How to Appeal a Denied Medical Claim (With Letter Template)

What an appeal actually is

Internal appeals come first. Your insurer reviews their own denial, with a different reviewer than the one who denied it originally. Federal law (29 CFR 2560.503-1) requires this for ERISA-covered plans, ACA-compliant plans, and individual marketplace plans. The reviewer must give no deference to the original denial.

External review comes second. After your internal appeal is exhausted, you can request that an independent review organization look at the claim. The independent reviewer's decision is binding on your insurer, which is why this stage matters.

Reversal rates depend heavily on plan type and denial reason. For ACA marketplace plans, KFF's analysis of 2024 federal data found insurers upheld 66% of internal appeals, meaning roughly a third were overturned. For Medicare Advantage prior authorization denials, more than 80% of appeals were partially or fully overturned in 2024.

The number that should bother you: in the ACA marketplace in 2024, fewer than 1% of denied claims were appealed at all. People walk away from money the insurer was prepared to pay if asked.

Why claims get denied

KFF's 2024 analysis breaks down marketplace denial reasons. The largest single bucket (36%) is "other," which is exactly as informative as it sounds. Administrative reasons account for 25%. Lack of prior authorization or referral accounts for 9%. Medical necessity, the reason most people associate with denials, accounts for only 5%.

Administrative denials (missing information, wrong provider ID, claim filed late) are usually the easiest to overturn. Often a corrected resubmission by the provider's billing department is enough, no formal appeal needed. Call the provider first.

Prior authorization denials require either retroactive authorization (sometimes possible if you can show the service was urgent) or a medical necessity argument from your physician.

Medical necessity denials are the hardest fight but also where the strongest cases get won. You need a letter of medical necessity from the treating physician, the relevant clinical guidelines (often from professional societies), and ideally peer-reviewed evidence supporting the treatment.

"Other" denials require you to push for specifics. Federal law requires the denial notice to state the reason. If the notice just says "not covered" or "denied per plan terms," call the insurer and ask for the specific policy section, criterion, or guideline they applied. They have to tell you.

I see this constantly at the women's health company I work at. Patients get a denial that says nothing useful, assume the insurer knows something they don't, and pay the bill. Most of the time the underlying reason is something fixable.

What to do about it

1. Read the denial letter carefully. It must include the specific reason for denial, the clinical criteria used (if applicable), your appeal deadline, and instructions for filing. If any of these are missing, the denial may itself be invalid under federal claims procedure rules.

2. Request the full claim file and the criteria the insurer used. You're legally entitled to this for free. Call your insurer and say: "I'd like to request the complete claim file for [claim number], including the medical necessity criteria, clinical guidelines, or rules used to deny this claim. Please send these to me in writing." Get a confirmation number.

3. Get supporting documentation from your provider. This is where appeals are won. Ask your doctor's office for: a letter of medical necessity that addresses the specific denial reason, relevant chart notes, test results, and any prior treatments that failed. If your provider has a billing or appeals coordinator, ask for them by name. They do this all day.

4. Write the appeal letter. Use the template below. Keep it formal, factual, and specific. Do not editorialize.

5. Submit by certified mail or the insurer's appeal portal. Keep proof of submission. The 180-day clock is hard. Miss it and the appeal is dead.

6. Follow up in 15 business days if you haven't heard back. Under federal claims procedure rules, the insurer must respond within 30 days for pre-service claims, 60 days for post-service claims, and 72 hours for urgent care. If they miss the deadline, you can declare "deemed exhaustion" and proceed directly to external review.

7. If denied internally, file for external review immediately. You have 4 months from the final internal denial to request external review. The external review organization is independent of your insurer and their decision is binding.

Appeal letter template

When to escalate

If your internal appeal is denied, file for external review immediately. The instructions are in your final internal denial letter, and you have 4 months to request it. For ERISA self-funded plans, you can also file a complaint with the Department of Labor's Employee Benefits Security Administration at askebsa.dol.gov or 1-866-444-3272. For ACA marketplace plans, your state's Consumer Assistance Program (where one exists) and your state insurance department both take complaints. For Medicare or Medicare Advantage, file with 1-800-MEDICARE.

If your appeal involves a No Surprises Act issue (emergency care, out-of-network providers at in-network facilities, air ambulance), file with CMS or call 1-800-985-3059.

Related reading

• What does CPT code 99285 mean? (And why your ER bill is so high)
• EOB shows a different amount than the bill: what's actually going on
• Balance billing vs. surprise billing under the No Surprises Act