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When a Medicare audit ends with denied or adjusted claims, the first place many providers see the result is not a letter but a line on the remittance advice. A denied or adjusted claim usually shows up as a short string of letters and numbers, such as CO-50 with a remark code, that can be hard to interpret. Yet those codes contain important information: who is financially responsible for the unpaid amount, why the claim was adjusted, whether the adjustment came from a review or audit, and sometimes what appeal rights apply.

Understanding Medicare denial codes is essential after any audit, whether it was conducted by a Medicare Administrative Contractor (MAC), a Recovery Audit Contractor (RAC), the Supplemental Medical Review Contractor (SMRC), the Comprehensive Error Rate Testing (CERT) contractor, or a Unified Program Integrity Contractor (UPIC). Reading the codes correctly helps you decide whether to correct and resubmit, appeal, write off the amount, or bill the patient, and it helps you track patterns that may signal larger compliance problems.

This plain-English guide explains what a Medicare remittance advice is, how the three code sets work together, how to read codes in the right order, which codes commonly appear after audits, how remittance codes relate to overpayment demand letters, and how to build a denial-code review process into your audit response. It is educational and does not replace professional billing, coding, or legal advice.

What Is a Medicare Remittance Advice?

After Medicare processes a claim, the MAC issues a remittance advice (RA) that explains how the claim was paid, adjusted, or denied. Medicare remittance advice comes in two main forms:

• The Electronic Remittance Advice (ERA), which uses the HIPAA standard ASC X12 835 Health Care Claim Payment/Advice transaction.

• The Standard Paper Remittance (SPR), a paper version of the same information.

CMS offers free software to help providers read ERAs in a more human-readable format. Medicare Remit Easy Print (MREP) is used by professional providers and suppliers, and PC Print is used by institutional providers. CMS regularly instructs its system maintainers to update MREP and PC Print with the latest code lists, so providers should make sure they are using current versions.

When an audit results in a claim adjustment, the MAC typically reprocesses or adjusts the original claim, and the adjustment appears on a later remittance advice. That is why reading remittance codes is often the first step in understanding the financial result of an audit.

The Three Code Sets: Group Codes, CARCs, and RARCs

CMS explains that, for any line or claim level adjustment, three sets of codes may be used:

1. Claim Adjustment Group Code (Group Code)

2. Claim Adjustment Reason Code (CARC)

3. Remittance Advice Remark Code (RARC)

Under HIPAA, Medicare and other health plans must use these standard national code sets in remittance advice transactions instead of their own proprietary codes. Each set has a distinct job.

Group Codes: Who Is Responsible?

Group codes assign financial responsibility for the unpaid portion of the claim balance. CMS gives the two most important examples: CO, Contractual Obligation, assigns responsibility to the provider, and PR, Patient Responsibility, assigns responsibility to the patient. CMS also states that Medicare beneficiaries may be billed only when Group Code PR is used with an adjustment.

The group codes commonly seen on Medicare remittance advice are:

• CO (Contractual Obligation). The provider is financially responsible for the adjusted amount and may not bill the beneficiary for it.

• PR (Patient Responsibility). The amount may be billed to the beneficiary or to another payer on the beneficiary's behalf, such as deductible and coinsurance amounts, or a denial for which the beneficiary was properly notified of liability in advance.

• OA (Other Adjustment). Used when neither CO nor PR applies.

The X12 standard also includes a PI (Payer Initiated Reduction) group code, but Medicare contractors generally use CO, PR, and OA. In an audit context, most denied amounts appear with group code CO, meaning the provider absorbs the loss unless the denial is overturned.

Claim Adjustment Reason Codes (CARCs): Why Was It Adjusted?

CMS explains that CARCs provide an overall explanation for the financial adjustment. CARCs are numeric or alphanumeric codes, such as 16, 50, or 96. Medicare uses this national code set to explain denials, payment reductions, and payment increases. CARCs are maintained at the national level and are updated regularly, so the exact wording of a code can change over time.

