Reference
Original notes for common interface work. This is not a copy of any standards body specification.
HL7
Message types
An HL7 v2 message starts with MSH. MSH-9 usually carries a message type and trigger event (for example ADT^A01). Common families on this site include ADT (patient administration), ORM (orders), ORU (results), and SIU (scheduling).
Trigger events
Triggers describe why a message was produced: admit, discharge, update, register, and similar workflow events. Always read both the type and the trigger; an A08 is not an A01.
Common segments
MSH is the header. PID identifies a patient record in the message. PV1 describes the visit. ORC/OBR/OBX carry orders and observations. NK1 and GT1 appear when next-of-kin or guarantor data is sent. Field numbers are 1-based; MSH numbering is special because MSH-1 is the field separator character itself.
Common data types
Composite values use the component separator from MSH-2. Names, coded values, and identifiers are typically composites. Repeating values use the repetition separator. Empty components still occupy a slot.
Explore messages with the HL7 Message Analyzer.
Provider
An NPI is a 10-digit identifier. Format and checksum can be checked locally. Registry search uses the official CMS NPPES API. An NPI is not proof of current licensure.