Claim denial codes (CARC), explained
Every line a payer adjusts on an 835/ERA or EOB carries a standardized X12 Claim Adjustment Reason Code (CARC) and a group code that says who absorbs the dollars. Pick a code below for a plain-English explanation, the real root causes, the exact steps to fix or appeal it, and a prevention checklist — then generate a ready-to-send appeal letter for free.
Source: X12 CARC & Claim Adjustment Group Codes. Maintained by the D3rx Clinical Billing Team.
Contractual Obligation (CO)
Provider write-offs — these amounts cannot be billed to the patient. Most are fee-schedule differences or fixable billing errors.
- CO-109Claim/service not covered by this payer/contractor. You must send the claim/service to the correct payer/contractor.
You billed the wrong payer or contractor — the claim needs to go to whoever is actually responsible for this patient/service.
- CO-16Claim/service lacks information or has submission/billing error(s).
Something required is missing or wrong on the claim itself — this is a paperwork/data error, not a clinical decision.
- CO-197Precertification/authorization/notification/pre-treatment absent.
The service required prior authorization (or notification) that wasn't obtained before it was provided, so the payer denied it.
- CO-22This care may be covered by another payer per coordination of benefits.
The payer believes another insurance is primary and should be billed before them.
- CO-29The time limit for filing has expired.
The claim was submitted after the payer's filing deadline, so it was denied for timeliness.
- CO-4The procedure code is inconsistent with the modifier used or a required modifier is missing.
The procedure was billed with a modifier that doesn't belong on it, or it needed a modifier that wasn't there.
- CO-45Charge exceeds fee schedule/maximum allowable or contracted/legislated fee arrangement.
Your billed charge was higher than the payer's allowed/contracted amount — the difference is a contractual write-off, not a real denial.
- CO-96Non-covered charge(s).
The service isn't covered, and under the CO group code the provider — not the patient — absorbs the charge (often because no valid advance notice was given).
- CO-151Payment adjusted because the payer deems the information submitted does not support this many/frequency of services.
The payer thinks you billed too many units, or the service too often, for what the documentation supports.
- CO-50These are non-covered services because this is not deemed a 'medical necessity' by the payer.
The payer decided the service wasn't medically necessary for the diagnosis submitted, so it won't pay — and under CO the provider absorbs it unless a prior notice shifted liability.
- CO-97The benefit for this service is included in the payment/allowance for another service/procedure that has already been adjudicated.
This is a bundling denial — the payer says the service is already paid for as part of another code you billed, so it won't pay separately.
- CO-11The diagnosis is inconsistent with the procedure.
The diagnosis on the claim doesn't clinically support the procedure you billed, so the payer rejected the pairing.
- CO-167This (these) diagnosis(es) is (are) not covered.
The diagnosis code itself isn't on the payer's covered list for this service — the issue is the diagnosis, not the procedure's necessity.
- CO-18Exact duplicate claim/service.
The payer sees this as an exact duplicate of a claim/line it already received for the same patient, date, and service.
Patient Responsibility (PR)
Balances that may be billed to the patient — deductibles, coinsurance, copays, or properly noticed non-covered services.
- PR-204This service/equipment/drug is not covered under the patient's current benefit plan.
The item or drug isn't covered under the patient's current benefit plan as billed, and under the PR group code the patient is responsible.
- PR-96Non-covered charge(s).
The service isn't covered under the patient's plan, and under the PR group code the patient is responsible for the charge.
- PR-1Deductible amount.
This amount is the patient's responsibility because it applies to their unmet annual deductible — the claim processed correctly.
Other Adjustment (OA)
Neither a provider write-off nor a patient balance — typically coordination-of-benefits accounting between payers.
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Medical billing disclaimer
CARC/RARC definitions are the standardized X12 set; group-code semantics follow the X12 Claim Adjustment Group Code standard. Filing and appeal windows vary by payer, plan, and state — always confirm the rule in the payer's provider manual before acting. D3rx is not responsible for claim outcomes.