HL7 ADT Message: Event Types & Structure

HL7 ADT (Admit, Discharge, Transfer) messages are the most widely used HL7 v2 message type, handling patient movement and registration events across healthcare facilities. Every hospital runs ADT interfaces — they are the backbone of patient census, bed management, and downstream system synchronization.

An HL7 ADT message is the message type that communicates patient administration events — admissions, discharges, transfers, registrations, and demographic updates. A single ADT message code carries dozens of distinct trigger events (A01 through A44), each identifying a specific event in the EVN and MSH-9 fields. Downstream systems use ADT feeds to stay synchronized with the patient census.

A typical ADT message contains these segments in order:

Segment	Name	Required	Purpose
MSH	Message Header	Yes	Sender, receiver, message type, version
EVN	Event Type	Yes	Trigger event code and timestamp
PID	Patient Identification	Yes	MRN, name, DOB, demographics
PD1	Additional Demographics	No	Primary care provider, living will
PV1	Patient Visit	Yes	Patient class, location, attending MD
PV2	Patient Visit (Additional)	No	Expected length of stay, admit reason
NK1	Next of Kin	No	Emergency contacts (repeating)
IN1	Insurance	No	Primary/secondary payer information
IN2	Insurance (Additional)	No	Extended insurance details
GT1	Guarantor	No	Guarantor demographics and employer
AL1	Allergy	No	Patient allergies (repeating)
DG1	Diagnosis	No	Diagnosis codes (repeating)

HL7 v2 defines over 40 ADT trigger events. These are the most commonly implemented:

Event	Name	Description
A01	Admit/Visit Notification	Patient admitted as inpatient or begins an encounter
A02	Transfer a Patient	Patient moved from one location to another
A03	Discharge/End Visit	Patient discharged from facility
A04	Register a Patient	Outpatient or emergency registration
A05	Pre-Admit a Patient	Future admission scheduled
A06	Change Outpatient to Inpatient	Patient class change
A07	Change Inpatient to Outpatient	Patient class change
A11	Cancel Admit	Reversal of A01
A12	Cancel Transfer	Reversal of A02
A13	Cancel Discharge	Reversal of A03

Event	Name	Description
A08	Update Patient Information	Demographics or visit data changed
A28	Add Person Information	New person record (not visit-specific)
A31	Update Person Information	Person-level update (not visit-specific)
A40	Merge Patient	Two patient records merged into one
A41	Merge Account	Two accounts merged
A44	Move Account	Account reassigned to different patient

Event	Name	Description
A09	Patient Departing (Tracking)	Temporary departure from assigned location
A10	Patient Arriving (Tracking)	Arrival at temporary location
A15	Pending Transfer	Transfer requested but not yet executed
A16	Pending Discharge	Discharge requested but not yet executed
A21	Patient Goes on Leave	LOA from inpatient bed
A22	Patient Returns from Leave	LOA return
A25	Cancel Pending Discharge	Reversal of A16
A26	Cancel Pending Transfer	Reversal of A15

The most commonly implemented HL7 ADT message types are HL7 ADT A01 (admission), HL7 ADT A04 (registration), and HL7 ADT A08 (patient update). These three events account for the majority of ADT interface traffic in most hospitals.

A complete admission message with all common segments:

Browse the full HL7 sample messages catalog for more ADT examples — each one ready to copy as valid HL7 or download as a .hl7 file.

The distinction between A08 and A31 trips up many implementers:

• A08 updates patient information in the context of a visit (PV1 segment present). Use when the patient has an active encounter.
• A31 updates person-level information independent of any visit. Used by master patient index (MPI) systems for demographic corrections.

Some EHR vendors (notably Epic) use A08 for both scenarios, while others strictly separate them. Always confirm with the sending system which events they fire and under what conditions.

Merge events are among the most complex ADT operations. When two patient records are merged, the surviving MRN absorbs the deprecated MRN. The A40 message contains:

• MRG segment: The deprecated (old) patient identifier being merged away
• PID segment: The surviving (correct) patient identifier

Receiving systems must update all references from the old MRN to the new MRN — lab results, orders, documents, and any other linked data. Incorrect merge handling is a patient safety risk.

In a large hospital, ADT feeds can generate thousands of messages per hour during shift changes, morning registrations, and discharge waves. Key design considerations:

• Message queuing: Buffer ADT messages during downstream system outages
• Idempotency: Handle duplicate messages gracefully (same control ID)
• Order preservation: ADT events must be processed in chronological order per patient
• Filtering: Not every downstream system needs every ADT event — route selectively