Medical Billing Denial Codes: Complete CARC & RARC Reference Guide (2026)

If you work in medical billing, denial codes are the language you need to speak fluently. Every denied claim comes with a code that tells you exactly what went wrong and what to do about it. This guide covers every common denial code you will encounter, organized by category, with clear explanations and actionable fix instructions.

How Denial Codes Work: CARCs and RARCs Explained

Two coding systems appear on your remittance advice (ERA/EOB). Understanding how they work together is the key to resolving denials quickly.

Claim Adjustment Reason Codes (CARCs) are the primary codes that tell you why a claim was denied or adjusted. Every denial has at least one CARC. They are maintained by the CARC/RARC committee under CMS.

Remittance Advice Remark Codes (RARCs) provide additional detail. They accompany CARCs and give you the specific context you need to fix the problem. A CARC might say "missing information" while the RARC tells you exactly which piece of information is missing.

Group Codes prefix every CARC and tell you who is financially responsible for the adjustment:

• CO (Contractual Obligation) — The provider is responsible. The patient cannot be billed for this amount. This is the most common group code.

• PR (Patient Responsibility) — The patient owes this amount. Includes deductibles, copays, and coinsurance.

• OA (Other Adjustment) — Neither the provider nor the patient is responsible. Often used for coordination of benefits.

• PI (Payer Initiated) — The payer made a reduction that is not the patient's responsibility but may be appealable.

• CR (Correction/Reversal) — Used when the payer is correcting a previous claim payment.

The combination of Group Code + CARC + RARC tells the full story. Always read all three together before deciding how to respond.

Billing and Submission Error Codes

These are the most common and most preventable denial codes. They indicate something was wrong with how the claim was submitted.

CO-4: The Procedure Code Is Inconsistent with the Modifier Used

The modifier you attached does not match the procedure code according to payer editing rules. Common causes include using modifier 25 on a code that does not support it, missing a required modifier like 59 or XE for distinct procedural services, or using bilateral modifier 50 when the payer expects two line items with RT/LT modifiers. Fix: verify the correct modifier for the CPT code using the payer's specific editing rules (not just CCI edits).

CO-16: Claim/Service Lacks Information or Has Submission/Billing Error

The most common and most vague denial code. It covers missing or invalid NPI numbers, incorrect place of service codes, missing required modifiers, date of service formatting errors, invalid diagnosis code combinations, and missing referring provider information. Fix: create a CO-16 checklist. Systematically verify NPI, place of service, modifiers, dates, and diagnosis codes before resubmitting. Practices that implement a pre-submission checklist reduce CO-16 denials by up to 60%.

CO-18: Exact Duplicate Claim/Service

The payer believes this claim was already submitted. But be careful: sometimes the system flags claims as duplicates when they are actually separate services on the same date. Fix: check your practice management system claim tracking first. If submitting a corrected claim, use claim frequency code 7 (replacement) or 8 (void). If these are genuinely separate services on the same date, make sure your modifiers clearly distinguish them (59, XE, XS, XP, XU).

CO-22: This Care May Be Covered by Another Payer per Coordination of Benefits

The payer believes another insurance is primary. This happens when coordination of benefits information is missing, outdated, or incorrect. Fix: verify the patient's COB information is current. If your payer is truly secondary, submit with the primary EOB attached. If your payer should be primary, contact the payer to update their COB records.

CO-29: The Time Limit for Filing Has Expired

Timely filing denial. Every payer has a filing deadline ranging from 90 days to one year from date of service. Fix: if you filed on time, gather proof of timely submission (clearinghouse reports, fax confirmations, certified mail receipts) and submit a timely filing appeal. If you genuinely missed the deadline, check for exceptions such as retroactive eligibility changes, delayed claim processing from a primary payer, or state prompt pay laws that may extend the deadline.

CO-B9: Patient Is Enrolled in a Hospice

The patient is enrolled in hospice and the service you billed is related to the terminal condition. Hospice benefit supersedes most other Part A and Part B coverage. Fix: if the service is unrelated to the hospice diagnosis, resubmit with modifier GW (service not related to hospice patient's terminal condition) and documentation supporting that the service was for a separate condition.

Prior Authorization and Notification Codes

CO-15: The Authorization Number Is Missing, Invalid, or Does Not Apply

The claim requires a prior authorization number and it was either not included or is incorrect. Fix: verify the auth number is in the correct field (usually Box 23 on CMS-1500). Check that the auth has not expired and that the dates of service, CPT codes, and provider on the claim match the authorization exactly.

