HL7 v2 Message Type Reference
This reference covers the most commonly implemented HL7 v2 message types and trigger events. Message types are identified by their three-letter code in MSH-9.1, and trigger events are identified by the event code in MSH-9.2. Segment notation uses brackets for optional segments and braces for repeating groups.
Segment Notation Key
MSH -- Required segment[NK1] -- Optional segment{OBX} -- Repeating segment groupADT -- Admit/Discharge/Transfer
ADT messages communicate patient movement events throughout the healthcare facility. These are typically the highest-volume message type in a hospital interface engine, driving census systems, downstream notifications, and patient tracking.
| Event | Name | Description | Key Segments |
|---|---|---|---|
| ADT^A01 | Admit/Visit Notification | Patient is admitted as inpatient or begins a visit. Triggers bed assignment, census update, and downstream notifications. | MSH, EVN, PID, PV1, [PV2], [NK1], [DG1], [IN1] |
| ADT^A02 | Transfer a Patient | Patient is transferred from one nursing unit or location to another within the facility. | MSH, EVN, PID, PV1 |
| ADT^A03 | Discharge/End Visit | Patient is discharged from the facility or the visit ends. | MSH, EVN, PID, PV1, [DG1] |
| ADT^A04 | Register a Patient | Outpatient or emergency registration. No bed assignment. Used for clinic visits, ED arrivals. | MSH, EVN, PID, PV1, [NK1], [IN1] |
| ADT^A05 | Pre-Admit a Patient | Reservation of a bed or service in advance of patient arrival. | MSH, EVN, PID, PV1, [NK1], [IN1] |
| ADT^A06 | Change Outpatient to Inpatient | Patient status changes from outpatient to inpatient (e.g., ED to admit). | MSH, EVN, PID, PV1 |
| ADT^A07 | Change Inpatient to Outpatient | Patient status changes from inpatient to outpatient (e.g., observation to discharge). | MSH, EVN, PID, PV1 |
| ADT^A08 | Update Patient Information | Demographics or visit information updated. Most commonly sent ADT event in many systems. | MSH, EVN, PID, PV1, [NK1], [IN1], [DG1] |
| ADT^A11 | Cancel Admit | Reversal of a previously sent A01 admit event. | MSH, EVN, PID, PV1 |
| ADT^A12 | Cancel Transfer | Reversal of a previously sent A02 transfer event. | MSH, EVN, PID, PV1 |
| ADT^A13 | Cancel Discharge | Reversal of a previously sent A03 discharge event. | MSH, EVN, PID, PV1 |
| ADT^A17 | Swap Patients | Two patients swap bed assignments (room/bed exchange). | MSH, EVN, PID, PV1, PID, PV1 |
| ADT^A18 | Merge Patient Information | Merge two patient records into one (deprecated; use A40 instead). | MSH, EVN, PID, PV1, MRG |
| ADT^A28 | Add Person Information | New patient record created in the MPI (no visit context). | MSH, EVN, PID, [NK1], [IN1] |
| ADT^A31 | Update Person Information | Update patient demographics in the MPI (no visit context). | MSH, EVN, PID, [NK1], [IN1] |
| ADT^A34 | Merge Patient ID Only | Merge patient identifiers from one record to another (MRN merge). | MSH, EVN, PID, MRG |
| ADT^A40 | Merge Patient - Patient ID List | Primary patient merge event. Merges one patient into another, combining all identifiers and history. | MSH, EVN, {PID, MRG, PV1} |
Orders and Results
Order and result messages drive the clinical workflow between ordering systems (CPOE, EHR), departmental systems (LIS, RIS, Pharmacy), and results repositories. ORM/ORU are the most widely deployed; newer message types (OML, OUL, RDE) provide more structured encoding.
