HL7 v2 Message Types Reference

An HL7 message type is a three-character code that identifies the category of healthcare communication — ADT for patient administration, ORU for results, ORM for orders, and so on. HL7 v2 defines dozens of message types, each paired with a trigger event code in MSH-9 (e.g., ADT^A01, ORU^R01, ORM^O01) that specifies the exact clinical event.

Message Type Format

Every HL7 message type follows the pattern:

MSH-9: MessageType^TriggerEvent^MessageStructure

For example, ADT^A01^ADT_A01 means:

ADT: Message category (Admit, Discharge, Transfer)
A01: Specific event (Admit/Visit Notification)
ADT_A01: Abstract message definition (which segments to include)

Patient Administration

ADT — Admit, Discharge, Transfer Patient movement, registration, and census events. A01 through A44 event types.

ADT messages are the most widely implemented HL7 v2 message type. They carry patient registration, admission, transfer, discharge, and demographic update events that drive census, bed management, and downstream system synchronization.

Orders & Results

ORM — General Order Lab, radiology, and ancillary service orders. ORC/OBR segments and order lifecycle.

ORU — Observation Result Lab results, radiology reports, and clinical observations. OBR/OBX segments and result status.

ORM messages place orders; ORU messages return results. Together they form the order-result lifecycle that connects ordering systems (EHRs) with fulfilling systems (lab, radiology, pharmacy).

Scheduling

SIU — Schedule Information Unsolicited Appointment booking, rescheduling, cancellation, and no-show events. S12 through S26.

SIU messages manage the appointment lifecycle between scheduling systems and clinical departments. They coordinate provider calendars, room availability, and patient scheduling.

Financial

DFT — Detailed Financial Transaction Charge posting, billing codes, and revenue cycle data. FT1 segment and CPT coding.

DFT messages carry charge data from clinical systems to billing platforms. Each message may contain multiple FT1 (Financial Transaction) segments representing individual charge line items.

Clinical Documents

MDM — Medical Document Management Document creation, authentication, addenda, and cancellation. TXA segment and document lifecycle.

MDM messages manage clinical document lifecycle events — operative notes, discharge summaries, consultation reports, and pathology reports flowing between transcription systems and EHRs.

System Messages

ACK — Acknowledgment Message receipt confirmation, error handling, and retry strategies. MSA/ERR segments.

ACK messages confirm receipt and processing of every other HL7 message type. Proper acknowledgment handling is essential for reliable message delivery across healthcare interfaces.

Message Type Summary

Type	Name	Common Events	Primary Segments
ADT	Admit/Discharge/Transfer	A01, A02, A03, A04, A08, A31, A40	MSH, EVN, PID, PV1
ORM	General Order	O01	MSH, PID, ORC, OBR
ORU	Observation Result	R01	MSH, PID, ORC, OBR, OBX
SIU	Schedule Information	S12-S26	MSH, SCH, PID, RGS, AIS, AIP, AIL
DFT	Financial Transaction	P03	MSH, PID, FT1, DG1, IN1
MDM	Medical Document	T02, T03, T11	MSH, EVN, PID, TXA, OBX
ACK	Acknowledgment	—	MSH, MSA, ERR

HL7 Segment Reference Field-by-field guides for MSH, PID, PV1, ORC, OBR, OBX, and 10+ additional segments.

HL7 Sample Messages Complete annotated sample messages for hands-on learning.

HL7 Workbench Parse and validate HL7 messages with segment highlighting and field lookup.

HL7 Resource Center Message types, segments, reference guides, and integration tools.

HL7 Integration Services HL7 v2 interface development, message routing, and integration architecture.