How to Appeal an Insurance Claim Denial (Step-by-Step Guide)

Getting a claim denied doesn't mean the answer is no. It means you need to ask again — the right way. Studies show that most patients who appeal a denial win more than half the time. The problem is that most people never appeal at all. Here's how to change that.

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Why insurance claims get denied

Understanding the reason for the denial is the most important step. Denials fall into a few broad categories:

• Not medically necessary — the insurer doesn't agree that the treatment was required
• Out-of-network provider — the physician or facility wasn't in your plan's network
• Prior authorization required — the procedure needed pre-approval that wasn't obtained
• Coding or administrative error — a wrong CPT code, wrong date, or other clerical mistake
• Coverage exclusion — the service isn't covered under your specific plan
• Duplicate claim — the same claim was submitted twice
• Timely filing — the claim was submitted after the insurer's filing deadline

Each type of denial requires a different appeal strategy. A “not medically necessary” denial needs a letter from your doctor. A coding error needs a corrected claim. Knowing which one you're dealing with shapes everything that follows.

Step 1: Read the denial reason carefully

Your Explanation of Benefits (EOB) contains the denial code and a brief reason. Don't skim it — read it word for word.

Look for:

• The specific denial code (e.g., CO-4, CO-97, PR-1)
• The stated reason in plain language
• The deadline to appeal (usually 180 days from the denial date)
• The instructions for how to file an appeal

If anything is unclear, call your insurer and ask them to explain exactly why the claim was denied and what documentation would reverse the decision. Write down the representative's name and the date of the call.

Step 2: Gather your appeal documents

A strong appeal is a documented appeal. Depending on the denial reason, you'll need:

• Your doctor's letter of medical necessity — for “not medically necessary” denials, this is essential. Ask your doctor to write a detailed letter explaining why the treatment was required.
• Clinical guidelines — reference the same medical guidelines your insurer uses to define medical necessity (often found in your plan documents or on the insurer's website)
• Medical records — relevant chart notes, test results, referrals, and prior treatment history that support the claim
• The original itemized bill — showing all charges and codes
• Prior authorization records — if authorization was obtained, include confirmation numbers and approval letters
• Your insurance policy — highlight any language that supports your coverage

For coding errors, you may not need all of the above — sometimes a corrected claim submitted by the provider is enough to fix the issue without a formal appeal.

Step 3: File an internal appeal

An internal appeal goes back to your insurance company. You're asking them to reconsider their decision in light of the documentation you've provided.

How to submit it

Most insurers accept appeals by mail, fax, or through their online portal. Use certified mail if you're mailing — you want proof of delivery and the date it was sent.

What your appeal letter should include

Keep the letter concise and factual. Structure it as:

1. Your name, member ID, and claim number
2. The date of service and the service that was denied
3. The reason given for the denial
4. Why the denial was wrong (e.g., “the service meets the plan's medical necessity criteria because...”)
5. A list of the supporting documents attached
6. A request for a specific outcome (e.g., “I am requesting that this claim be reconsidered and covered in full”)

Attach all supporting documentation. Keep a copy of everything you send.

Timelines

Insurers are required to respond to internal appeals within specific timeframes — typically 30 days for non-urgent claims, 60 days for standard appeals, and 72 hours for urgent/expedited cases. If your health is at immediate risk, request an expedited appeal and note that clearly in your letter.

Step 4: Request an external review if needed

If your internal appeal is denied, you have the right to an independent external review. An external reviewer is an organization not affiliated with your insurer — their decision is binding.

To request an external review, contact your insurer or your state's insurance commissioner. Under the Affordable Care Act, you're entitled to external review for most commercial health plan denials.

External reviews overturn insurer decisions roughly 40% of the time. It's worth pursuing — especially for high-cost procedures like surgery, cancer treatment, or mental health care.

What to say on the call

Before or during the appeal process, a phone call to your insurer can sometimes resolve the issue faster than a paper appeal. Use this script:

“I'm calling about claim number [X] for a service on [date]. The claim was denied for [reason]. I'd like to understand exactly what documentation or criteria would be needed to approve this claim, and I'd like to note that I intend to file an appeal.”

Always get the representative's name and a reference number. Ask them to note your call in the account. This documentation can support your appeal.

When to get help

Appeals can be time-consuming — gathering records, writing letters, tracking deadlines, and following up with multiple departments. If the amount is significant or the denial complex, Agent Loop can handle the investigation and appeal process for you. Our clever fox digs into the denial reason, identifies the right documentation strategy, and fights the case directly with your insurer. No savings, no fee.

For more on understanding what your insurer actually paid (versus what you owe), see our guide on disputing a medical bill with your insurance company.

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