An HL7 message is a way to transmit data between disparate systems. A message consists of a group of segments in a defined sequence, with these segments (or groups of segments) being optional, required, and/or repeatable.

The HL7 message type defines the purpose for the message being sent and is present in every HL7 message. Message types are identified by a three-character code, and are used in conjunction with a trigger event. An HL7 trigger event is a real-world event that initiates communication and the sending of a message, and is shown as part of the message type. Both the message type and trigger event are found in the MSH-9 field of the message.

For example, the MSH-9 field might contain the value ADT-A01. This means that ADT is the message type, and A01 is the trigger event. In the HL7 Standard, an ADT-A01 message is known as a &qout;patient admit&qout; message.

Each message type and trigger event within a specific HL7 version has a defined format.

There are some message types and triggers that have the exact same format, such as
ADT-A01, ADT-A04, ADT-A05 and ADT-A08. But in many cases, the formats vary widely.

When sending or receiving messages, it is crucial that the correct message type and trigger are selected for use when setting up message processing. If a different message type and trigger format is received than is expected, data can be lost and/or misinterpreted.

The most commonly used HL7 message types include:

• ACK – General acknowledgement
• ADT – Admit discharge transfer
• BAR – Add/change billing account
• DFT – Detailed financial transaction
• MDM – Medical document management
• MFN – Master files notification
• ORM – Order (Pharmacy/treatment)
• ORU – Observation result (Unsolicited)
• QRY – Query, original mode
• RAS – Pharmacy/treatment administration
• RDE – Pharmacy/treatment encoded order
• RGV – Pharmacy/treatment give
• SIU – Scheduling information unsolicited

For a full list of all HL7 message types, refer to Appendix A of the HL7 Standard.