How to Appeal an Insurance Denial
Your insurance company denied your claim. That feels like a final answer — but it may still be reviewable. Start by getting the denial reason, plan language, dates, and missing documents in one place.
THE SHORT VERSION
- 1. Read your denial letter — find the denial code and reason
- 2. Request your full claim file from the insurer
- 3. File an internal appeal within 180 days (30 days for urgent)
- 4. If internal appeal fails, request an external review (independent third party)
- 5. Escalate to your state insurance commissioner if needed
IN THIS GUIDE
- 1. Why Claims Get Denied (and Why Most Denials Are Wrong)
- 2. Understanding Your Denial Letter
- 3. Step 1: File an Internal Appeal
- 4. Step 2: Request an External Review
- 5. Common Denial Codes and How to Beat Them
- 6. Critical Deadlines You Cannot Miss
- 7. What to Say in Your Appeal Letter
- 8. When to Escalate Beyond the Insurer
- 9. FAQ
1. Why Claims Get Denied (and Why Most Denials Are Wrong)
Insurance companies deny claims for dozens of reasons, but most fall into a few categories. Understanding why your claim was denied is the first step to overturning it.
Wrong patient info, missing referral number, incorrect provider ID, duplicate submission. These are the easiest to overturn — often a simple correction and resubmission.
Service wasn't pre-approved. For emergencies, prior auth is NOT required under federal law. For non-emergencies, retroactive auth is often possible within 14 days.
Insurer says the service wasn't medically necessary. This is where a letter from your treating physician is most powerful — they know your condition better than a desk reviewer.
Service not covered under your plan, or you weren't eligible on the date of service. Check your plan documents carefully — what the insurer says isn't covered sometimes is.
The remaining ~10% are coding disputes (wrong CPT or diagnosis code), timely filing issues, or coordination of benefits problems. Every one of these is appealable.
Sources: KFF analysis of ACA marketplace data; AHIP claims data
2. Understanding Your Denial Letter
Every denial letter must include specific information by law. Here's what to look for:
A code like CO-50, PR-1, or N265. This tells you exactly why the claim was denied. Our tool decodes these automatically.
The letter must explain in plain English why the claim was denied. If it's vague ('not medically necessary' with no detail), that itself is grounds for appeal.
The letter must tell you HOW to appeal and the deadline. Under ACA §2719, you have at least 180 days for internal appeal.
The letter must mention your right to an independent external review if the internal appeal fails. This is federal law — the insurer cannot skip this.
Pro tip:If your denial letter doesn't include all of the above, the insurer may be violating ACA §2719 requirements. Mention this in your appeal — it strengthens your position significantly.
3. Step 1: File an Internal Appeal
An internal appeal is reviewed by the insurance company itself — but by a different person than the one who denied your claim. Under ACA §2719, all non-grandfathered health plans must provide at least one level of internal appeal.
How to file:
State the denial reason, ask for the plan language or evidence used, and include your policy number, claim number, date of service, and denial date. Review any draft before sending.
Get a letter of medical necessity from your treating physician. Include medical records, test results, peer-reviewed literature supporting the treatment, and your plan's own coverage criteria showing the service should be covered.
You have 180 days from the denial for a standard internal appeal. For urgent/pre-service denials, you can request an expedited review (insurer must respond within 72 hours). Send via certified mail with return receipt.
Under ACA §2719(b)(2)(B), you have the right to access and copy your entire claim file, including the clinical rationale used to deny your claim. This often reveals weak reasoning you can challenge directly.
Don't write your appeal letter from scratch
Clever Dispute helps decode denial language and organize review questions. It does not calculate a real win probability or provide legal advice.
Generate My Appeal Letter — Free4. Step 2: Request an External Review
If your internal appeal is denied, you have the right to an external review — an independent third party (not affiliated with your insurer) reviews the decision. This is one of the most powerful protections in the ACA.
Under ACA §2719, after exhausting internal appeals (or simultaneously for urgent cases), you can request that an independent review organization (IRO) evaluate your claim. The IRO is chosen by the state or a federal authority — not your insurer.