CO-197: Precertification/Authorization/Notification Absent

Prior authorization was required and not obtained, or notification was not provided within the required timeframe. This is different from CO-15 where an auth number was simply missing from the claim. With CO-197, the auth itself does not exist. Fix: some payers allow retroactive authorization within a limited window (24-72 hours for urgent services). If the auth was actually obtained, resubmit with the correct auth number. If no auth was obtained, you may need to appeal based on medical necessity or emergent circumstances.

CO-204: This Service/Equipment/Drug Is Not Covered Under the Patient's Current Benefit Plan

The specific service is excluded from the patient's plan. This is not a medical necessity denial; the plan simply does not cover this category of service. Fix: verify benefits and check for alternative codes. Some services may be covered under a different benefit category. If the patient has secondary insurance, submit there. If the service should be covered based on the plan documents, file a formal appeal with the plan contract language.

Medical Necessity and Clinical Denial Codes

These codes require clinical review and often need a formal appeal with supporting documentation.

CO-50: These Are Non-Covered Services Because This Is Not Deemed a Medical Necessity

The payer determined that the service was not medically necessary based on the documentation submitted. Important: this does not mean the service was not necessary. It means the documentation did not demonstrate necessity to the reviewer. Fix: obtain the payer's specific medical policy for the procedure. Write an appeal letter that addresses each criterion in the policy, citing peer-reviewed evidence and clinical guidelines. Include all relevant clinical notes, test results, and failed conservative treatment documentation. AI-powered appeal generators like EZAppeal can help structure clinical arguments to match each payer's specific criteria.

CO-51: Non-Covered Services Due to Being a Pre-Existing Condition

Under current ACA rules, pre-existing condition exclusions are prohibited for most commercial plans. However, this code may still appear on grandfathered plans, short-term limited duration plans, or certain employer self-funded plans. Fix: verify whether the plan is subject to ACA pre-existing condition protections. If it is, file an appeal citing the applicable ACA provisions.

CO-55: Procedure/Treatment/Drug Is Deemed Experimental, Investigational, or Unproven

The payer considers the service to be experimental or not yet established as standard of care. Fix: gather evidence that the procedure has become standard of care since the payer's policy was last updated. Submit peer-reviewed studies, FDA approvals, specialty society guidelines, and any applicable state mandates requiring coverage. This often requires a strong clinical appeal.

CO-150: Payer Deems the Information Submitted Does Not Support This Level of Service

Upcoding denial. The payer believes the documentation supports a lower level of service than what was billed. Common with E/M codes (99213 vs 99214) and inpatient admission levels. Fix: review the documentation against the payer's specific requirements for the billed level. If the documentation supports the code, appeal with highlighted sections showing the key elements. If the documentation is genuinely insufficient, work with the provider to improve documentation going forward.

CO-151: Payment Adjusted Because the Payer Deems the Information Submitted Does Not Support This Many/Frequency of Services

The payer believes the service was performed too frequently. Many payers have published frequency limits for common services such as chiropractic adjustments, physical therapy visits, and lab panels. Fix: check the payer's frequency limitations for the specific service. If your utilization is within limits, appeal with documentation. If you are exceeding published limits, ensure documentation clearly supports medical necessity for the additional frequency.

Payment and Fee Schedule Codes

CO-45: Charge Exceeds Fee Schedule/Maximum Allowable

Your billed amount exceeds the payer's fee schedule. But this code can also indicate that you are billing the wrong code for the service, the payer has incorrect provider information on file, or there is a contract rate discrepancy. Fix: do not automatically lower your fees. First verify you are using the correct CPT code. Then check that the payer has your correct provider type and specialty on file, as different specialties have different fee schedules. Finally, compare against your contract rates.

CO-59: Processed Based on Multiple or Concurrent Procedure Rules

The payer applied a multiple procedure reduction, typically paying the second and subsequent procedures at 50% of the fee schedule. Fix: verify that the reduction is being applied correctly per your contract. Check that the highest-paying procedure is being paid at 100%. If modifier 59 or an X modifier should prevent bundling, make sure it is present.

CO-97: The Benefit for This Service Is Included in the Payment/Allowance for Another Service/Procedure

Bundling denial. The payer considers this service to be part of another service that was already paid on the same claim or a related claim. Fix: check CCI (Correct Coding Initiative) edits to verify if the codes are truly bundled. If the services are truly separate and distinct, add modifier 59 or the appropriate X modifier (XE, XS, XP, XU) with documentation supporting that the services were performed independently.

