A segment is a group of fields that then contain varying types of data. Each segment exists independently and can be utilized in multiple messages, in varying sequences, throughout the HL7 standard. Segments may be required for a particular message or they may be optional.
A unique three-character code called the “Segment ID” identifies each segment. Segment ID codes beginning with the letter Z are reserved for locally defined Z-segments that are not part of the HL7 standard.
You can remove one or more segments from a message if desired. But because HL7 rules state that unexpected or unused segments should be ignored, most systems will ignore the unexpected segments without issue (therefore, manually removing these segments is not necessary). You might wish to remove segments only if one of your trading partners’ systems might have problems when unexpected segments are received.
The most commonly utilized segments in real-world HL7 processing include:
- DG1 – Diagnosis
- EVN – Event type
- GT1 – Guarantor
- IN1 – Insurance
- MSH – Message header
- NK1 – Next of kin/associated parties
- NTE – Notes and comments
- OBR – Observation request
- OBX – Observation result
- ORC – Common order
- PID – Patient identification
- FT1 (for DFT messages) – Financial transaction
For a full list of all HL7 segments, refer to Appendix A of the HL7 standard.