The Most Common Medicare and Commercial Claim Denial Codes (and How to Appeal Each One)

Denial Rates by the Numbers

According to CMS data from 2024, the average claim denial rate across commercial and Medicare payers sits at 10.5%. That means roughly 1 in 10 claims your practice submits comes back rejected or denied. For practices processing 3,000 claims per month, that’s 315 denials requiring staff time, appeal effort, and cash flow delays.

The American Medical Association’s 2023 Prior Authorization Survey found that 34% of physicians reported a prior auth denial led to a serious adverse event for their patient. But there’s a financial reality too: the Medical Group Management Association (MGMA) estimates the average cost to rework a denied claim ranges from $25 to $118 per claim, depending on complexity.

Here’s what catches most billing teams off guard. A KFF analysis of Affordable Care Act marketplace claims showed that only 0.2% of denied claims were appealed. That’s not a typo. Fewer than 1 in 500 denials gets challenged, even though appeal success rates range from 40% to 65% depending on the payer and denial type.

From what we see across our placements, the practices that track denial codes systematically and appeal strategically recover 15% to 25% more revenue than those that write off denials as a cost of doing business. This guide is built to be your desk reference for exactly that.

How CARC and RARC Codes Work Together

Every denial comes with two code types, and understanding both is critical to crafting an effective appeal.

CARC (Claim Adjustment Reason Codes) tell you why the payer adjusted the claim. These are standardized codes maintained by the X12 organization and used by all payers. When you see “CO-16” or “PR-1” on a remittance advice, that’s a CARC.

RARC (Remittance Advice Remark Codes) provide additional detail. They explain the CARC in context. For example, a CARC of CO-16 (“claim lacks information”) might be paired with RARC N382 (“missing/incomplete/invalid patient identifier”). The RARC tells you exactly what’s missing.

The prefix matters too:

• CO (Contractual Obligation): The adjustment is the provider’s responsibility based on the contract. Neither the provider nor the patient should be billed for the difference.
• PR (Patient Responsibility): The patient owes the adjusted amount (deductible, copay, coinsurance).
• OA (Other Adjustment): Catch-all for adjustments that don’t fall into CO or PR.
• CR (Correction and Reversal): Used when a previous claim line is being corrected.

When you read a denial, always check both codes. The CARC gives you the category; the RARC gives you the fix.

The 10 Most Common Denial Codes

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

This denial fires when the CPT/HCPCS code and its modifier don’t match according to the payer’s editing logic. National Correct Coding Initiative (NCCI) edits are the most frequent trigger.

Why it happens: A modifier 25 appended to an E/M code when no separate, identifiable service was documented. A modifier 59 used where the payer expected an X-modifier (XE, XS, XP, or XU). Or a bilateral modifier (50) on a procedure the payer considers inherently bilateral.

How to appeal:

1. Pull the operative note or visit documentation.
2. Verify the modifier against NCCI edits at cms.gov/medicare-coding.
3. If the modifier is correct, write a letter citing the specific clinical documentation that supports the modifier’s use.
4. If the modifier was wrong, correct it and resubmit (this is a corrected claim, not technically an appeal).

Documentation needed: Operative report, medical record supporting the distinct procedure or anatomical site, NCCI edit reference showing the modifier is valid for the code pair.

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

CO-16 is the most common denial code in the industry. It’s a broad category that means something is missing or incorrect on the claim form. The paired RARC code is your actual diagnostic tool here.

Why it happens: Missing subscriber ID, incorrect date of birth, invalid NPI, missing referring provider information, wrong place of service code, incomplete diagnosis codes. Any data field that doesn’t match the payer’s records can trigger CO-16.

How to appeal:

1. Read the RARC code first. Common pairings include MA130 (“your claim contains incomplete and/or invalid information”), N382 (missing patient identifier), and N362 (missing/incomplete service date).
2. Compare your claim data against the patient’s insurance card and eligibility response.
3. Fix the specific field identified by the RARC.
4. Resubmit as a corrected claim using the appropriate frequency code (bill type “XX7” for institutional, or resubmission code on the 837P).

Documentation needed: Copy of insurance card (front and back), eligibility verification printout, corrected claim form with the fix highlighted.

CO-18: Exact Duplicate Claim/Service

The payer’s system flagged your submission as a duplicate of a previously processed claim. Same patient, same provider, same date of service, same procedure code.

Why it happens: Staff resubmitted a claim that was already paid or pending. The clearinghouse sent a duplicate transmission. Or a corrected claim was submitted without the proper resubmission indicators, so the system treated it as a new (duplicate) claim.

How to appeal:

1. Check your payment records. Was the original claim actually paid? If yes, no appeal is needed.
2. If the original was denied and you’re resubmitting, use frequency code 7 (replacement) or 8 (void) with the original claim number in the appropriate field.
3. If these are genuinely separate services on the same date (e.g., bilateral procedures), append modifier 59 or an X-modifier and resubmit with documentation showing distinct services.

