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