Remittance Advice Remark Codes (RARCs): What Else Do I Need to Know?

RARCs supplement CARCs with a more specific explanation or with information about remittance processing. CMS has long served as the national maintainer of the RARC list. RARCs usually begin with letters, such as M, MA, or N, followed by numbers.

There are two general types of RARCs:

• Supplemental RARCs, which give more detail about a specific adjustment.

• Informational or "Alert" RARCs, which begin with the word "Alert" and convey general information that is not tied to a specific adjustment, such as appeal rights.

CMS policy has historically required at least one RARC to be used with certain CARCs that are general in nature, such as CARC 16 (missing information or submission errors), CARC 96 (non-covered charges), CARC 125 (submission or billing errors), and CARC A1 (claim or service denied), because those CARCs alone do not fully explain the adjustment.

How Often the Codes Change

CMS issues recurring change requests to update the CARC and RARC lists. CMS has explained that updates follow a schedule three times per year, around March 1, July 1, and November 1. Because codes are added, modified, and deactivated, providers should always look up the current description of any code rather than relying on memory or old cheat sheets.

How to Read a Remittance Code Combination

A typical adjustment appears as a group code, followed by a CARC, often with one or more RARCs. For example, an entry might show CO-50 with a remark code. Read it in this order:

Step 1: Read the Group Code

Start with the two-letter group code. It tells you who is responsible for the unpaid amount:

• CO means the provider absorbs the amount unless the denial is corrected or overturned.

• PR means the amount may be billed to the patient, subject to Medicare rules.

• OA means another type of adjustment, often related to coordination of benefits or other situations.

Step 2: Read the CARC

Next, look up the CARC to understand the general reason for the adjustment, such as medical necessity, non-covered service, missing information, or a duplicate claim.

Step 3: Read the RARC

Then read any RARC to get the specific explanation, such as missing documentation, an invalid code, an adjustment based on a recovery audit, or information about appeal rights.

Step 4: Read the Amounts

Note the billed amount, allowed amount, adjusted amount, and paid amount. The adjustment amount tied to each group code and CARC tells you how much was denied or reduced for that reason.

Step 5: Check Claim-Level Versus Line-Level Adjustments

Adjustments may apply to the entire claim or to individual service lines. Make sure you know which lines were affected, especially when only some services on a claim were denied after review.

Step 6: Look for Provider-Level Adjustments

The 835 transaction also includes a provider-level adjustment section, sometimes called the PLB segment, which shows adjustments that are not tied to a single claim, such as overpayment recoveries or offsets. After an audit, recoupments of overpayments may appear here rather than, or in addition to, claim-level entries.

Common CARCs Seen After Medicare Audits

The following CARCs often appear after medical review and audits. Descriptions are summarized in plain English, and providers should always check the current official wording.

• CARC 50. The services are non-covered because they were not deemed medically necessary by the payer. This is one of the most common codes after medical review, especially when documentation did not support medical necessity under applicable coverage policies.

• CARC 96. Non-covered charges. This general code is used with a RARC that explains the specific reason, such as a service not covered under the benefit or a policy requirement not met.

• CARC 16. The claim or service lacks information or has submission or billing errors. A RARC identifies what is missing or invalid. When a claim is rejected as unprocessable, it generally must be corrected and resubmitted rather than appealed.

• CARC 151. Payment was adjusted because the payer deems the information submitted does not support the number or frequency of services billed.

• CARC 18. Exact duplicate claim or service.

• CARC 97. The benefit for this service is included in the payment or allowance for another service or procedure that has already been adjudicated.

• CARC 29. The time limit for filing has expired.

In audit situations, the CARC often reflects the reviewer's finding. For example, a claim denied because documentation did not support medical necessity may appear with CARC 50, while a claim for which no records were received may appear with a different code accompanied by a RARC explaining that requested information was not received.

