Understanding your denial: CARC and RARC codes explained
If a claim or authorization comes back denied or adjusted, the explanation usually arrives as a short code, not a sentence. Here's what those codes actually mean and where the real reason is hiding.
Two code sets, two jobs
Denials and adjustments in U.S. healthcare claims use two standardized code sets, both maintained by X12, the same body behind the EDI transaction formats plans and providers exchange claims through:
- CARC (Claim Adjustment Reason Code) -- a numeric code that says why a claim line was paid differently than billed, denied, or adjusted. Example: CARC 50 means "these are non-covered services because this is not deemed a medical necessity by the payer."
- RARC (Remittance Advice Remark Code) -- an alphanumeric code that adds detail the CARC alone doesn't capture. RARCs often point to something specific, like a missing prior authorization number or an incomplete referral.
They're designed to work together: the CARC gives the category, the RARC narrows it down. A denial explanation of "CARC 50, RARC N115" is more specific than either code alone.
A few CARC codes you'll actually see
| Code | What it means |
|---|---|
CO-50 | Not deemed a medical necessity by the payer |
CO-18 | Duplicate claim or service already adjudicated |
CO-197 | Precertification/authorization/notification absent |
PR-1 | Deductible amount (patient responsibility) |
CO-45 | Charge exceeds the fee schedule/maximum allowable |
The prefix (CO, PR, PI, OA) is a "claim adjustment group code" that says who is financially responsible for the adjustment: CO means contractual obligation (the provider absorbs it), PR means patient responsibility (you owe it), and so on.
Where to find the actual, human-readable reason
Codes are required on the electronic remittance, but every plan is also required to give a written explanation of an adverse determination in plain language, including the specific clinical rationale for a UM denial and your appeal rights and deadline. If your explanation of benefits (EOB) or denial letter shows only a code, you're entitled to ask for the full written rationale, and for prior-authorization denials specifically, the criteria the reviewer applied.
Why this matters for an appeal
An appeal that responds to the code ("this wasn't medically necessary") is weaker than one that responds to the actual criteria cited in the full determination. If your plan can't produce the specific rule it applied when it denied you, that's worth noting in the appeal itself.
See a real denial explained
The demo shows a skilled-nursing denial with its CARC code, the plain-English explanation, and the exact rule it was checked against.