Medical Billing Denial Codes: Complete List & Meanings

When a payer denies a claim, they send a reason a standardized code that tells you exactly why the claim was not paid. The problem is that these codes were written for payer systems, not for the billers and practice managers who need to act on them. Most practices receive a denial, look up the code, find a vague description, and still do not know what to actually do next.

This reference covers the most common medical billing denial codes CO codes, PR codes, and OA codes with plain-English explanations and a specific first action step for each. For the five codes that account for the largest share of denied revenue at most practices, I have written full dedicated guides that walk through the resolution and appeal process in detail. Links to those are in the table below.

The codes on this page are Claim Adjustment Reason Codes (CARCs), maintained by the Washington Publishing Company (WPC) on behalf of CMS. They appear on every Electronic Remittance Advice (ERA) and Explanation of Benefits (EOB) you receive from any payer in the U.S.

What Are Claim Adjustment Reason Codes (CARCs)?

Claim Adjustment Reason Codes (CARCs) are standardized codes that explain why a payer adjusted, reduced, or denied a claim. They appear on every ERA and EOB issued by Medicare, Medicaid, and commercial payers in the United States. CMS mandates their use under HIPAA transaction standards (ASC X12 835).

There are three main CARC prefixes, and understanding what each prefix signals tells you immediately where the problem sits:

• CO (Contractual Obligation): The adjustment is based on a contractual agreement between the provider and payer. The patient cannot be billed for a CO adjustment. These are the most common denial codes.
• PR (Patient Responsibility): The amount is the patient’s responsibility their deductible, copay, or coinsurance. Bill the patient, not the payer.
• OA (Other Adjustment): Adjustments that do not fall into the CO or PR categories, including coordination of benefits and Medicare secondary payer situations.

CARCs are often accompanied by Remittance Advice Remark Codes (RARCs), which provide additional detail. When you see an N-series code (N115, N130, etc.) alongside a CARC, that is the RARC adding specificity. Both are published and maintained by CMS on their CARC/RARC reference page.

The Most Common Denial Codes Reference Table

This table covers the denial codes that appear most frequently across U.S. physician practices, based on CMS data and our internal denial tracking across 200+ active client practices.

Code	Meaning	What the Payer Is Telling You	First Action Step
CO-45	Charges exceed fee schedule / maximum allowable	Your billed charge is above the contracted rate. The payer pays the contracted amount and writes off the rest.	No action needed if write-off is correct. Flag if payment is below contracted rate that is an underpayment, not a CO-45.
CO-97	Benefit for service included in payment for another service	The procedure is bundled with another code billed on the same claim NCCI edit triggered.	Review NCCI edit. If services were genuinely separate, add correct modifier (59, XS, XE, XP, or XU) and resubmit with documentation.
CO-4	Service not consistent with modifier	The modifier on the claim does not align with the procedure code or does not justify separate billing.	Review modifier use against payer policy. Resubmit with correct modifier or remove incorrect modifier.
CO-16	Claim lacks information / incomplete	Missing or incomplete data on the claim often a required field, attachment, or supporting document.	Read the RARC code alongside CO-16 it will identify the specific missing element. Correct and resubmit.
CO-22	Payment adjusted for other coverage	Primary payer has already paid. This payer is adjusting as secondary.	Confirm EOB from primary payer was submitted. Resubmit with primary payer’s ERA/EOB attached if missing.
CO-29	Timely filing limit expired	The claim was submitted after the payer’s filing deadline.	Check timely filing limit for this payer. If within limit, submit proof of timely filing (clearinghouse report). If outside limit, write off unless exception applies.
CO-50	Non-covered service not deemed medical necessity	Payer does not consider the service medically necessary based on the diagnosis codes submitted.	Review clinical documentation. If necessity is supported, add supporting diagnosis codes and resubmit or appeal with clinical notes.
CO-57	Prior authorization required	The service required prior authorization that was not obtained before the service was rendered.	Confirm whether authorization was obtained. If yes, resubmit with auth number. If no, appeal with clinical urgency documentation if applicable.
CO-96	Non-covered charge(s)	The specific service or charge is not covered under the patient’s plan.	Verify patient’s benefit plan. If covered, resubmit with corrected coding. If not covered, bill patient if ABN was signed; write off if not.
CO-167	Diagnosis is not covered	The ICD-10 diagnosis code is not covered by the patient’s plan for the service billed.	Review diagnosis coding. Resubmit with a covered, accurate diagnosis code if documentation supports it.
CO-170	Payment denied healthcare not performed by required provider	Service must be performed by a specific provider type or specialty per plan requirements.	Confirm provider type meets plan requirements. If an in-network specialist was used, resubmit with correct provider NPI and credentials.
CO-197	Precertification / authorization absent	Authorization was required but missing from the claim.	Locate the auth number and resubmit. If authorization was not obtained, submit retroactive authorization request to payer.
PR-1	Deductible amount	The amount applied to the patient’s annual deductible.	Bill the patient for the deductible amount. No payer action needed.
PR-2	Coinsurance amount	The patient’s coinsurance portion of the allowed amount.	Bill the patient for the coinsurance amount. No payer action needed.
PR-3	Co-payment amount	The patient’s copay per their plan.	Bill the patient for the copay. Ensure copay was collected at time of service per your policy.
PR-204	Service not covered by this payer patient is responsible	This payer does not cover the service; patient is liable.	Confirm the service is genuinely non-covered. If so, bill patient — but only if an ABN or financial agreement was signed before service. If payer error, appeal.
OA-18	Duplicate claim	An identical or near-identical claim was already submitted and processed.	Verify the original claim was processed and paid correctly. If yes, no action needed. If the original was denied, note ‘Not a duplicate original claim denied’ in resubmission.
OA-23	Payment adjusted due to coordination of benefits	This claim involves COB another payer is primary.	Confirm primary payer and submit primary EOB with this claim for secondary processing.