Common RARCs Seen After Medicare Audits

RARCs are especially useful after audits because they can tell you which program or process caused the adjustment. Examples include:

• N432. Adjustment based on a Recovery Audit. CMS's Financial Management Manual instructs MACs that, when possible, the remittance advice shall contain an N432 code to let the provider know that an adjustment is the result of a RAC audit. MACs such as Noridian advise providers to check remittance advice for N432 to identify claims recouped by a Recovery Auditor.

• N469. This remark code relates to the limitation on recoupment under Section 935 of the Medicare Prescription Drug, Improvement, and Modernization Act of 2003. MAC guidance explains that N469 indicates a Section 935 overpayment and that an overpayment demand letter will follow. CMS has noted that in some systems N469 may appear in place of N432 on RAC-adjusted claims.

• MA01 and MA02. These Alert codes inform providers of their right to appeal and how to request a review.

• MA130. This code indicates that the claim contains incomplete or invalid information and that no appeal rights are afforded because the claim is unprocessable. It must be corrected and resubmitted.

• M127 and similar M-series codes. These identify missing or incomplete documentation, such as a missing patient medical record or missing certification.

Because RARC wording changes over time, always look up the current description in the official code list referenced by CMS's remittance advice resources and your MAC.

How Remittance Codes Relate to Audit Letters and Demand Letters

Remittance codes are only one part of the audit paper trail. After a postpayment audit, providers typically receive:

• A review results letter from the auditing contractor, explaining the findings for each claim.

• An overpayment demand letter from the MAC, stating the amount owed, the claims involved, the due date, interest information, and appeal rights.

• One or more remittance advices showing claim adjustments and, later, any recoupment of the overpayment.

CMS's Financial Management Manual instructs MACs to include the accounts receivable number, the adjusted claim number, and the adjusted claim amounts when notifying providers of RAC-related adjustments. Providers should match these identifiers across the review results letter, the demand letter, and the remittance advice so that every adjusted claim is accounted for.

Why the Codes Alone Are Not Enough

A CARC and RARC may say "not medically necessary," but they do not explain which documentation element the reviewer found lacking. For that detail, review the audit results letter, the reviewer's rationale, and the claim-specific findings. Use the remittance codes to confirm the financial impact and to categorize denials, and use the audit letters to build your appeal.

Recoupment and the Limitation on Recoupment

Under Section 1893(f)(2) of the Social Security Act and 42 CFR 405.379, CMS may not recoup certain overpayments while a timely first-level appeal (redetermination) or second-level appeal (reconsideration) is pending, as long as the provider files the redetermination within 30 days of the demand letter and the reconsideration within 60 days of the redetermination decision. Remark codes such as N469 can signal that the overpayment falls under these rules. Providers that want to limit recoupment should calendar these deadlines from the demand letter, not from the remittance advice.

What to Do After Reading the Codes

Option 1: Correct and Resubmit

If the claim was rejected as unprocessable (for example, CARC 16 with MA130), it generally cannot be appealed. Correct the error and submit a new claim, as long as timely filing rules allow.

Option 2: Reopen

For minor clerical errors, a reopening may be appropriate. MACs have specific processes for clerical error reopenings, which can sometimes be requested through their portals or by phone.

Option 3: Appeal

If you disagree with a denial based on medical review, file an appeal through the Medicare fee-for-service appeals process:

• Redetermination by the MAC, generally within 120 days of receiving the initial determination.

• Reconsideration by a Qualified Independent Contractor (QIC), generally within 180 days of the redetermination decision.

• Administrative Law Judge (ALJ) hearing, generally within 60 days of the reconsideration decision.

• Medicare Appeals Council review.

• Judicial review in federal district court.

Option 4: Accept and Correct the Process

If the denial is correct, accept it, make sure the beneficiary is not improperly billed when the group code is CO, and fix the underlying process so the error does not repeat.

