Message Parts
An HL7 message contains the following parts: segments, data fields, components, and optionally subcomponents. HL7 messages have the following hierarchical structure:
Segment
Data Field
Component
Subcomponent (optional)
Segments
A segment is a logical grouping of data fields. Segments are at the highest level (depth 1) of the message hierarchy. Each segment has a name that includes a three-character literal value. The following table shows examples of segment names contained by ADT message types.
| Segment code | Description |
|---|---|
| MSH | Message Header |
| EVN | Event Type |
| PID | Patient Information |
HL7 messages have a message header and body. The MSH segments define the header of the message and all other types of segments form the body of the message.
Data Fields
A data field is a string of characters that occur at depth 2 of the message hierarchy. Data types define data fields: Simple and Complex.
The following example shows the hierarchical message structure of the MSH and EVN segments containing the MSH.1 and EVN.1 data fields:
MSH
MSH.1
EVN
EVN.1
Components and Subcomponents
Components and subcomponents further contain the data within data fields. Components and subcomponents can repeat within the same field.
See Also
Processing HL7 Messages
Message Processing
Using HL7 2.X Schemas