If the IRO rules in your favor, the insurer must cover the claim. They cannot appeal the decision. This is why external review success rates are so meaningful — a win here is final.
You typically have 4 months (or 60 days in some states) from the date of the internal appeal denial to request external review. For urgent cases involving ongoing treatment, you can request expedited external review within 72 hours.
5. Common Denial Codes and How to Beat Them
Every denial has a reason code. Understanding yours is the key to a successful appeal. Here are the most common codes and the strategy for each:
| Code | Meaning | Win Rate | Strategy |
|---|---|---|---|
| CO-50 | Not medically necessary | 58-72% | Letter of medical necessity from physician + peer-reviewed literature |
| CO-16 | Missing info to adjudicate | 80-88% | Request specific list of missing items, resubmit complete |
| N265 | Prior auth required | 45-68% | Retroactive auth or cite No Surprises Act for emergencies |
| CO-97 | Bundled with another service | 60-72% | Show services were distinct encounters with NCCI documentation |
| CO-29 | Filing deadline expired | 55-70% | Provide proof of timely submission (certified mail receipt, EDI log) |
| CO-18 | Duplicate claim | 75-85% | Show services were separate encounters on different dates |
| CO-197 | Missing preauthorization | 50-68% | Request retroactive auth; emergency care doesn't need prior auth |
| PR-1 | Applied to deductible | 30-50% | Verify deductible balance; check 501(r) charity care eligibility |
Our denial appeal tool covers 35+ denial codes with specific strategies, win rates, and auto-generated appeal letters for each.
6. Critical Deadlines You Cannot Miss
Miss a deadline and you lose your right to appeal — regardless of how strong your case is. Mark these dates immediately when you receive a denial:
7. What to Say in Your Appeal Letter
An effective appeal letter is specific, evidence-based, and cites the law. Here's the structure:
State your name, policy number, claim number, date of service, and denial date. Reference the specific denial code and reason language from the denial letter.
Address the specific denial reason with evidence. If 'not medically necessary' — include physician statement and clinical guidelines. If 'not covered' — cite the specific plan benefit that covers it. If admin error — explain the correction.
Cite ACA §2719 (appeal rights), the No Surprises Act (for emergency/out-of-network), your state's insurance regulations, and any applicable specialty guidelines (AMA, ACR, etc.).
List all attachments: physician letter, medical records, peer-reviewed studies, plan documents showing coverage, prior authorization records if applicable.
Request reversal of the denial, reprocessing of the claim, and written confirmation. State your intent to pursue external review and regulatory complaints if the appeal is denied.
Let AI write your appeal letter
Enter your denial code to organize a plain-English appeal review draft. Check your plan documents and deadlines before sending anything.
Generate My Appeal Letter — Free8. When to Escalate Beyond the Insurer
If your internal and external appeals fail — or if the insurer is violating the process — you have additional options:
Every state has an insurance commissioner who investigates consumer complaints against insurers. Filing a complaint creates a regulatory paper trail that insurers take seriously. Many denials are reversed at this stage because insurers don't want regulatory scrutiny.
Find yours at naic.org/state-insurance-regulators
For Medicare Advantage plans or ACA marketplace plans, file a complaint at cms.gov/nosurprises. CMS has enforcement authority and can order the insurer to reprocess your claim.
File at cms.gov/nosurprises
If the denied claim has resulted in medical debt or collections activity, the Consumer Financial Protection Bureau can investigate. Particularly effective if the billing is unfair, deceptive, or abusive.
File at consumerfinance.gov/complaint
For patterns of bad faith denial (e.g., the insurer routinely denies a specific treatment that's clearly covered), your state AG's consumer protection division may investigate.
Find yours at naag.org
9. Frequently Asked Questions
Can I appeal a denial after the deadline has passed?+
Do I need a lawyer to appeal an insurance denial?+
What if my employer's plan denied the claim?+
Can the insurer deny my appeal without reviewing new evidence?+
What if my claim is for emergency care?+
Should I pay the bill while I'm appealing?+
Your insurer denied your claim. Fight back.
Denials can be worth reviewing. Clever Dispute helps you decode the reason, collect the record, and prepare cautious wording before you submit an appeal.
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