for the wider standards family.

Alternatives to ADT messaging

Approach	What it is	Relationship to ADT
HL7 v2 ADT	Event-driven messages for patient administration	The subject of this page
HL7 FHIR	HL7 International’s REST/JSON interoperability standard built from resources and implementation guides	Complements rather than replaces v2; both commonly run side by side. See what FHIR is.

For the records these messages feed, see EHR vs EMR; patient matching across systems relates to the two patient identifiers requirement.

FAQ

What are the HL7 ADT message types?

ADT is the message type; the variants are trigger events. The common ones are A01 admit, A02 transfer, A03 discharge, A04 register an outpatient, A05 pre-admit, A06 and A07 for outpatient/inpatient status changes, A08 update patient information, and A11 to A13 for cancellations.

What does ADT mean in HL7?

Admit, discharge, transfer: the Patient Administration messages in chapter 3 of HL7 v2.

What is the difference between A01 and A04?

A01 is intended for admitted patients only. A04 signals that a patient has arrived or checked in as a one-time or recurring outpatient.

Which ADT message updates demographics?

A08. Where a transfer and a demographics change occur together, the standard recommends an A02 followed by an A08.

Which ADT events does syndromic surveillance use?

The CDC NSSP message guide covers A01, A03, A04 and A08.

Sources

• HL7 v2.6, chapter 3, Patient Administration (HL7 Europe copy): hl7.eu/HL7v2x/v26/std26/ch03.html, read 2026-11-08.
• HL7 v2.3.1, chapter 3 (HL7 Europe copy): hl7.eu/HL7v2x/v231/std231/CH3.html, read 2026-11-08.
• CDC National Syndromic Surveillance Program implementation guides (PHIN Messaging Guide, Release 2.0, April 2015).
• Ringholm, HL7 ADT messages overview: ringholm.com (secondary, context only).