Full Denial Code Guides Step-by-Step Resolution

The five codes below account for the largest volume of denied revenue at most practices. Each has a dedicated guide covering root cause analysis, resolution steps, appeal letter guidance, and prevention protocol.

📋  Individual Denial Code Deep-Dive Guides • CO-45 Denial Code: What It Means and How to Handle It: Contractual obligation adjustment; the most common CARC on any ERA. • CO-97 Denial Code: Bundled Service Explained: NCCI bundling denials; when and how to use modifier 59 or the X-series. • PR-204 Denial Code: Non-Covered Service: Patient responsibility denials; ABN requirements and patient billing rules. • CO-16 Denial Code: Missing or Incomplete Information: Incomplete claim denials; how to read RARC codes alongside CO-16. • CO-29 Denial Code: Timely Filing Expired: Timely filing denials; proof of timely filing and exception criteria.

Denial Prevention: What the Codes Are Really Telling You

Most denial codes are symptoms of upstream process failures, not isolated billing errors. After tracking denial patterns across our client base, three root causes account for over 70% of all denials:

• Front-end eligibility failures:  CO-96, CO-50, PR-204, and PR-1/2/3 denials often trace back to eligibility not being verified before the visit, or being verified but not reviewed for plan-specific benefit limitations.
• Coding and modifier errors: CO-4, CO-97, and CO-16 denials are almost always coding issues: wrong modifier, missing modifier, bundling that should have been unbundled, or a diagnosis code that does not support the procedure.
• Administrative failures: CO-29 (timely filing), CO-197 (missing auth), and CO-22 (COB) denials are administrative, not clinical. They are preventable with the right front-end workflow.

If you are seeing the same denial code appear on more than 5% of your claims in any given month, that is a workflow problem, not a random error. Our denial management team runs a root-cause denial analysis as the first step of every new engagement identifying which codes are appearing, at what frequency, and which front-end or coding process is generating them.

Working With Denial Codes Systematically

A denial code is a message. Most practices treat each denial as an isolated event look up the code, fix the claim, resubmit. That approach recovers some revenue, but it does not fix the underlying problem generating the denial in the first place.

The practices that consistently maintain denial rates below 4% treat denial codes as data. They track which codes appear, at what frequency, on claims from which providers or service types, and they trace each pattern back to its root cause. That is how a 19% denial rate becomes a 4% denial rate in 90 days not by working faster, but by fixing the right upstream process. Our denial management services are built around exactly that approach.

If your practice is seeing recurring denial codes that are not being resolved, or you want a clear picture of what your denial pattern actually looks like, schedule a free denial analysis with the Globill Medical Resources LLC team. We will pull your top denial codes by frequency and dollar value and tell you exactly which processes are generating them.

Frequently Asked Questions About Denial Codes