Documentation needed: Original claim number and status, proof that services were distinct (if applicable), corrected claim with proper resubmission coding.

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

The payer believes another insurance plan is primary and should pay first. This is a coordination of benefits (COB) issue.

Why it happens: The patient has dual coverage and the payer order is wrong in the system. A spouse’s plan should be primary per birthday rule. Medicare Secondary Payer (MSP) rules apply and the commercial plan should pay first. Or the payer’s COB records are outdated.

How to appeal:

1. Verify the patient’s coverage order. Ask the patient directly about other insurance.
2. If your claim was sent to the correct primary payer, submit proof: the other payer’s EOB showing they’ve already paid or denied, or a letter from the other insurer confirming your payer is primary.
3. If the other plan IS primary, submit there first. Then submit to this payer as secondary with the primary payer’s EOB attached.

Documentation needed: Primary payer’s EOB or denial letter, patient’s insurance verification showing coverage order, updated COB information from the patient.

CO-29: The Time Limit for Filing Has Expired

The payer received the claim after their filing deadline. Medicare allows 12 months from the date of service. Commercial plans range from 90 days to 365 days, depending on the contract.

Why it happens: Delayed charge entry. Waiting for a prior claim to process before submitting a related one. Clearinghouse transmission failures that went unnoticed. Staff turnover with no one monitoring the aging report.

How to appeal:

1. Check your clearinghouse logs for proof of timely submission. A transmission report showing the claim was accepted before the deadline is your strongest evidence.
2. If you can prove timely filing via electronic records, submit the appeal with the clearinghouse acceptance report, including date/time stamps and transaction IDs.
3. If the delay was caused by the payer (e.g., they took 90 days to process a prior auth that delayed the service), document that timeline.
4. Some payers allow “good cause” exceptions. Check the provider manual for your specific payer.

Documentation needed: Clearinghouse transmission reports with timestamps, payer acknowledgment receipts, any correspondence showing payer-caused delays, proof of original submission date.

CO-29 has the lowest appeal success rate of any code on this list. Prevention is everything here. Run a weekly report of claims older than 60 days that haven’t been acknowledged.

CO-45: Charges Exceed Your Contracted/Legislated Fee Schedule Amount

This code appears on almost every remittance advice, and in most cases it’s not a true denial. It’s an adjustment showing the difference between your billed charges and the payer’s allowed amount per your contract.

Why it happens: Your charge master rates are higher than the payer’s fee schedule (this is normal and expected). The payer applied the wrong fee schedule. Or the payer paid at a lower rate than your contract specifies.

How to appeal:

1. Compare the allowed amount on the EOB against your contracted rate for that specific CPT code.
2. If the payment matches your contract, CO-45 is informational. No appeal needed. Do not bill the patient for this difference if you’re in-network.
3. If the payment is below your contracted rate, submit a formal appeal with your fee schedule and contract attached. Reference the specific contract section and effective date.

Documentation needed: Copy of payer contract with fee schedule, EOB showing the underpayment, rate comparison spreadsheet showing expected vs. actual payment.

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

The payer determined the service wasn’t medically necessary based on the diagnosis codes submitted, the patient’s condition, or their coverage criteria. After CO-16, this is the denial that generates the most revenue loss.

Why it happens: The diagnosis code doesn’t support the procedure per the payer’s LCD (Local Coverage Determination) or NCD (National Coverage Determination). The documentation doesn’t establish medical necessity. The service exceeds frequency limits. Or the payer requires a specific diagnosis code that wasn’t included.

How to appeal:

1. Review the applicable LCD/NCD on the CMS website (for Medicare) or the payer’s medical policy database (for commercial plans).
2. Check whether a different, more specific ICD-10 code better supports the service.
3. Request a peer-to-peer review with the payer’s medical director. This is your right under most contracts and under Medicare rules.
4. Submit a detailed appeal letter including the clinical rationale, supporting medical records, and references to published clinical guidelines (AMA, specialty society guidelines).

Documentation needed: Full medical record for the date of service, applicable LCD/NCD printout, letter of medical necessity from the treating physician, peer-reviewed literature supporting the treatment if applicable.

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

The payer bundled your service into another service on the same claim or a prior claim. NCCI edits and the payer’s own bundling rules drive this one.

Why it happens: Two procedures were billed that NCCI considers part of the same service. The payer has internal bundling edits beyond NCCI. A global surgical period includes the denied service. Or a “column 1/column 2” edit applies and no unbundling modifier was used.