PR-1: Deductible Amount

The amount applied to the patient's deductible. This is not a denial but an adjustment. Fix: bill the patient for the deductible amount. If you believe the deductible was applied incorrectly, verify the patient's benefits and accumulated deductible for the plan year.

PR-2: Coinsurance Amount

The patient's coinsurance responsibility. Like PR-1, this is not a denial. Fix: bill the patient for the coinsurance amount per your contract terms.

PR-3: Co-Payment Amount

The patient's copay. Fix: collect at time of service when possible. Bill the patient for any uncollected copay amounts.

Eligibility and Enrollment Codes

CO-27: Expenses Incurred After Coverage Terminated

The patient's coverage was not active on the date of service. Fix: verify the exact coverage dates with the payer. If the patient had coverage, submit proof of eligibility. If coverage had truly lapsed, check for retroactive eligibility or bill the patient directly. Always verify eligibility before rendering services.

CO-109: Claim/Service Not Covered by This Payer/Contractor

The claim was sent to the wrong payer entirely. Fix: verify the patient's current insurance information and resubmit to the correct payer. This commonly happens when patients change insurance plans and the old information is still on file.

CO-B7: This Provider Was Not Certified/Eligible to Be Paid for This Procedure/Service on This Date of Service

The provider was not credentialed or enrolled with the payer on the date of service. Fix: verify your provider enrollment status. If enrollment was pending, check if the payer allows retroactive claims once enrollment is complete. If the provider is not in network, bill as out-of-network or have the patient assign benefits.

Commonly Confused RARC Codes

N1-N999 (Alert Codes)

N-series RARCs are informational alerts. They do not change payment but provide additional context. Common examples: N30 (missing or incomplete service facility name or address), N56 (procedure code billed is not correct), N479 (missing or invalid data — check the specific data element referenced).

MA01-MA130 (Medicare Alert Codes)

Medicare-specific informational codes. MA01 indicates an appeal was received but documentation is pending. MA130 tells you the claim was submitted to the DME MAC when it should have gone to the carrier or vice versa.

Building a Denial Management Strategy

Understanding individual codes is essential, but the real impact comes from tracking patterns. Here is a proven approach to reducing denials systematically:

Track your top 5 denial codes by volume each month. Most practices find that 80% of their denials come from just 5-10 codes. Fix those and your overall denial rate drops dramatically.

Create payer-specific playbooks. The same denial code can mean different things with different payers, and the fix is often payer-specific. Keep a reference sheet for your top 5 payers.

Implement pre-submission checks. Automated claim scrubbing catches most billing and submission errors (CO-4, CO-16, CO-18) before they become denials.

Appeal every medical necessity denial. Studies show that 50-70% of medical necessity denials are overturned on appeal when properly documented. AI-powered tools like EZAppeal can generate payer-specific appeal letters in 60 seconds, citing each payer's own medical policy criteria and peer-reviewed clinical evidence.

Measure your appeal success rate. Track which codes and which payers have the highest overturn rates. Focus your appeal efforts where the ROI is highest.

Every denied claim represents revenue that belongs to you. Understanding these codes is not just about compliance; it is about getting paid for the care you provide.

Detailed Denial Code Guides

Need a deeper dive into a specific denial code? These guides include step-by-step fix instructions, appeal letter tips, and real-world examples:

• CO 50 Denial Code: What It Means & How to Fix It — Medical necessity denials


• CO 11 Denial Code: What It Means & How to Fix It — Diagnosis inconsistent with procedure


• CO 16 Denial Code: What It Means & How to Fix It — Missing information for adjudication


• CO 97 Denial Code: What It Means & How to Fix It — Bundling and NCCI edit denials


• MA01 Remark Code: What It Means & How to Fix It — Medicare coordination of benefits


• N479 Remark Code: What It Means & How to Respond — Missing clinical documentation

Payer-Specific Appeal Guides

When a denial code leads to a formal appeal, these payer-specific guides walk you through the process:

• UHC Denied Your Lumbar MRI? Appeal Guide


• BCBS Denied Ambulance Transport? Appeal Guide


• Humana Denied Skilled Nursing? Appeal Guide


• Aetna Denied Knee Arthroscopy? Appeal Guide


• ER Claim Denied? How to Appeal


• External Review: Your Last Resort