Why Claims Get Denied: The Top 5 Reasons
Before you can appeal effectively, you need to understand why the denial happened. Payers communicate denial reasons through Claim Adjustment Reason Codes (CARC codes) on the Explanation of Benefits (EOB) or Electronic Remittance Advice (ERA). See the full CARC denial code reference for detailed breakdowns of each code.
The five most common reasons claims get denied — accounting for roughly 50–60% of all denials — are:
| CARC Code | Denial Reason | Share of Denials | Appeal Win Rate |
|---|---|---|---|
| CO-16 | Missing or incomplete information — required field blank or incorrect | 15–20% | 70–75% |
| CO-18 | Duplicate claim — payer believes claim was submitted twice | 10–12% | 90%+ |
| CO-11 | Diagnosis inconsistent with procedure — ICD-10 doesn't support CPT billed | 8–10% | 65–70% |
| CO-4 | Missing modifier — required CPT modifier not included | 6–8% | 75–80% |
| CO-9 | No prior authorization — PA required but not obtained | 4–5% | 65–75% |
Notice what these have in common: all five are highly appealable. CO-16, CO-18, and CO-4 are administrative errors that can usually be corrected and resubmitted within days. CO-11 and CO-9 require clinical documentation but have strong win rates when that documentation is properly assembled.
The strategy for appealing CO-16 (billing error) is completely different from CO-9 (missing prior auth) or CO-5 (not covered). Don't write a generic appeal letter — identify the exact CARC code on the EOB and tailor your appeal to that specific denial reason. Generic appeals fail more often.
The Appeal Process Explained
The medical claim appeal process has two main levels for commercial payers, and up to five levels for Medicare. Understanding the hierarchy matters because you must exhaust lower-level appeals before escalating.
Commercial Payer Appeal Levels
- Internal Appeal (Level 1): Formal request to the payer to reconsider the denial. This is the standard written appeal most billing teams file. Decision timeline: 30–60 days for routine, 72 hours for urgent/expedited.
- Peer-to-Peer Review: Optional but powerful — the treating physician calls the payer's medical reviewer directly. Not technically a formal appeal level, but often resolves medical necessity denials faster than written appeals. Available before or during the formal appeal process with most payers.
- External Independent Review: If the internal appeal fails, patients have the right to request review by an independent organization under the ACA (for non-grandfathered plans). External reviewers overturn 40–50% of coverage denials. The payer must comply with the external reviewer's decision.
- State Insurance Department Complaint: Final option. File a formal complaint with your state's insurance regulator. Low success rate for specific claims but creates regulatory pressure, especially for pattern issues.
Medicare Appeal Levels
- Redetermination: Request to the Medicare Administrative Contractor (MAC). File within 120 days. Decision in 60 days.
- Reconsideration: Request to a Qualified Independent Contractor (QIC). Decision in 60 days.
- Office of Medicare Hearings and Appeals (OMHA): ALJ hearing. Used for claims over $180 (2026 threshold).
- Medicare Appeals Council: Administrative review of ALJ decisions.
- Federal District Court: Judicial review. Reserved for claims over $1,870 (2026 threshold).
Successful appeals typically involve: a billing specialist who identifies the denial reason and gathers documentation; a physician or clinical staff member who writes the medical necessity letter if needed; and an appeal coordinator who tracks deadlines and follows up. For high-volume practices, assigning a dedicated denial manager measurably improves recovery rates.
The 10-Step Appeal Walkthrough: From Denial Notice to Resolution
This is the exact workflow to follow from the moment a denial arrives through to resolution. Move through these steps quickly — every day you wait is a day off your appeal deadline.
Identify the denial immediately — flag it by deadline
When you receive a denial EOB or ERA, record the denial date and the payer name immediately. Calculate your appeal deadline from that date (65 days for UHC, 120 days for Medicare, 180 days for most commercial). Add the deadline to your tracking system that day. Don't wait.
Read the denial — find the CARC code and reason
Locate the Claim Adjustment Reason Code (CARC) on the EOB. Also look for a Remittance Advice Remark Code (RARC) — this often specifies the exact field or requirement the payer is flagging. Don't assume you know why it was denied; confirm it from the denial document. See the full CARC reference guide for every code.
Decide: appeal or resubmit a corrected claim?