How to appeal:

1. Check NCCI edits at the CMS NCCI lookup tool. Determine if the code pair has a “1” indicator (modifier allowed) or “0” (not allowed).
2. If the indicator is 1, resubmit with the appropriate modifier (59, XE, XS, XP, or XU) and documentation showing the services were distinct.
3. If the indicator is 0, the services cannot be unbundled. Review whether the higher-paying code was the one that was paid.
4. For payer-specific bundling rules beyond NCCI, reference your contract and the payer’s published policy.

Documentation needed: NCCI edit lookup results, operative note or procedure documentation showing distinct services, modifier justification letter.

PR-1: Deductible Amount

The patient hasn’t met their annual deductible. The adjusted amount is the patient’s responsibility. This isn’t a denial in the traditional sense; it’s a patient responsibility transfer.

Why it happens: The service was rendered before the patient met their deductible. The payer applied the allowed amount to the deductible correctly. Or (less commonly) the payer’s deductible tracking is wrong.

How to appeal:

1. Verify the patient’s deductible status through the payer portal or a real-time eligibility check.
2. If the deductible was already met (you have an EOB from another claim showing the deductible satisfied before this date of service), submit that EOB as proof.
3. If the deductible application is correct, this is a collection issue, not an appeal. Bill the patient.

Documentation needed: Real-time eligibility printout, EOBs from other claims showing deductible accumulation, payment records from the patient if a dispute arises.

From what we see across our placements, PR-1 accounts for the largest dollar volume of all adjustments. The appeal rate is low because most PR-1 adjustments are correct. But verifying deductible status before the visit reduces patient surprise bills and collection headaches significantly.

CO-197: Precertification/Authorization/Notification/Pre-Treatment Absent

The payer required prior authorization for this service, and either no auth was obtained, the auth expired, or the auth didn’t match the service performed.

Why it happens: Prior auth wasn’t obtained before the service. The auth was for a different CPT code than what was billed. The auth expired before the date of service. The number of authorized units was exceeded. Or the auth was obtained but the reference number wasn’t included on the claim.

How to appeal:

1. If auth was obtained, locate the authorization number and resubmit the claim with it included in the correct field (Box 23 on the CMS-1500, or the REF segment on the 837P).
2. If auth was not obtained, check whether the payer allows retroactive authorization. Many commercial plans allow retro auth within 24 to 72 hours for urgent/emergent services.
3. If the auth was for a different code, request a modification from the payer’s utilization management department.
4. For expired authorizations, contact the auth department and request an extension with clinical documentation.

Documentation needed: Copy of the authorization letter or reference number, clinical documentation supporting medical necessity, proof of emergent circumstances if requesting retro auth, updated clinical notes if requesting auth modification.

Denial Code Quick Reference Table

Code	Description	Primary Cause	Appeal Approach	Typical Filing Limit
CO-4	Inconsistent modifier	Wrong modifier for CPT code per NCCI edits	Correct modifier or submit documentation supporting use	60-180 days from denial
CO-16	Claim lacks information	Missing or invalid data field on claim	Identify missing field via RARC, correct, resubmit	60-180 days from denial
CO-18	Duplicate claim	Same claim submitted twice without resubmission code	Verify original status; resubmit with frequency code 7	60-180 days from denial
CO-22	Coordination of benefits	Wrong payer order; other insurance is primary	Submit to correct primary payer; attach primary EOB	60-365 days from denial
CO-29	Timely filing expired	Claim received after payer’s filing deadline	Submit clearinghouse proof of timely submission	Limited; varies by payer
CO-45	Charges exceed fee schedule	Billed amount above contracted rate (usually informational)	Appeal only if paid below contract; attach fee schedule	60-180 days from denial
CO-50	Not medically necessary	Diagnosis doesn’t support procedure per LCD/NCD	Peer-to-peer review; submit clinical documentation	60-180 days from denial
CO-97	Bundled with another service	NCCI or payer-specific bundling edit	Add unbundling modifier if allowed; document distinct services	60-180 days from denial
PR-1	Deductible amount	Patient hasn’t met annual deductible	Verify deductible status; bill patient if correct	N/A (patient responsibility)
CO-197	Precertification absent	Prior auth missing, expired, or mismatched	Locate auth number and resubmit; request retro auth if applicable	60-180 days from denial

How to Build a Denial Tracking System

Tracking denials without a system is like reading lab results without reference ranges. You see numbers but can’t act on them. Here’s a step-by-step approach that works whether you’re using a spreadsheet or a full practice management system.

Step 1: Capture every denial the day it arrives. Set up an automated feed from your clearinghouse or practice management system that flags every non-zero adjustment. Don’t wait for the paper EOB. Electronic remittance advice (ERA/835) files contain CARC and RARC codes that can be parsed automatically.

Step 2: Categorize by CARC code. Group denials into buckets: eligibility issues (CO-22, PR-1), coding issues (CO-4, CO-97), missing information (CO-16), medical necessity (CO-50), authorization (CO-197), timely filing (CO-29), and duplicates (CO-18). This tells you where your process is breaking down.

