Denial Codes and Remark Codes on Your EOB

When your insurer processes a medical claim and pays less than the billed amount — or nothing at all — the Explanation of Benefits (EOB) you receive will include codes explaining why. These codes are standardized across the US health insurance system, and understanding them is the first step toward knowing whether to accept the result or take action. This page covers the two main code sets: Claim Adjustment Reason Codes (CARCs) and Remittance Advice Remark Codes (RARCs).

For a broader introduction to reading your EOB, visit the Reading an EOB and a Medical Bill guide.

The four adjustment group codes

Every claim adjustment on an EOB begins with a two-letter group code that tells you who bears responsibility for the adjusted amount. Understanding these prefixes tells you immediately whether a charge is something you owe, something the provider absorbs, or something that requires further investigation.

Group Code	Name	What it means
CO	Contractual Obligation	The provider agreed by contract to accept less than billed; neither you nor the insurer owes this portion
PR	Patient Responsibility	This amount is legitimately yours — deductible, copay, or coinsurance
OA	Other Adjustments	Adjustments not covered by CO or PR — commonly coordination of benefits or Medicare rules
PI	Payer Initiated	The insurer is reducing the payment for an administrative reason not related to the contract

The CO group is the most common. When you see a large CO adjustment alongside an in-network provider visit, it reflects the negotiated discount — the gap between what the provider billed and what the contracted rate allows. You should not be billed for this amount. If a provider bills you for a CO-coded adjustment, that is improper balance billing.

Claim Adjustment Reason Codes: the numbered explanation

After the group code, a numeric Claim Adjustment Reason Code (CARC) gives a more specific explanation. These codes are maintained by the Washington Publishing Company under contract with CMS and updated quarterly. The most common codes you will encounter include:

• CO-45: Charges exceed your contracted or fee schedule amount. This is the standard in-network discount — routine and expected.
• CO-97: Payment is included in the allowance for another service or procedure. The billed service was “bundled” into a different code.
• CO-4: The service was billed under the incorrect procedure code. This is a fixable error — the provider should resubmit.
• PR-1: Deductible amount. The amount applied toward your annual deductible.
• PR-2: Coinsurance amount. Your percentage share after the deductible is met.
• PR-3: Copay amount.
• CO-50: These are non-covered services because this is not deemed a ‘medical necessity’ by the payer. This code triggers the right to appeal if you believe the service was medically necessary.
• CO-29: The time limit for filing has expired. This is a timely filing denial and is covered in more detail in the Timely Filing and Late Bills guide.

Remark codes: the second layer of explanation

A Remittance Advice Remark Code (RARC) is a supplementary code that appears alongside a CARC when the reason code alone is not sufficient. RARCs begin with a letter: M codes are informational messages, N codes are alert or supplemental information codes.

Common remark codes you may encounter:

• M51 — Missing or incomplete/invalid procedure code. The provider submitted a claim without a valid CPT or HCPCS procedure code. The provider can resubmit with the corrected code.
• M76 — Missing or incomplete/invalid diagnosis or condition. The claim did not include the required ICD-10 diagnosis code.
• M15 — Separately billed services/tests have been bundled. Services that should be billed together were submitted individually.
• N30 — Patient cannot be identified as our insured. The patient information on the claim does not match what the insurer has on file — often a name, date of birth, or ID number error.
• N115 — This decision was based on a Local Coverage Determination (LCD). The service falls under a policy that defines when it is covered by Medicare. You can look up the specific LCD on cms.gov.

Documents and terms you’ll see

When reviewing your EOB for adjustment and remark codes, you will encounter these terms:

• Denial code — also called a Claim Adjustment Reason Code (CARC); the standardized number that identifies why a payment was reduced or denied
• Remark code — a supplementary RARC that gives additional detail when the denial code alone does not fully explain the adjustment
• EOB (Explanation of Benefits) — the document from your insurer summarizing how the claim was processed; consumer EOBs often translate codes to plain language
• Adjustment — any difference between the billed amount and the amount the insurer pays; adjustments are classified by group code (CO, PR, OA, PI)
• Claim — the formal request submitted by your provider to the insurer for payment; each claim line can carry its own set of adjustment codes

How to act on a denial code

The action you should take depends heavily on the reason code category. Here is a practical framework:

CO-4, M51, N30, and similar administrative or coding errors: These indicate a fixable mistake. Contact your provider’s billing department and ask them to correct the error and resubmit the claim. You should not pay the bill while a corrected resubmission is pending.

CO-50 (medical necessity) or CO-96 (non-covered charge): These codes reflect the insurer’s coverage decision. You have the right to file an internal appeal, providing clinical documentation from your treating provider explaining why the service was medically necessary. If the internal appeal fails, you can request an external review.

PR-1, PR-2, PR-3 (deductible, coinsurance, copay): Verify that the amounts are consistent with your plan’s Summary of Benefits and Coverage. If your deductible has already been partially met earlier in the year, confirm the insurer has applied prior payments correctly.

CO-45 (contractual discount): Routine — you do not owe this amount. If the provider is billing you for a CO-45 adjustment amount, tell them in writing that CO-45 adjustments are a contractual write-off and ask for a corrected bill.

When remark codes point to a provider error

Several remark codes specifically indicate that the claim was rejected or reduced because the provider submitted incorrect or incomplete information. If you see codes like M51, M76, or N30 on your EOB alongside a zero-payment result, the problem is almost certainly on the provider’s side. The provider should resubmit at no cost to you — this is their billing error, not a coverage decision that requires you to pay.

Keep a record of the date you requested a corrected resubmission and follow up if the provider does not act within a few weeks. If the claim remains unresolved and you receive a bill for the amount, write to the billing department referencing the specific remark code and asking for the corrected claim to be submitted before you will consider any payment.

Tracking codes across multiple EOBs

If you received care from multiple providers on the same day — for example, a surgeon, an anesthesiologist, and a facility — you will receive separate EOBs for each claim. Each EOB can carry different adjustment codes. Review them together: what one provider is billing as patient responsibility should align with what the other providers are billing, and all should align with your plan’s cost-sharing structure.

If you see conflicting PR amounts across EOBs for the same encounter, contact your insurer’s member services to reconcile the totals before paying any of them.