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How to Dispute an Insurance Denial

March 2026

Your doctor ordered an MRI. Your insurance company pre-authorized it. You got the scan. Then three weeks later, a denial letter arrives saying the $3,100 claim won't be paid. The reason given is vague — "not medically necessary" or "additional documentation required." This is one of the most frustrating situations in medical billing, and it happens constantly.

Why pre-authorized claims get denied

Pre-authorization isn't a guarantee of payment. Insurers can (and do) deny claims after the fact for reasons like coding mismatches, missing documentation the provider was supposed to submit, or retroactive medical necessity reviews. But that doesn't mean you're stuck paying. The Affordable Care Act gives you specific appeal rights, and insurers overturn denials more often than most people realize. According to KFF, patients who appeal win roughly 40-50% of the time.

The appeal process

Your denial letter must include instructions for filing an internal appeal. You typically have 180 days from the denial date. For the internal appeal, you'll want your pre-authorization reference number, the denial letter with its specific reason code, and a letter from your ordering physician explaining why the procedure was medically necessary. If the internal appeal fails, you have the right to an external review by an independent third party — the insurer doesn't get to make the final call.

Sample dispute letter

This is the type of escalation letter Simpler Disputes generates for insurance denials. This example covers a $3,100 MRI that was pre-authorized but denied post-service.

Re: Appeal of Claim Denial — Claim #[Claim Number] Member ID: [Your Member ID] Pre-Authorization Reference: [Auth Number] Dear Appeals Department, I am filing a formal internal appeal of your denial of claim #[Claim Number] for an MRI of the lumbar spine (CPT 72148) performed on [date] at [facility name]. This procedure was pre-authorized by your office on [pre-auth date] under reference number [auth number]. Despite this prior authorization, the claim was denied on [denial date] with the explanation "[denial reason from letter]." I am requesting reversal of this denial for the following reasons: 1. Your organization issued prior authorization for this specific procedure, creating a reasonable expectation of coverage. Retroactive denial of a pre-authorized service shifts costs to the patient in a manner inconsistent with the purpose of the pre-authorization process. 2. Under the Affordable Care Act (42 U.S.C. § 300gg-19), I have the right to appeal any adverse benefit determination. I am exercising that right. 3. The ordering physician, Dr. [Name], determined this MRI was medically necessary for [diagnosis/reason]. I have attached a letter of medical necessity from Dr. [Name]'s office supporting this determination. I request that you: 1. Reverse the denial and process the $3,100 claim as approved 2. Provide written confirmation of the reversal within 30 days If this internal appeal is denied, I intend to exercise my right to external review under federal and state law. I will also file a complaint with [State] Department of Insurance. Sincerely, [Your Name] [Your Address] [Date] Enclosures: Pre-authorization confirmation, denial letter, physician letter of medical necessity

Tips for a stronger appeal

Get your doctor involved. A letter of medical necessity from the ordering physician carries significant weight. Ask them to reference specific clinical findings — radiology reports, exam notes, failed conservative treatments — that led to the MRI order. Generic "this was necessary" statements don't help. Specifics do.

Also note the pre-authorization reference number prominently. This is your strongest piece of evidence. The insurer approved this procedure in advance, and their own records should confirm it.

Appealing a denied claim? Get your letter in under a minute.

Generate your letter

Simpler Disputes writes appeal letters that reference your specific denial reason, pre-authorization details, and the laws that protect your right to appeal. One-time payment, no subscription.