HL7 ADT_A04 Register a Patient

The patient has arrived or checked in, but this is not an inpatient admission.
HL7 message structure ADT_A01 groups and segments from HL7 v2.5.1 Hide structure

These are the generated groups and segments for the version selected at the top of the page. The article explains the workflow, and this panel follows the chosen HL7 version.

Message Structure

SegmentNameRequiredRepeatable
MSH
 Message Header Yes No
SFT
 Software Segment No Yes
EVN
 Event Type Yes No
PID
 Patient Identification Yes No
PD1
 Patient Additional Demographic No No
ROL
 Role No Yes
NK1
 Next of Kin / Associated Parties No Yes
PV1
 Patient Visit Yes No
PV2
 Patient Visit - Additional Information No No
ROL
 Role No Yes
DB1
 Disability No Yes
OBX
 Observation/Result No Yes
AL1
 Patient Allergy Information No Yes
DG1
 Diagnosis No Yes
DRG
 Diagnosis Related Group No No
ADT_A01.PROCEDURE
 Procedure group No Yes
PR1
 Procedures Yes No
ROL
 Role No Yes
GT1
 Guarantor No Yes
ADT_A01.INSURANCE
 Insurance group No Yes
IN1
 Insurance Yes No
IN2
 Insurance Additional Information No No
IN3
 Insurance Additional Information, Certification No Yes
ROL
 Role No Yes
ACC
 Accident No No
UB1
 UB82 No No
UB2
 UB92 Data No No
PDA
 Patient Death and Autopsy No No

ADT_A04 is the register-patient event. In many interfaces it means an outpatient, emergency, clinic, recurring outpatient, or one-time visit has started. The patient is known to the facility now, the visit exists now, and downstream systems need the demographics and encounter context before orders, results, charges, labels, meals, notifications, or tracking workflows start to move.

The message usually appears on the wire as ADT^A04^ADT_A01. That third component matters: the trigger event is A04, but the structure used for validation is ADT_A01. The structure panel below shows the ADT_A01 groups because that is the local HL7 v2.5.1 message structure backing this event.

A small A04 example

MSH|^~\&|REG|CITYHOSP|EHR|CITYHOSP|20260715081500||ADT^A04^ADT_A01|ADT040001|P|2.5.1 EVN|A04|20260715081400|||REGCLERK^Riley^Morgan PID|1||123456^^^CITYHOSP^MR||Smith^Jane^Anne^^Ms^^L||19800314|F|||12 High Street^^Auckland^^1010^NZ^H||^PRN^CP^^64^21^5551234 PV1|1|O|ED^TRIAGE^1^CITYHOSP||||12345^Careful^Clara|||||||||||VST440001^^^CITYHOSP^VN|||||||||||||||||||||||||20260715081200 NK1|1|Smith^Alex|SPO|12 High Street^^Auckland^^1010^NZ^H|^PRN^CP^^64^21^5559876

What systems do with it

The sender is usually registration, an ED tracking board, a practice-management system, or an ADT feed from the EHR. Receivers use PID to match or create the patient, PV1 to create the visit, and optional NK1, IN1, GT1, AL1, and OBX data to prepare the rest of the visit workflow.

A04 is common in emergency and outpatient feeds because those visits often need ancillary systems before the patient is admitted to a bed. A lab system may need the visit number, a radiology system may need the ordering location, and a notification platform may need enough demographics to match the patient.

How to read the structure

The generated panel shows ADT_A01 because HL7 uses the same broad message structure for A01, A04, A08, and A13. For A04, the practical core is MSH, EVN, PID, and PV1. In PV1, patient class is commonly outpatient or emergency, and PV1-44 is commonly used as the visit start or admit date/time.

Optional groups are not decorative. Insurance, guarantor, accident, diagnosis, allergy, and observation segments may be needed by billing, public health, dietary, pharmacy, and ancillary receivers. Send the data the receiver actually supports, and avoid stuffing uncertain registration details into every optional segment just because the structure allows it.

Implementation traps

The most common trap is treating A04 like A01. A01 is an admit or visit notification, often inpatient. A04 is registration, commonly non-admitted. If a receiver turns every A04 into an inpatient admission, bed boards, census, billing class, and patient-status logic will drift fast.

The second trap is not agreeing on visit identity. For A04, the visit number in PV1 is often the key that downstream systems use later when orders, results, DFT charges, and discharge/end-visit messages arrive. A patient identifier alone is not enough to file visit-specific data safely.

Reference notes

The HL7 patient-administration chapter describes A04 as the event used when a patient arrives or checks in as a one-time or recurring outpatient and is not assigned to a bed. The CDC syndromic surveillance addendum also shows ADT^A04^ADT_A01 as the constrained message type for emergency department registration. IHE Patient Encounter Management mappings published by Oracle similarly map A04 to ADT_A01 for register-outpatient workflows.