PID-3 in HL7 v2: A Field Guide to Patient Identifier List
PID-3 is the field almost every downstream system keys off of, yet it is also one of the most misparsed — because it is a repeating field that can carry an MRN, an account number, and identifiers from other facilities all in the same message.
PID-3 is the third field of the HL7 v2 PID segment — a repeating list of patient identifiers, each with its own ID number, assigning authority, and identifier type code, most commonly the medical record number.
What this field contains
Example
Three identifiers are packed into PID-3, separated by ~: an MRN, an account number, and an MRN from a second facility. Synthetic patient — not a real person.
Where this trips people up
Interfaces built and tested against a single identifier per patient often hardcode "take the first repetition" logic. That works until a merged or referred patient arrives with identifiers from two facilities, and the system silently keeps the wrong one — or drops the identifier the receiving application actually needed to match on.