Option 5: Evaluate Broader Overpayment Obligations

If audit findings or denial patterns suggest that similar claims outside the audit sample were also paid in error, providers should evaluate their obligations under the 60-day overpayment rule at 42 CFR 401.305, which requires reporting and returning identified overpayments within specified timeframes.

Building a Denial Code Review Process After an Audit

Create a Denial Log

Record every adjusted or denied claim from the audit, including the claim number, beneficiary, date of service, group code, CARC, RARC, adjusted amount, audit contractor, and the related review results and demand letters.

Categorize Denials

Group denials by root cause, such as documentation, medical necessity, coding, missing orders, signature issues, or technical billing errors. CARCs and RARCs help create these categories consistently.

Assign Responsibility

Assign each category to an owner: clinical documentation issues to clinical leaders, coding issues to coding staff, and billing errors to the billing team.

Track Deadlines

Calendar appeal deadlines and recoupment-protection deadlines from the dates on the demand letter and initial determination.

Monitor Trends

Review denial trends monthly. A rise in CARC 50 or documentation-related RARCs after an audit may indicate a systemic problem that needs education or process changes.

Keep Code References Current

Make sure MREP, PC Print, and practice management systems are updated with current code lists, and train staff to look up the official descriptions rather than relying on outdated references.

Common Mistakes When Reading Medicare Denial Codes

• Billing the patient for amounts adjusted under group code CO.

• Reading the CARC without the RARC and missing the specific reason.

• Appealing a claim that was rejected as unprocessable instead of correcting and resubmitting it.

• Ignoring Alert RARCs that contain appeal information.

• Relying on outdated code descriptions.

• Missing provider-level recoupments that do not appear at the claim level.

• Using remittance dates instead of demand letter dates for appeal and recoupment deadlines.

• Failing to match remittance adjustments to audit results letters and demand letters.

A Plain-English Example

Consider a hypothetical outpatient therapy provider that went through a postpayment review. A few weeks later, its remittance advice shows several claims with group code CO, CARC 50, and a remark code indicating that the adjustment was based on a recovery audit.

Reading the codes in order:

• CO tells the provider that it, not the patient, is responsible for the denied amount.

• CARC 50 tells the provider that the reviewer found the services not medically necessary.

• The remark code tells the provider that the adjustment resulted from a Recovery Audit.

The provider then matches the claims to the RAC review results letter, which explains that the plan of care was not certified for some dates of service, and to the MAC's demand letter, which lists the overpayment amount and due date. The provider calendars the 30-day deadline to file a redetermination to limit recoupment, gathers the signed certifications that existed at the time of service, and files the appeal. At the same time, it reviews its certification tracking process to prevent future denials.

Frequently Asked Questions

What is the difference between a CARC and a RARC?

A CARC gives the overall reason for a payment adjustment. A RARC adds more specific detail or general information, such as appeal rights.

Can I bill the patient for a denied claim?

Only when the adjustment is assigned group code PR. CMS states that Medicare beneficiaries may be billed only when Group Code PR is used with an adjustment. Amounts with group code CO are the provider's responsibility.

How can I tell if a denial came from a RAC audit?

CMS instructs MACs, when possible, to include remark code N432 on remittance advice for RAC adjustments. In some cases N469 may appear instead. Always compare with your RAC review results letter and MAC demand letter.

Can I appeal a claim denied as unprocessable?

Generally no. Claims returned as unprocessable, often shown with CARC 16 and remark code MA130, must be corrected and resubmitted.

Where can I find the current code descriptions?

CMS's remittance advice resources and your MAC's website direct providers to the official CARC and RARC lists, which are updated about three times per year.