Step 3: Calculate denial rate by category, payer, and provider. Your overall denial rate is less useful than knowing that Dr. Smith’s CO-50 denial rate with UnitedHealthcare is 18% while every other provider is at 6%. That specificity points to a documentation problem with one physician and one payer’s LCD criteria.

Step 4: Track appeal outcomes. For every appeal submitted, record the date sent, method (phone, fax, portal), turnaround time, and result (overturned, upheld, partial). After 90 days of data, you’ll know which denial types are worth appealing and which are better addressed through prevention.

Step 5: Run a monthly denial review meeting. Fifteen minutes, once a month. Review the top 5 denial codes by volume and dollar amount. Assign corrective actions. Track whether last month’s actions reduced this month’s denial rate. This meeting alone can cut denial rates by 20% to 30% within two quarters.

So what does a good tracking system actually measure? At minimum: denial rate by payer, denial rate by CARC code, average days to appeal, appeal overturn rate, and net revenue recovered from appeals.

When to Escalate vs. Write Off

Not every denial is worth fighting. But too many practices write off claims that should be appealed, and too few escalate when first-level appeals fail.

Appeal when:

• The claim value exceeds $75 (your breakeven point for staff time on most appeals)
• You have clear documentation supporting your position
• The denial was caused by a payer error or system glitch
• The CARC code has a historical overturn rate above 30% at your practice

Escalate to a second-level appeal when:

• The first-level appeal was denied without addressing your evidence
• The claim value exceeds $500
• The denial affects multiple claims with the same issue (a pattern denial)
• You’ve identified a contract interpretation dispute

Consider writing off when:

• The claim is under $25 and the denial reason is valid
• You missed the timely filing window and have no proof of earlier submission
• The service truly isn’t covered under the patient’s plan
• Three levels of appeal have been exhausted

For Medicare claims, remember that you have the right to escalate through five levels: redetermination, reconsideration by a Qualified Independent Contractor (QIC), Administrative Law Judge (ALJ) hearing, Medicare Appeals Council review, and federal district court. The ALJ level has the highest overturn rate at approximately 70%, according to HHS Office of Medicare Hearings and Appeals data. But the backlog means you may wait 12 to 18 months to get there.

And here’s a number that should inform your strategy: practices that appeal at least 60% of clinical denials (CO-50, CO-197) recover an average of $12,000 to $48,000 per physician per year, based on MGMA benchmarking data. That’s money already earned for services already rendered.

Frequently Asked Questions

What is the difference between a rejected claim and a denied claim?

A rejected claim never entered the payer’s adjudication system. It was kicked back by the clearinghouse or the payer’s front-end edits because of a formatting error, invalid code, or missing required field. A denied claim was received, processed, and the payer made a payment decision (which was to not pay). Rejections can be corrected and resubmitted immediately. Denials require a formal appeal with documentation.

How long do I have to appeal a denied claim?

It varies by payer. Medicare allows 120 days from the date on the Medicare Redetermination Notice (MRN) for a first-level appeal. Commercial payers set their own timelines, with 60 to 180 days being the most common range. Always check your provider contract for the specific deadline. Some states have laws requiring minimum appeal windows, so state insurance regulations may override a payer’s shorter timeline.

Can I appeal a CO-45 adjustment?

In most cases, CO-45 is not a denial. It’s the difference between your billed charges and the payer’s allowed amount per your contract. If you’re in-network, you agreed to accept the allowed amount and cannot bill the patient for the difference (balance billing). However, if the allowed amount is lower than what your contract specifies, that’s an underpayment, and you should absolutely appeal. Pull your contract’s fee schedule and compare it line by line against the payment.

What should I do if the same denial code keeps appearing across multiple claims?

A recurring denial code points to a systemic issue. If CO-16 appears on 15% of claims to a specific payer, you likely have a data mapping problem in your practice management system. If CO-50 spikes for a particular CPT code, your documentation may not meet that payer’s LCD criteria. Pull a report of all claims with that CARC code for the past 90 days. Group them by payer, provider, and procedure code. The pattern will tell you where to focus your fix. One corrective action upstream (a template change, a training session, a workflow update) can prevent hundreds of future denials.

Is it worth hiring a denial management specialist?

The math is straightforward. A dedicated denial management specialist working a denial volume of 300+ per month can recover $150,000 to $400,000 annually for a mid-size practice, based on MGMA and HFMA benchmarking data. If the specialist’s fully loaded cost (salary plus benefits plus overhead) is $55,000 to $75,000, the ROI is 2:1 to 7:1. For practices with fewer than 150 denials per month, a part-time or remote billing specialist focused on denials makes more financial sense than a full-time hire. In our placements, we’ve seen practices cut their denial write-off rate by 40% to 60% within the first six months of dedicated denial management staffing.