Reading an HL7 v2 message

Segments and fields

An HL7 v2 message is plain text: one segment per line, fields separated by |. The first segment, MSH, sets the delimiters (| ^ ~ \ &), sender, receiver and message type.

MSH|^~\&|EHR|Hospital|LIS|Lab|202411181200||ADT^A01|12345|P|2.3
PID|1||123456^^^Hospital||Smith^John||19800101|M
PV1|1|I|ICU^01^01

• MSH — header (who, what, when).
• PID — patient identification (demographics).
• PV1 — visit (location, clinician).

If a required segment like PID is missing, the receiver can't identify the patient and rejects the message — a common cause of interface failures.