Message types and trigger events
How HL7v2 names and categorizes the messages it exchanges, and how a single message type can carry many distinct events.
At its most basic, HL7v2 is composed of a set of messages. A sending application creates and transmits each message; a receiving application accepts and processes it. Each message exists to serve a specific purpose in healthcare communication.
Messages are discrete units of information exchange. Their format and structure are defined by the HL7v2 standard. The content of any given message depends on its message type and its trigger event — the two properties that together describe what the message is for.
Message type
The message type is a category that describes what kind of healthcare information the message carries. HL7v2 defines a fixed set of message types, each identified by a three-letter code.
Common HL7v2 message types include:
| Message type | Title | Description |
|---|---|---|
| ADT | Admit / Discharge / Transfer | Patient admission, discharge, or transfer information. |
| ORM | Order | Orders for laboratory tests, medications, or other healthcare services. |
| ORU | Observation Result | Clinical observation results. |
| DFT | Detailed Financial Transaction | Financial transactions, such as billing and claims information. |
Each message type has a predefined structure — a set of segments and fields appropriate for the information it conveys.
Trigger event
The trigger event is the specific occurrence or action that prompts the transmission of an HL7v2 message. It defines the reason the message exists.
For ADT messages, trigger events include A01 — admit a patient, A02 — transfer a patient, and A03 — discharge a patient. Each trigger event indicates a different action within the patient's journey, and the corresponding message carries the data associated with that action.
The trigger event is closely tied to the message type. Together they give the message its full identity: the type defines the broad purpose and structure; the trigger event narrows it to a specific action so the receiver can react appropriately.
Message types and trigger events together
By combining message type with trigger event, HL7v2 provides a standardized framework for transmitting specific types of healthcare information. The combination is what makes the standard precise enough to be useful.
The HL7v2 standard defines numerous message types and trigger events, covering a wide range of healthcare scenarios. The same message type can be paired with different trigger events to represent distinct actions — a single type therefore covers many situations without losing specificity.
Use case
Consider the ADT message type. ADT can carry many different trigger events: A01, A02, A03, A04, A05, and more. Each trigger event represents a different action within a patient's journey — admitting, transferring, discharging, updating demographics, merging records.
The diagram below shows how the ADT message type is used during a patient admission. Each event triggered generates a different ADT message, carrying the data relevant to that specific event. The receiving system processes the data based on the action conveyed.
By associating different trigger events with the same message type, HL7v2 accommodates a wide range of actions within a healthcare system. Understanding the relationship between message type and trigger event is therefore essential to interpreting HL7 messages correctly: it lets a receiving system respond appropriately to the specific action that the message describes.
For the full catalog of message types and trigger events, see the HL7v2 events reference.
The next page takes a closer look at the structure of HL7v2 messages and the components that make up an HL7v2 message.
Introduction to HL7v2
HL7v2 is the messaging standard that has carried clinical and administrative data between healthcare systems since 1987. This page places it within the wider HL7 family.
Message structure
How HL7v2 messages are organized — segments, fields, components, sub-components, and the delimiters that separate them.