Join a US health plan project and within a day you'll hear transaction numbers used as nouns: "the 834s are late", "the 835 doesn't balance", "we're rejecting 837s". They are X12 EDI transactions (version 5010), the standard files that move data between employers, providers, clearinghouses, banks and payers. Learn what each one is for and the meetings start making sense.
834 — enrollment
The employer or the government exchange tells the payer who is covered: adds, changes and terminations for each subscriber and their dependents, with plan and coverage dates. A payer saying worth remembering: if it's not on the enrollment file, the member doesn't exist. Late or partly loaded 834s are the biggest single source of avoidable denials.
820 — premium payment
How an employer or government programme pays premiums to the plan, matched against the invoice for that group.
270/271 — eligibility
A provider asks, before the visit, "is this member covered today, for this service?" (270). The payer answers (271) with coverage status, copay, how much of the deductible is met and the primary care provider.
278 — prior authorisation
Some services need approval before they happen. The 278 carries the request and the decision. An approved 278 comes with an authorisation number, and the claim must later carry it with matching dates and units or it will deny.
837 — the claim
The provider bills the payer. There are three flavours: 837P for professional (doctor) claims, 837I for institutional (hospital) claims and 837D for dental. The claim carries the member, billing and rendering provider, diagnosis and procedure codes, dates of service, charges and any authorisation number.
999 and 277CA — acknowledgements
The 999 says whether a file was structurally valid. The 277CA says, claim by claim, whether each one was accepted into the payer's system. A file can pass the 999 and still have claims rejected on the 277CA.
276/277 — claim status
"Where is my claim?" The provider asks with a 276 and the payer replies with a 277.
835 — remittance advice
What was paid, per claim and per line, and why the rest wasn't, using CARC and RARC adjustment codes. The 835 must balance against the EFT that lands in the provider's bank account.
Where FHIR fits
US rules (the CMS Interoperability and Prior Authorization final rule, CMS-0057-F) require Medicare Advantage, Medicaid, CHIP and federal-exchange plans to offer FHIR APIs, including for prior authorisation, phased in from 2026–2027. X12 isn't going away; the two will run side by side for years.
Testing the files, not just the screens
- Every EDI change needs a test file for each transaction set before release. The trading partner won't test it for you.
- Test the companion guide rules — each payer's own additions to the standard — not only the X12 structure.
- Reconcile: records in versus records loaded for the 834, and claims-to-remittance-to-bank for the 835.
- Use masked or synthetic member data in every lower environment.