Insurance companies deny claims using specific reason codes and standardized language. Knowing which denial type you're facing is the first step to building a winning appeal. Here are the ten most common denial reasons, what each actually means, and the specific arguments that work for each.
Before you appeal: request your full claim file
Under federal law, your insurer must provide all documents used in the denial decision, free of charge, before deciding your appeal. Request the "complete claim file and all clinical criteria referenced in the denial" in writing. This often reveals the specific clinical guideline the insurer used — and gives you the exact language you need to counter it.
The 10 Denial Types
1. Not Medically Necessary
The most common denial. The insurer's medical reviewer determined that the treatment doesn't meet their definition of "medically necessary" — usually based on internal clinical guidelines called MCGs (Milliman Care Guidelines) or InterQual criteria.
How to fight it: Request the specific clinical criteria the insurer used. Then have your physician write a letter directly addressing those criteria, explaining point-by-point why your case meets them. Attach peer-reviewed literature from major medical societies (ACS, AHA, ACP, etc.) supporting the treatment. If the insurer's criteria are outdated or conflict with current standards of care, say so explicitly with citations.
2. Experimental or Investigational
The insurer claims the treatment lacks sufficient evidence and is not yet established as standard of care. This is frequently applied to newer therapies, off-label drug uses, and genetic treatments — even when they're widely used by specialists.
How to fight it: Research whether the treatment has FDA approval for your indication, is included in NCCN or other major guidelines, and is reimbursed by Medicare. If it is any of these, say so. The definition of "experimental" in most plan documents includes explicit carve-outs for FDA-approved treatments — read your plan language carefully and quote it back to the insurer.
3. Prior Authorization Not Obtained
The treatment required advance approval from the insurer that wasn't obtained before service — or was obtained but with errors in the documentation.
How to fight it: If the care was urgent or emergency and prior auth wasn't possible, cite the ACA emergency care provisions and your state's surprise billing laws. If the PA was obtained but the denial claims otherwise, provide proof (fax confirmation, reference numbers, call logs). If the error was the provider's, appeal on grounds that the treatment was medically necessary and you shouldn't be penalized for administrative errors outside your control.
4. Out-of-Network Provider
The insurer denies or reduces payment because you saw a provider outside your plan's network.
How to fight it: If no in-network provider with the needed specialty is available within a reasonable distance (network adequacy failure), your insurer may be required to cover out-of-network care at in-network rates. Request documentation of the in-network providers they claim are available — often these lists are outdated. The No Surprises Act (2022) provides additional protections for emergency care and certain facility-based situations.
5. Benefit Not Covered Under Your Plan
The insurer says the service simply isn't part of your plan's covered benefits.
How to fight it: Pull your Summary of Benefits and Coverage and read the exact exclusion language. Many denials citing "not covered" are incorrect — the benefit exists but was wrongly coded or categorized. If you have an ACA marketplace plan, check whether the denied service is an Essential Health Benefit (EHB) — EHBs cannot be excluded. Also check your state's mandated benefit laws.
6. Duplicate Claim
The insurer claims it already processed a claim for the same service.
How to fight it: This is usually an administrative error. Provide documentation showing the services were distinct (different dates, different procedures, different providers). If there was a legitimate rebilling after a coding correction, explain the timeline. These appeals are often resolved quickly once you provide clear documentation.
7. Coding Error
The claim was denied because of incorrect procedure codes (CPT), diagnosis codes (ICD-10), or modifier codes submitted by the provider.
How to fight it: Work with your provider's billing department to review and correct the codes. Request a corrected claim be resubmitted. In your appeal letter, note that the denial was based on a billing code error, not on the medical merits of the claim, and that the corrected codes accurately reflect the services provided. See our
full guide on coding error appeals.
8. Coverage Lapsed or Eligibility Issues
The insurer claims you weren't covered on the date of service — possibly due to a premium payment gap, enrollment error, or employer reporting delay.
How to fight it: Gather proof of your coverage: insurance cards, enrollment confirmation emails, employer verification letters. If coverage lapsed due to an administrative error (employer failed to enroll you, payment was delayed by mail), document the error and escalate to HR. State insurance regulators can intervene if a coverage lapse was caused by insurer or employer error rather than your own failure to pay.
9. Mental Health or Substance Use Parity Violation
Mental health or substance use treatment is being covered more restrictively than comparable medical/surgical benefits — which violates federal parity law.
How to fight it: The Mental Health Parity and Addiction Equity Act (MHPAEA) requires health plans to cover mental health and substance use disorder benefits no more restrictively than comparable medical/surgical benefits. If your insurer requires step therapy (try cheaper treatment first) for mental health drugs but not for cardiac drugs, that's a parity violation. File a complaint with your state insurance department and the Department of Labor simultaneously with your internal appeal.
10. Custodial Care Exclusion
Long-term care, nursing home stays, or assisted living is denied as "custodial" rather than "skilled nursing" care — even when skilled care is being provided.
How to fight it: Document all skilled nursing activities being performed (wound care, IV medications, physical therapy, medication management). The distinction between custodial and skilled care is fact-specific. Your physician and facility should provide detailed documentation of the skilled care services being delivered that meet Medicare or plan-specific definitions of skilled nursing care.
What to Do After Any Denial
- Request the full claim file and clinical criteria used
- Identify the specific denial type from the list above
- Use the targeted strategy for your denial type
- File your internal appeal within your plan's deadline (usually 180 days)
- If internal appeal fails: request external review — independent reviewers reverse internal decisions ~40% of the time
Use our free appeal letter generator to create a customized letter for your specific denial type, or use our appeal probability checker to estimate your chances before investing time in an appeal.
Disclaimer: This article provides general information only. Individual plan terms vary. For complex cases, consider consulting a patient advocate or attorney. Last updated: March 2026.