Common Claim Denial Codes in Medical Billing: A Decoder

Denial codes tell you why claims don't pay. Every denied or adjusted claim carries a Claim Adjustment Reason Code, called a CARC, often paired with a Remittance Advice Remark Code, called a RARC, that adds detail. The two-letter prefix in front of the CARC, CO, PR, OA, or PI, tells you who is financially responsible for the adjusted amount. Read all three together and the code stops being alphabet soup. It tells you what to do next: fix and resubmit the claim, appeal it, write it off, or bill the patient.

This is a decoder, not a causes-and-fixes guide. For the common reasons behind denials and how practices prevent them, see our guide to claim denials. For help building a process around this, see denial management. BillingFirms is a directory, not a billing company. We have no favorite among the firms listed here.

How to read a denial code

A denial code has three parts that each answer a different question.

The group code answers: who absorbs the amount?

• CO (Contractual Obligation): the provider absorbs it under the payer contract, and the patient cannot be billed.
• PR (Patient Responsibility): the patient owes it.
• OA (Other Adjustment): neither by default.
• PI (Payer Initiated Reduction): the payer reduced the payment on its own initiative.

The CARC answers: what was the reason? CO-50 is the classic example: the payer decided the service was not medically necessary.

The RARC answers: what exactly? RARCs add payer-specific detail on top of the CARC. Two claims can both carry CO-16 (claim lacks information) while different RARCs name the missing item, one a rendering provider NPI, the other a referral number. That detail decides what you fix.

A typical remittance line might read: CO-16 with RARC N290. Translation: contractual obligation (the provider eats this, cannot bill the patient), missing claim information, and specifically the rendering provider identifier was missing or invalid. The action writes itself: add the rendering NPI, resubmit as a corrected claim.

CARC lists are maintained by X12 and updated three times a year. RARC lists are maintained by CMS. The code means the same thing whether it comes from Medicare, Medicaid, or a commercial payer.

CARC vs RARC

The group codes, and what each routes to

The group code is the first thing to check. It decides which queue the claim belongs in. A CO denial belongs in resubmit-or-appeal. A PR line belongs in patient billing. Sending a PR balance through the appeals queue wastes the hours that should go toward recoverable revenue, and billing a patient for a CO adjustment can violate the payer contract.

Fix and resubmit: the correctable codes

These are the good-news codes. Nothing about the service itself was challenged. Something on the claim was missing, inconsistent, or wrong. Fix it and resubmit as a corrected claim.

CO-16: Claim lacks information or has a submission or billing error. The claim is missing required data or carries something invalid: patient ID, provider NPI, modifier, place of service, payer ID. The paired RARC usually names the missing item. Check the remittance before you touch the claim, because the RARC tells you exactly what to fix.

CO-4: Procedure code inconsistent with the modifier used. The modifier does not fit the procedure code, or a required modifier is missing. Common with bilateral, staged, and multi-surgeon procedures. Verify the modifier against the payer's edit rules and resubmit.

CO-11: Diagnosis inconsistent with the procedure. The ICD-10 code billed does not support medical necessity for the CPT or HCPCS code. This is a linking problem, not a documentation problem: the right diagnosis is in the chart but the wrong one made it onto the claim line. Correct the pairing and resubmit.

CO-97: Service included in the payment for another service. The payer bundled the billed service into the payment for another service already adjudicated, per its bundling edits. This usually comes from modifier use the payer did not accept or from services the payer's edits treat as one. Review the payer's bundling and NCCI edits before billing; if a modifier was valid and documented, appeal with the records.

CO-15: Authorization number missing, invalid, or not for this service or provider. Authorization existed, or existed for a different date of service, but the number on the claim is wrong. Add the correct authorization number and resubmit. If the authorization itself was never obtained, that is CO-197, below.

CO-22: Care may be covered by another payer. Coordination of benefits. Another payer should have been billed first. Verify the patient's primary coverage and send the claim to the primary payer.

CO-252: Attachment or other documentation required to adjudicate the claim. The payer needs a specific document, an operative report, an invoice, medical records, before it can decide. The RARC names the document. Send exactly that document through the payer's preferred channel. This is a request to complete the claim, not a denial to appeal.

Appeal with records: the judgment-call codes

These codes mean the payer judged something about the service or the medical record. You do not fix them with a corrected claim. You argue them, with documentation.

CO-50: Service not deemed medically necessary. The payer decided the diagnosis and documentation do not support the service. Appeals run on clinical records and a letter of medical necessity. The best defense is before the fact: check the payer's coverage criteria (Medicare's local and national coverage determinations, commercial medical policies) before the service, and document why the service was needed in the note.

