The Complete Insurance Denial Code Library — What Every Code Means

The ClaimCompass Denial Code Library translates every major insurance denial code and tells you exactly what to do.

The Insurance System Runs on Codes

When your health insurance claim is denied, the denial does not arrive in plain English. It arrives as a code — CO-50, PR-96, CO-4, CO-97 —accompanied by boilerplate language that tells you almost nothing about why your specific claim was denied or what you can do about it.

These codes are not random. They follow a standardized system maintained by the National Uniform Claim Committee (NUCC) and the American Medical Association. Understanding them is the first step to challenging them.

How Denial Codes Work

Health insurance denial codes consist of a two-letter prefix and a number.

The prefix tells you who is responsible:

• CO — Contractual Obligation: a reduction based on the provider's contract with the insurer. The provider typically cannot bill you for CO denials.
• PR — Patient Responsibility: the insurer is indicating you — the patient — are responsible for this charge.
• OA — Other Adjustment: an adjustment that doesn't fit into CO or PR categories.
• PI — Payer Initiated: a reduction the payer made that is not based on a contract or patient responsibility.

The number identifies the specific reason:

Numbers are standardized across most insurers, though some insurers use proprietary codes. The most common codes appear across virtually all plans.

The Most Common Denial Codes — Quick Reference

CO-45 — Charge Exceeds Fee Schedule

Highest search volume of any denial code. This is a contracted rate write-off — you do not owe the billed-charge difference. Check the rest of your EOB for codes that are actually appealable.

CO-50 — Not Medically Necessary

The insurer determined the service did not meet their clinical necessity criteria. One of the most common and most frequently overturned denial codes.

PR-96 — Non-Covered Charge

The insurer says the charge is excluded under your plan. Coverage exclusions must be explicitly stated.

PR-97 — Bundled Charge Billed to You

A bundled service was billed to you as patient responsibility. That is often wrong.

CO-4 — Procedure / Modifier Mismatch

The procedure code and modifier do not match, or a required modifier is missing. Usually a coding fix, not a coverage fight.

CO-97 — Bundling Issue

The service is considered included in another billed service. Often correctable with billing modifiers.

CO-167 — Diagnosis Inconsistent with Procedure

The diagnosis code submitted does not support the procedure billed. Often a documentation gap, not a clinical dispute.

CO-29 — Timely Filing Limit Exceeded

The claim was submitted after the filing deadline. Harder to overturn without proof of timely submission.

CO-197 — Prior Authorization Missing

Required prior authorization was not on file. Retroactive authorization and emergency exceptions reverse this often.

CO-16 — Claim Lacks Information

Required information was missing from the claim. Often correctable through resubmission.

B15 — Missing or Invalid Authorization

An authorization number was missing or incorrect on the claim. Usually correctable at the billing level.

OA-23 — Coordination of Benefits

Payment adjusted due to multiple insurance coverage. Submit to the primary insurer first.

CO-58 — Treatment Not Covered

The service is excluded from your plan. Exclusions must be explicitly stated in the plan document.

CO-109 — Claim Sent to the Wrong Payer

The claim went to the wrong insurance company. Usually a routing error, not a coverage dispute.

See the full denial code library or start with how to appeal a health insurance denial.

Frequently Asked Questions

Where do I find the denial code on my EOB?

The denial code appears in the Claim Adjustment Reason Code (CARC) column of your Explanation of Benefits or electronic remittance advice (835 transaction). On a paper EOB, it is usually labeled "reason code," "remark code," or "adjustment code." It may appear as a standalone number or with a prefix like CO-, PR-, or OA- that indicates the group responsible for the adjustment.

What does the CO group code mean on an insurance denial?

CO stands for Contractual Obligation. When a denial carries the CO prefix, the financial responsibility sits with the healthcare provider — they agreed in their contract with the insurer to write off that amount and cannot bill you for it. If you receive a bill for a CO-coded amount, dispute it with the provider.

What is the difference between a denial code and a remark code?

A denial code (Claim Adjustment Reason Code or CARC) identifies the primary reason a claim was adjusted — for example, CO-50 for not medically necessary or CO-16 for missing information. A remark code (Remittance Advice Remark Code or RARC) provides additional detail about the specific issue. CO-16 denials, for example, always come with a remark code (like N245 or N3) that specifies exactly what information is missing.

Don't Know Your Code? Let ClaimCompass Decode It

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