For billing errors (CO-16, CO-4, CO-18), a corrected claim resubmission is often faster than a formal appeal. Call the payer's provider line to confirm their preferred method — some payers process corrected claims in 2 weeks vs. 60 days for formal appeals. For medical necessity denials (CO-9, CO-5, CO-11), a formal appeal with clinical documentation is required.
Pull the claim and patient chart
Gather the original claim form, the denial EOB, the patient's insurance card from the date of service, and the medical chart notes for the visit. You need to understand exactly what was billed, what the payer denied, and what the clinical documentation shows. Don't write the appeal letter until you've reviewed all three.
Identify the specific gap the denial is citing
The appeal will fail if you don't address the exact reason for denial. CO-16 requires identifying which field was missing. CO-11 requires identifying which diagnosis/procedure pairing the payer flagged. CO-9 requires knowing which authorization was missing. Get specific — "the claim was denied" is not a basis for appeal. "The claim was denied due to missing referring provider NPI in field 17a, which is now corrected" is.
Gather your documentation package
Assemble everything before writing the appeal letter. For billing errors: corrected claim, original denial EOB, and documentation proving the correction. For medical necessity: physician chart notes, a letter of medical necessity from the treating physician, relevant clinical guidelines (payer LCD/NCD), and any prior authorization records. See the full documentation checklist below.
Consider a peer-to-peer review first (for medical necessity denials)
If the denial is for medical necessity (CO-5, CO-9, or a payer medical policy denial), request a peer-to-peer review before submitting a written appeal. The treating physician calls the payer's medical reviewer and explains the clinical rationale directly. P2P reviews with Cigna and Aetna often resolve the denial on the same call. This is faster and more effective than written appeals alone.
Write a precise, evidence-based appeal letter
Your appeal letter should: (1) state the claim information (patient name, date of service, claim number, CARC code), (2) clearly state why the denial is incorrect with specific reference to the payer's own guidelines or the corrected documentation, (3) list every supporting document attached, and (4) request a specific outcome (reversal/payment). Keep it under 2 pages — reviewers read dozens of appeals. Clarity beats length.
Submit via the payer's required method and confirm receipt
Most payers accept appeals via their provider portal, fax, or mail. Portal submission is safest — it generates a confirmation timestamp. If faxing, use a cover page with a fax confirmation request. If mailing, use certified mail with return receipt. Always get a tracking or confirmation number. Follow up 5 business days after submission to confirm receipt — some payers "lose" faxes and your clock is still ticking.
Track and follow up — escalate if the deadline passes
Most commercial payers must respond within 30–60 days. Set a follow-up reminder at 30 days. If you haven't received a decision at 45 days, call the appeals department. If the appeal is denied, review the denial reason and decide whether to escalate to external review or file a state insurance department complaint. Document every communication with the payer — dates, names, reference numbers.
Documentation Checklist: What to Gather Before Filing
Missing documentation is the most common reason appeals fail after they're filed. Don't submit until you have everything. Here's what you need by denial category:
For Billing Error Denials (CO-16, CO-4, CO-18, CO-11)
- Original claim (CMS-1500 or 837P) with the specific error identified
- Corrected claim with the fix applied (field corrected, modifier added, code updated)
- Original denial EOB / ERA showing the CARC and RARC codes
- Supporting document for the correction (patient insurance card, NPI registry confirmation, clearinghouse submission log)
- Brief cover letter explaining exactly what was wrong and what was corrected
For Medical Necessity Denials (CO-5, CO-9, CO-11 with clinical dispute)
- Physician's detailed letter of medical necessity (treating physician, on letterhead, signed)
- Relevant chart notes from the visit documenting the condition and clinical decision-making
- Applicable payer LCD/NCD (Local/National Coverage Determination) supporting the service
- Relevant clinical guidelines from medical societies (AMA, specialty boards)
- Any peer-reviewed literature supporting the treatment approach (for experimental/investigational disputes)
- Documentation of prior treatment attempts and clinical rationale for escalation
- Prior authorization records if PA was obtained (for CO-9 appeals)
For Coverage/Eligibility Denials (CO-27, CO-22)
- Real-time eligibility verification from the payer on the date of service
- Copy of patient's insurance card collected at the visit
- Signed patient intake form with insurance information
- COB determination documents (for CO-22 coordination of benefits denials)
- Primary payer's EOB showing their payment or denial (if secondary payer appeal)
At minimum, every appeal must include (1) the original denial EOB with the CARC code clearly visible, and (2) a cover letter that explicitly states why the denial is incorrect. Appeals received without these are typically returned without review, burning deadline days.