| Event | Name | Description | Key Segments |
|---|---|---|---|
| ORM^O01 | General Order Message | New order, order update, or order cancellation for lab, radiology, pharmacy, or other departments. | MSH, PID, PV1, {ORC, OBR, [OBX], [NTE]} |
| ORU^R01 | Unsolicited Observation Result | Lab results, radiology reports, or other observation results sent from the performing system to the ordering system. | MSH, PID, PV1, {OBR, {OBX}, [NTE]} |
| OML^O21 | Laboratory Order | Laboratory-specific order message (newer alternative to ORM for lab orders). | MSH, PID, PV1, {ORC, OBR, [SPM]} |
| OUL^R22 | Unsolicited Lab Result (Specimen) | Lab results organized by specimen (newer alternative to ORU for lab results). | MSH, PID, {SPM, {OBR, {OBX}}} |
| OMG^O19 | General Clinical Order | Radiology-specific and general clinical orders. | MSH, PID, PV1, {ORC, OBR} |
| RDE^O11 | Pharmacy/Treatment Encoded Order | Pharmacy order with full medication encoding (drug, dose, route, frequency). | MSH, PID, PV1, {ORC, RXO, RXE, {RXR}, {RXC}} |
| RDS^O13 | Pharmacy/Treatment Dispense | Pharmacy dispense notification including dispensed medication and quantity. | MSH, PID, PV1, {ORC, RXD, {RXR}, {RXC}} |
| RAS^O17 | Pharmacy/Treatment Administration | Medication administration record from nursing or pharmacy. | MSH, PID, PV1, {ORC, RXA, RXR} |
Scheduling, Documents, Financial, and Other
Additional message types support scheduling workflows, clinical document management, financial transactions, master file updates, and immunization registry reporting.
| Event | Name | Description | Key Segments |
|---|---|---|---|
| SIU^S12 | Notification of New Appointment Booking | New appointment has been booked in the scheduling system. | MSH, SCH, PID, PV1, {RGS, {AIG}, {AIL}, {AIP}} |
| SIU^S13 | Notification of Appointment Rescheduling | Existing appointment has been rescheduled to a new date/time. | MSH, SCH, PID, PV1, {RGS} |
| SIU^S14 | Notification of Appointment Modification | Appointment details modified without changing the date/time. | MSH, SCH, PID, PV1, {RGS} |
| SIU^S15 | Notification of Appointment Cancellation | Previously booked appointment has been cancelled. | MSH, SCH, PID, PV1 |
| SIU^S26 | Notification that Patient Did Not Show | Patient did not arrive for a scheduled appointment. | MSH, SCH, PID, PV1 |
| MDM^T02 | Original Document Notification and Content | New clinical document created with full content (narrative text). | MSH, EVN, PID, PV1, TXA, {OBX} |
| MDM^T06 | Document Addendum Notification and Content | Addendum added to an existing document. | MSH, EVN, PID, PV1, TXA, {OBX} |
| MDM^T08 | Document Edit Notification and Content | Existing document has been edited/replaced. | MSH, EVN, PID, PV1, TXA, {OBX} |
| DFT^P03 | Post Detail Financial Transaction | Charge posting from clinical system to billing/financial system. | MSH, EVN, PID, PV1, {FT1}, [DG1], [IN1] |
| BAR^P01 | Add Patient Accounts | New patient account creation for billing purposes. | MSH, EVN, PID, PV1, [DG1], [IN1], [GT1] |
| MFN^M02 | Master File - Staff/Practitioner | Master file notification for practitioner/staff directory updates. | MSH, MFI, {MFE, STF, PRA} |
| VXU^V04 | Unsolicited Vaccination Record Update | Immunization record sent to an immunization registry. | MSH, PID, [NK1], {ORC, RXA, [RXR], {OBX}} |
Implementation Notes
Actual segment usage varies by implementation. Interface specifications (IHE profiles, vendor documentation, and site-specific interface agreements) define which optional segments and fields are populated for a given integration. The message structures shown here reflect the standard definitions; consult your interface engine documentation for site-specific variations. Z-segments (custom segments starting with Z) are commonly used for site-specific extensions not covered by the standard.