When your health insurance claim is denied or adjusted, your Explanation of Benefits (EOB) contains one or more reason codes that explain why. These codes look like "CO-50" or "PR-2" — and they're not random. They follow a standardized system maintained by the healthcare industry. Knowing what these codes mean is the first step toward a successful appeal. This guide decodes the most important ones.
The Two Code Systems: CARC and RARC
Two standardized code sets appear on EOBs and electronic remittance advice (ERA) documents:
- CARC (Claim Adjustment Reason Codes): Explain why a payment was adjusted or why the claim was not paid in full. Every adjustment must have a CARC.
- RARC (Remittance Advice Remark Codes): Provide supplemental information about the claim adjustment. They add detail to the CARC explanation.
CARCs begin with a group code prefix: CO (Contractual Obligation), PR (Patient Responsibility), OA (Other Adjustment), or PI (Payer Initiated). The most important distinction is CO vs. PR: CO means the provider cannot bill you; PR means you owe the amount.
Most Common Denial Codes and What to Do
Medical Necessity Denials
| Code | Plain-Language Meaning | Who Acts | Your Response |
|---|---|---|---|
| CO-50 | Not medically necessary per payer | Patient + Provider | Medical necessity appeal with physician letter and clinical evidence |
| CO-57 | Payment adjusted because the patient has not met the required eligibility, spending account, or benefit for this date of service | Patient | Verify eligibility and enrollment dates |
| CO-151 | Payment adjusted because the payer deems the information submitted does not support this level of service | Patient + Provider | Submit detailed clinical documentation supporting the level of service billed |
Coding and Billing Errors
| Code | Plain-Language Meaning | Who Acts | Your Response |
|---|---|---|---|
| CO-4 | Procedure code is inconsistent with the modifier | Provider | Provider resubmits corrected claim |
| CO-11 | Diagnosis is inconsistent with the procedure | Provider | Provider submits corrected claim with appropriate ICD-10 code |
| CO-16 | Claim/service lacks information which is needed for adjudication | Provider | Provider resubmits with missing information |
| CO-18 | Duplicate claim/service | Provider | Provider appeals with proof claim was not a duplicate |
| CO-22 | This care may be covered by another payer per coordination of benefits | Provider + Patient | Update coordination of benefits; file with primary insurer first |
| CO-29 | Timely filing period for submission has ended | Provider | Provider appeals with proof of original timely submission |
| CO-97 | Service is bundled with another service already paid | Provider | Provider appeals documenting that services are separately billable |
Authorization and Pre-Certification
| Code | Plain-Language Meaning | Who Acts | Your Response |
|---|---|---|---|
| CO-15 | The authorization number is missing, invalid, or does not apply to the billed services | Provider | Provider obtains correct authorization number and resubmits |
| CO-197 | Precertification/authorization absent | Provider + Patient | Appeal as emergency/urgent if applicable; provider requests retroactive authorization |
Patient Responsibility Codes
| Code | Plain-Language Meaning | Action |
|---|---|---|
| PR-1 | Deductible amount | You owe this — it applies to your annual deductible |
| PR-2 | Coinsurance amount | You owe your coinsurance percentage of the allowed amount |
| PR-3 | Co-payment amount | You owe your fixed copay for the service |
| PR-96 | Non-covered charge — patient responsibility | Service is not covered; you are responsible if you chose to proceed |
Network and Coverage Denials
| Code | Plain-Language Meaning | Your Response |
|---|---|---|
| CO-96 | Non-covered charge — contractual exclusion | Review plan exclusions; appeal if the exclusion is disputed |
| CO-109 | Claim not covered by this payer / contractor (sent to wrong payer) | Identify correct payer; provider resubmits |
| CO-242 | Services not provided by network/primary care providers | Verify network status; appeal if provider was in-network at time of service |
How to get a plain-language explanation
If you see a code not in this guide, call your insurer's member services line and ask them to explain the code in plain language. Also ask: "What specifically would be needed to reverse this denial?" This question often reveals the simplest path to resolution — sometimes a corrected claim, sometimes a physician letter, sometimes just a phone call between your doctor and the insurer's medical reviewer.
RARC Codes: Adding Detail to the Explanation
Remittance Advice Remark Codes add specificity to CARCs. Common RARCs include:
| Code | Meaning |
|---|---|
| M1 | X-ray not taken within the past 12 months or patient is over age limit |
| M15 | Separately billed services/tests have been bundled as they are related to the same procedure |
| N1 | Alert: you may appeal this decision |
| N130 | Consult our contractual agreement for further information |
| N479 | A facility is requesting payment for services rendered by a provider who was not credentialed by us at the time of service |
What to Do With Denial Codes: Your Action Plan
Once you've decoded the denial code, take these steps:
- Determine who needs to act: Is this a provider billing error (CO codes)? A medical necessity denial (CO-50, CO-151)? Or a legitimate patient responsibility (PR codes)?
- Contact your provider's billing department if the code suggests a billing error — corrected claims often resolve the issue without a formal appeal
- Request your insurer's written criteria if denied for medical necessity — you need to know exactly what clinical criteria you must meet
- File a formal appeal if the denial is disputed — your EOB will specify the appeal deadline and address
For help building your appeal based on your specific denial code, use our free appeal letter generator. To understand the full context of your EOB before appealing, read our EOB guide.
Sources: ANSI X12 Claim Adjustment Reason Code set (Washington Publishing Company) · CMS Remittance Advice Remark Code set · HFMA billing standards. Disclaimer: Code interpretations are general guides. Your specific claim may involve additional context. Last updated: March 2026.