X12 275 BGN Segment Guide: Fields, Format, and Testing Gotchas
Before a 275 can carry a single page of medical records, it has to introduce itself. BGN is that introduction — get the purpose code or reference number wrong and the receiving system either can't tell an original submission from a resend, or can't match the attachment back to the claim that asked for it.
BGN (Beginning Segment) is the header that opens the transaction set portion of an X12 275, sitting right after the ST segment. It states whether this transaction is an original submission or a resubmission, assigns the sender's own control number for tracking the transaction, and records the date the transaction was created. Everything else in the 275 — the attachment content, the claim identifiers it's tied to — follows this segment.
The elements that actually matter in practice
Example
A synthetic BGN opening an original 275 (BGN01=00), carrying sender control number ATC0098231, created on 2026-07-28.
Where this trips people up
Because BGN looks like boilerplate compared to the attachment content that follows it, a lot of synthetic 275 test files hard-code BGN01 to '00' and never generate a reissue scenario. In production, a payer requesting a resend after a rejected or incomplete attachment set will see a BGN01 of '18,' and a receiving system that only knows how to handle originals will process the resend as if it were a brand-new, unrelated submission — duplicating records instead of replacing them.