Payer-Specific Appeal Deadlines
Appeal deadlines are calculated from the denial date — the date on the EOB, not the date you received it. Know your deadline for every payer you bill. Missing it is permanent; there is no late-filing exception at most payers.
A UHC denial received on April 1st expires June 5th — less than 10 weeks. If your team reviews denials monthly, you may have already lost 30 days before anyone looks at the claim. Build a process that flags UHC denials for review within 48 hours of receipt. Every other payer gives you more time, but UHC cannot be treated like the others.
For a complete breakdown of specific CARC codes and their payer-specific appeal requirements, see the full denial code reference guide, which covers CO-16, CO-18, CO-4, CO-11, CO-9, CO-22, CO-27, and more — with payer-specific deadlines and step-by-step appeal guidance for each.
6 Common Mistakes That Kill Appeals
These are the patterns that consistently derail otherwise valid appeals. They're all avoidable.
1. Filing After the Deadline
Once the appeal window closes, the claim is permanently lost. There is no cure for a missed deadline. This is the single most preventable cause of unrecovered revenue — especially for UHC claims with the 65-day window. Build deadline tracking into day one of receiving a denial.
2. Writing a Generic Appeal Letter
Sending the same template letter for every denial is the fastest way to lose. Appeals must directly address the specific CARC code denial reason. A CO-16 appeal needs to name the exact field that was missing and show it's been corrected. A CO-9 appeal needs to address the specific authorization requirement. Generic letters are dismissed or denied without detailed review.
3. Appealing Non-Appealable Codes
CO-96 (charges exceed fee schedule) and CO-97 (bundled service) have 5–25% success rates because they're contractual issues, not billing errors. Spending two hours on a CO-96 appeal instead of a CO-18 appeal (90%+ win rate) is a net-negative use of staff time. Know which codes are worth appealing before you start. See the denial code reference for success rates by code.
4. Not Confirming Appeal Receipt
Faxed appeals regularly don't arrive. Portal submissions get lost in routing. If you don't confirm receipt within 5 business days of submission, you may be waiting 60 days only to discover the appeal was never logged — and your deadline has now passed. Always get a confirmation number or reference ID.
5. Missing the Physician Letter for Medical Necessity Denials
For CO-5, CO-9, and any medical necessity denial, an appeal without a physician letter has dramatically lower success rates. The payer's medical reviewer needs clinical justification from the treating physician — not the billing office. The letter needs to be specific to this patient, this service, and why the clinical guidelines support it. A one-sentence note does not count.
6. Treating All Payers the Same
UHC has a 65-day window. Medicare has a 5-level appeals process. Cigna prefers peer-to-peer over written appeals. Aetna requires specific documentation for medical necessity claims. Using the same process for every payer results in missed deadlines, wrong submission methods, and appeals sent to the wrong department. Know each payer's specific requirements before you submit.
Run a Free Denial Audit First
Before you start appealing individual claims, you need visibility into which of your current denied claims are worth the effort. Not all denials are created equal — and the most valuable use of your billing team's time is focusing on high-recovery codes with near-term deadlines.
A denial audit tells you:
- Which CARC codes account for the largest share of your denied revenue
- Which claims are approaching their appeal deadline and need immediate attention
- Your estimated recoverable revenue if you appealed the winnable claims
- Which denials are contractual adjustments (CO-96, CO-97) and not worth appealing
Your practice management system can export a denied claims CSV with claim ID, CARC code, payer, amount, and denial date. That's all you need to get a complete picture.
Upload your denied claims CSV at vigil-ai-2.polsia.app and see your recoverable revenue breakdown in 60 seconds. The audit identifies your highest-priority claims by code and deadline — no account required, data is never stored.
Denial management is not a quarterly project. It's a weekly discipline. The practices that consistently run below a 5% denial rate have systems that identify recoverable claims before deadlines expire — not processes that catch problems three months after the fact.
The 60% overturn rate for appealed claims is not an accident of luck. It's the result of filing on time, documenting correctly, and addressing the specific denial reason the payer cited. The barrier isn't knowledge — it's bandwidth and prioritization. Knowing which claims to work first makes all the difference.