Conclusion

Medicare denial reason and remark codes are a compact language that tells providers who is responsible for an unpaid amount, why a claim was adjusted, and often which program caused the adjustment. After an audit, reading these codes correctly, in the order of group code, CARC, RARC, and amounts, helps providers confirm the financial impact, match adjustments to audit and demand letters, protect appeal and recoupment rights, avoid improper patient billing, and identify the root causes that need to be fixed. Combined with careful review of audit findings, a disciplined denial-code process turns confusing remittance lines into actionable information.

How HealthBridge Can Help

HealthBridge is a healthcare management consulting firm that helps providers interpret Medicare remittance advice and audit results, build denial tracking and root-cause processes, match adjustments to review findings and demand letters, prepare appeals, and strengthen documentation and billing practices. If you need help understanding your denial codes after an audit, please get in touch with us. We will be happy to help you with this topic.

References

• Centers for Medicare & Medicaid Services. Health Care Payment and Remittance Advice. https://www.cms.gov/medicare/coding-billing/electronic-billing/health-care-payment-remittance-advice

• Centers for Medicare & Medicaid Services. Medicare Claims Processing Manual, Chapter 22, Remittance Advice. https://www.cms.gov/regulations-and-guidance/guidance/manuals/downloads/clm104c22.pdf

• Centers for Medicare & Medicaid Services. MLN Matters MM6229, Remittance Advice Remark Code and Claim Adjustment Reason Code Update. https://cms.gov/Outreach-and-Education/Medicare-Learning-Network-MLN/MLNMattersArticles/Downloads/MM6229.pdf

• Centers for Medicare & Medicaid Services. MLN Matters MM11252, RARC, CARC, MREP and PC Print Update. https://cms.gov/Outreach-and-Education/Medicare-Learning-Network-MLN/MLNmattersArticles/downloads/MM11252.pdf

• U.S. Department of Health and Human Services. MM12774, Remittance Advice Remark Code, Claims Adjustment Reason Code, MREP and PC Print Update. https://www.hhs.gov/guidance/sites/default/files/hhs-guidance-documents/MM12774.pdf

• Centers for Medicare & Medicaid Services. CMS Manual System, Pub. 100-04, Transmittal 1267 (RARC and CARC Update and MREP Enhancement). https://cms.gov/Regulations-and-Guidance/Guidance/Transmittals/Downloads/R1267CP.pdf

• Centers for Medicare & Medicaid Services. CMS Manual System, Pub. 100-06, Transmittal 167 (Recovery Audit Contractor Adjustments, N432). https://cms.gov/Regulations-and-Guidance/Guidance/Transmittals/Downloads/R167FM.pdf

• Centers for Medicare & Medicaid Services. Medicare Financial Management Manual, Chapter 3, Overpayments. https://www.cms.gov/regulations-and-guidance/guidance/manuals/downloads/fin106c03.pdf

• Centers for Medicare & Medicaid Services. Medicare Financial Management Manual, Chapter 4, Recovery Audit Program. https://www.cms.gov/regulations-and-guidance/guidance/manuals/downloads/fin106c04.pdf

• Centers for Medicare & Medicaid Services. Original Medicare (Fee-for-Service) Appeals. https://www.cms.gov/medicare/appeals-grievances/fee-for-service

• U.S. Social Security Administration. Social Security Act, Section 1893, Medicare Integrity Program. https://www.ssa.gov/OP_Home/ssact/title18/1893.htm

• Electronic Code of Federal Regulations. 42 CFR 405.379, Limitation on Recoupment of Provider and Supplier Overpayments. https://www.ecfr.gov/current/title-42/section-405.379

• Electronic Code of Federal Regulations. 42 CFR 401.305, Requirements for Reporting and Returning of Overpayments. https://www.ecfr.gov/current/title-42/chapter-IV/subchapter-A/part-401/subpart-D/section-401.305

• Centers for Medicare & Medicaid Services. Medicare Claim Review Programs (MLN Booklet). https://www.cms.gov/outreach-and-education/medicare-learning-network-mln/mlnproducts/downloads/mcrp-booklet-text-only.pdf