BillSherpa · Patient Advocacy · Updated 2026

My insurance denied my claim — step by step guide to fighting back

Getting an insurance denial feels like hitting a wall. But here's what the insurance company doesn't tell you: more than half of all denied claims that are appealed get reversed. The denial is not the end of the road. It's the beginning of a process you have every right to use.

First: understand why it was denied

Before you can fight back, you need to know exactly why the claim was denied. The denial letter must include a specific reason under federal law. Common denial reasons include:

• Not medically necessary — the insurer determined the service wasn't required for your condition
• Out of network — the provider isn't in your plan's network
• Prior authorization required — approval wasn't obtained before the service
• Coding error — a wrong procedure or diagnosis code caused the claim to fail
• Experimental or investigational — the treatment isn't covered under your plan
• Duplicate claim — the insurer believes this was already submitted

The denial reason determines your appeal strategy. A coding error is very different from a "not medically necessary" denial — and each requires a different response.

Step by step: how to appeal an insurance denial

1. Read the denial letter carefully. Look for the specific reason code, the claim number, the date of service, and the deadline to appeal. Write all of this down. The deadline is critical — missing it can forfeit your appeal rights.
2. Get your medical records. Request all records related to the denied service from your provider. You need these to build your case. Your provider is legally required to give them to you within 30 days under HIPAA.
3. Write a letter of medical necessity. Ask your doctor to write a letter explaining why the service was medically necessary for your specific condition, referencing your diagnosis and clinical guidelines. This is the most powerful document in your appeal.
4. Submit your internal appeal. Send your appeal letter, the doctor's letter, your medical records, and any supporting clinical literature (guidelines, studies) to the address on the denial letter. Send everything certified mail. Keep copies.
5. If internal appeal fails: request external review. You have the right to an independent external review by a third party that is not affiliated with your insurer. The external reviewer's decision is binding on the insurance company in most cases.
6. File a complaint with your state insurance commissioner. If your insurer is unresponsive or denies your appeal improperly, file a complaint. This creates a formal record and often prompts faster resolution.

The most common reason appeals succeed

Most successful appeals win on one of three grounds: (1) the denial was based on a coding error that can be corrected, (2) the doctor provides a letter of medical necessity that wasn't in the original claim, or (3) the insurer applied the wrong clinical criteria to the denial. Your doctor's letter is by far the most important weapon you have.

Deadlines you cannot miss

• Internal appeal deadline: Usually 180 days from the date of the denial letter. Some plans require earlier — check your denial letter.
• External review deadline: Usually 60 days after the internal appeal final decision.
• State complaint deadline: Varies by state — typically 1-2 years from the date of denial.