CO-151: Information does not support this many or this frequency of services. The payer believes the documentation does not justify the number of units billed or how often the service was billed, therapy visits above a frequency limit being the classic case. Check billed units against the documentation and the payer's frequency policy. Appeal with records when the extra services were necessary.

CO-167: Diagnosis code not on the covered list. The diagnosis billed is not covered for the service under the payer's policy. Correct the diagnosis if the claim carried the wrong one, or appeal with clinical documentation if the diagnosis is right and the policy should cover it.

CO-197: Precertification, authorization, or notification absent. A required prior authorization was never obtained, was expired, or did not match what was billed. Common for imaging, surgery, and high-cost treatments. If the authorization exists, submit it as the appeal. If it does not, ask whether the payer allows retro authorization. Our prior authorization guide covers the process in detail.

Check before you act: the terminal-looking codes

These codes look final, and some of them are. Each one needs a check before you decide whether anything can be done.

OA-18: Exact duplicate of a claim already processed. Same claim, submitted twice. X12 directs payers to pair CARC 18 with the OA group code (except state workers' compensation, where it can appear as CO-18). Duplicates usually come from resubmitting a claim that was still being processed. Do not resubmit again. Check claim status. If something on the original claim needs to change, send a corrected claim, not a new one.

CO-109: Claim not covered by this payer or contractor. The claim went to the wrong payer, or another payer should have been billed first. Update the insurance information, check coordination of benefits, and bill the right payer. Verify eligibility at every visit and this code shrinks.

CO-26 / CO-27: Expenses incurred before coverage began / after coverage ended. The patient's coverage was not active on the date of service. Verify the coverage effective date. If the patient had other coverage on that date, bill it. If there was no coverage, the balance becomes the patient's responsibility or self-pay.

CO-29: Time limit for filing has expired. The claim arrived after the payer's filing deadline. Commercial deadlines vary by contract; Medicare allows 12 months from the date of service. These denials are almost never appealable once triggered, unless you can prove timely filing, so keep submission confirmations and clearinghouse acceptance reports for every claim.

CO-96 / CO-204: Non-covered charges / service not covered under the patient's benefit plan. The service is excluded from the plan. CO-96 is the general non-covered code; CO-204 names a service, equipment, or drug the patient's plan does not cover. Neither is a medical-necessity judgment; the plan simply does not pay for this. Verify benefits before costly services, and get a signed financial responsibility form or Medicare advance beneficiary notice where the rules allow.

CO-45: Charge exceeds the fee schedule or contracted amount. This is not really a denial. The billed amount exceeds the payer's contracted or allowable rate, and the difference is a contractual adjustment. It belongs in the write-off queue, not the appeals queue.

CO-119: Benefit maximum reached. The patient's plan-year or lifetime benefit limit for the service is exhausted. Verify the remaining benefit before the service where the plan discloses it.

CO-B7: Provider not certified or eligible to be paid for this service on this date of service. A credentialing and enrollment problem. The provider was not enrolled with the payer, or not yet credentialed, on the date of service. Keep credentialing and enrollment current with every payer to prevent it.

Patient responsibility: not denials

PR-1: Deductible amount. The patient's deductible was not yet met. PR-2: Coinsurance amount. The patient's share under a coinsurance structure. PR-3 covers copays. These are not denials. They are the plan design working as intended, and they belong in patient billing, not in the appeals queue. They show up here because practices routinely misroute them into denial worklists, where they clog the queue that recoverable revenue is waiting in.

Where denial codes appear

On paper, they print on the explanation of benefits or remittance advice that comes with each claim. Electronically, they arrive in the ERA, the HIPAA 835 electronic remittance transaction, which posts directly into the practice management system. Most payer portals also show the group code, CARC, and RARCs on the claim detail view.

Work from the ERA or the portal whenever possible. Codes retyped by hand introduce errors, and the full code triple, group code plus CARC plus RARC, is what the routing decisions above depend on. If your system only shows the CARC, that is enough to start. The RARC layer helps, it's not required.

What to do when a code appears

Route by group code first. CO goes to the billing team: correct and resubmit, appeal with records, or write off per the contract. PR goes to patient billing. OA gets investigated, because it is where coordination-of-benefits and duplicate issues hide. Then match the CARC to the action above: fix-and-resubmit codes go back out corrected, judgment-call codes get an appeal file with documentation, terminal-looking codes get their one check before the write-off or patient-billing decision.

The habit that pays for itself is tracking codes by volume, not working them one at a time. A steady stream of CO-16s points at the front desk and the claim scrubber. A cluster of CO-197s points at the prior authorization process. Repeated CO-97s point at bundling edits and modifier use. The codes are a diagnostic for the revenue cycle. Read enough of them and they tell you where the process is broken.