How to Appeal Denied Insurance Claims: A Step-by-Step Guide

Published April 25, 2026 · 9 min read · By RCMAXIS Revenue Cycle Team

Claim denials are one of the most damaging — and preventable — threats to a medical practice's cash flow. According to the American Medical Association, practices write off billions in recoverable revenue every year simply because they lack the process to appeal effectively. Yet industry data consistently shows that 63% of denied claims are never appealed, even when the denial was wrong.

This guide walks through everything your practice needs: the most common denial reasons, how to distinguish soft from hard denials, the full appeals process from internal to external review, and the critical deadlines you cannot afford to miss.

Top 10 Reasons Insurance Claims Are Denied

Understanding why claims are denied is the foundation of any effective appeal strategy. The most frequently cited denial reasons across commercial, Medicare, and Medicaid payers are:

1. Missing or invalid prior authorization — service rendered without required pre-approval, or authorization number not included on the claim
2. Medical necessity not established — documentation does not support the diagnosis or level of care billed
3. Timely filing violation — claim submitted after the payer's deadline
4. Incorrect patient information — name, date of birth, or member ID does not match payer records
5. Duplicate claim submission — claim submitted more than once for the same date of service and procedure
6. Coordination of benefits (COB) issues — primary/secondary payer order not established or incorrect
7. Non-covered service — procedure not included in the patient's plan benefits
8. Bundling and unbundling errors — CPT codes that should be billed together are split, or vice versa
9. Provider not in-network — rendering provider is not contracted with the patient's plan
10. Lack of supporting documentation — operative report, office notes, or lab results not attached when required

Soft Denials vs. Hard Denials: Know the Difference

Not all denials are created equal. Before investing resources in an appeal, classify the denial type:

Soft denials are temporary. They indicate that the claim can be corrected and resubmitted without a formal appeal. Common soft denials include requests for additional information, coordination of benefits inquiries, and claims that were pended due to missing attachments. These can often be resolved within 5–10 business days by supplying the missing data.

Hard denials are final decisions by the payer that require a formal appeal process. Examples include medical necessity denials, timely filing denials, and non-covered service determinations. Hard denials require written appeal letters, clinical documentation, and adherence to strict deadlines.

Misclassifying a soft denial as a hard denial (and writing it off) or treating a hard denial as a simple resubmission (and missing the appeal window) are both costly mistakes. Your claims management workflow should require every denial to be classified before any action is taken.

Step-by-Step Appeals Process

Step 1: Obtain the Explanation of Benefits (EOB) or Remittance Advice (ERA)

Every denial generates an EOB or ERA that contains the denial reason code (CARC — Claim Adjustment Reason Code) and the remark code (RARC — Remittance Advice Remark Code). These codes tell you exactly why the claim was denied and what your next step should be. Pull the full EOB before taking any action.

Step 2: Verify the Denial Reason and Identify the Corrective Action

Map the CARC/RARC combination to a specific root cause. Was documentation missing? Was the auth number omitted? Was the patient's date of birth entered incorrectly? Each cause has a specific remedy. Resubmission without identifying the root cause simply generates a second denial.

Step 3: Determine Whether to Correct and Resubmit or File a Formal Appeal

If the error is a billing mistake (wrong code, missing modifier, incorrect patient info), correct the claim and resubmit as a corrected claim (frequency code 7 on the 837 transaction). If the denial is a coverage or medical necessity decision, file a formal written appeal.

Step 4: File the Internal Appeal

Most payers have a multi-level internal appeal process. Submit your appeal in writing within the payer's deadline, including:

• A cover letter clearly stating the claim number, date of service, patient name, and the specific basis for the appeal
• A letter of medical necessity signed by the treating physician
• Complete clinical documentation (office notes, test results, operative reports)
• Reference to the payer's own clinical coverage criteria that support the service
• Any relevant clinical literature or published guidelines (e.g., from AMA, ACOG, ACS)

Step 5: Request a Peer-to-Peer Review

When a claim is denied on medical necessity grounds, your physician has the right to speak directly with the payer's medical director in what is called a peer-to-peer review. This is one of the most effective tools available — peer-to-peer reviews overturn medical necessity denials at a 60–75% rate in most specialties. Request it within the first 10 days after receiving the denial.

Step 6: Escalate to External Appeal if Necessary

If the internal appeal is exhausted and the denial is upheld, most states and the ACA provide the right to an external independent review by an organization not affiliated with the payer. External review decisions are typically binding on the payer. For Medicare, this means progressing through the Redetermination, Reconsideration, ALJ Hearing, Medicare Appeals Council, and Federal District Court levels.

Key Deadlines and Timely Filing Limits by Payer

Missing a filing or appeal deadline is one of the few denial reasons that truly cannot be reversed. Below are standard timely filing and appeal deadlines by payer type. Always verify the specific plan's requirements, as individual plans within a payer family can vary.

What to Include in an Effective Appeal Letter

An appeal letter that wins has a specific structure. Payer reviewers process hundreds of appeals per week. Make their job easy by being precise and well-organized:

• Header: Payer name, address, claim number, patient name, member ID, date of service, CPT and ICD-10 codes, billed amount
• Opening statement: "We are appealing the denial of claim [number] for [patient name] dated [DOS]. The service was denied for [reason code]. We respectfully disagree for the following reasons."
• Clinical justification: Cite the patient's diagnosis, clinical history, and why the service was medically necessary. Reference the payer's own coverage criteria by name and page number.
• Supporting evidence: Reference attached documentation item by item. Number each exhibit.
• Closing request: Explicitly request reversal of the denial and payment of the claim. Provide a specific deadline for response if one applies.
• Physician signature: For medical necessity appeals, the treating physician should sign the letter directly — not just the billing office.

How to Track Denial Patterns and Reduce Future Denials

Appeals recover revenue retroactively. Denial pattern analysis prevents revenue loss prospectively. Every denial your practice receives should be logged by: date, payer, CPT code, denial reason code, billed amount, and resolution. Review this log monthly and look for patterns — if 30% of your denials from Aetna are for missing auth on a specific procedure code, the fix is a workflow change, not better appeals.

Our revenue recovery team at RCMAXIS conducts quarterly denial root cause analyses for every client, producing a written report with specific workflow changes that reduce denial rates period over period.

RCMAXIS Denial Recovery Performance

At RCMAXIS, we treat every denial as a recoverable revenue opportunity unless it is genuinely not payable. Our denial management results across our client base:

• First-pass claim acceptance rate: 98.1% — denials start low because errors are caught pre-submission
• Denial appeal success rate: 87% of appealed claims result in full or partial payment
• Average days to denial resolution: 18 days for internal appeals, 34 days for escalated cases
• Write-off rate: Under 0.8% of net patient revenue, versus industry average of 3–5%

When you partner with RCMAXIS, you get a dedicated claims management team that monitors every denial the day it is received, categorizes it, and initiates the appropriate workflow within 24 hours — not the end of the month. Contact us to learn how our free RCM audit can quantify exactly how much your practice is currently losing to unchallenged denials.