Why Insurers Deny Valid Claims, and How to Appeal in 3 Steps

You open your mailbox expecting a routine Explanation of Benefits (EOB) after a necessary medical procedure, only to find a notice saying your claim was denied. Below the heading, a brief standard sentence explains that the service was deemed “Not Medically Necessary” or “Experimental.”

Your stomach drops. Suddenly, you are left holding the financial burden for thousands of dollars in medical costs that you reasonably assumed your health insurance policy covered.

The Hidden Mechanics: Why Valid Claims Get Rejected

An initial denial does not always reflect a careful judgment about your treatment. Many claims are first processed by automated systems that compare procedure codes with diagnosis codes and plan rules, and a claim can be rejected at that stage without anyone reading the full medical record.

News investigations and lawsuits in recent years have alleged that some insurers used automated tools to deny large batches of claims with very little physician review. Insurers have disputed those descriptions. Whatever the cause in your case, a first denial is a decision you are entitled to question. Our article on how insurers use algorithms in claim decisions goes deeper.

Three common reasons a claim that should be payable gets denied are:

• Administrative & Coding Errors: A transposed digit in a procedure code or a diagnosis code that does not match the service can cause a claim to be rejected.
• “Not Medically Necessary” Decisions: Insurers apply clinical guidelines (such as InterQual or MCG criteria, or their own medical policies) that may be narrower than what your treating doctor considers appropriate.
• Missing Prior Authorization: If a required pre-service approval was not obtained, the insurer may deny payment on procedural grounds. Sometimes this can be corrected or appealed.

The 3-Step Appeal Process That Wins

A phone call to customer service can clear up a simple error, but representatives usually cannot overturn a formal denial. For that, follow the written appeal process below. No process guarantees a win, but a documented appeal gives you the best chance.

Step 1: Uncover the Denial Code & Request Your Complete Claim File

Under the Affordable Care Act (ACA) and ERISA regulations, health plans must tell you the specific reason for a denial. Look closely at your Explanation of Benefits (EOB) or denial letter for the denial reason code and its explanation.

Then send a written request for your complete claim file. For plans covered by the federal claims rules, you are entitled on request, and free of charge, to the documents relevant to your claim, including the clinical criteria relied on. Our guide on how to request your complete insurance claim file includes sample wording.

Step 2: Build a Bulletproof Clinical Evidence Package

Once you know why the claim was denied, work with your treating physician to answer it point by point. A strong internal appeal packet usually contains three items:

1. A Custom Letter of Medical Necessity: Ask your doctor to write a targeted letter explicitly refuting the insurer’s denial code. The letter must explain why alternative lower-cost treatments were inadequate or previously failed.
2. Peer-Reviewed Medical Literature: Attach clinical guidelines from major medical societies (e.g., American College of Cardiology, American Cancer Society) confirming that your treatment represents standard medical care.
3. Peer-to-Peer Discussion Log: Have your physician request a formal “Peer-to-Peer” phone conference with the health plan’s medical director. Some denials are reversed after this physician-to-physician conversation.

Step 3: Trigger the External Independent Review (The Ultimate Lever)

If your internal appeal is rejected, do not lose hope. You possess a powerful legal safeguard: The External Review Process.

For most non-grandfathered plans, if your insurer upholds a denial that involves medical judgment, such as medical necessity, you have the right to request an external review by an Independent Review Organization (IRO). The reviewers are independent clinicians who do not work for the insurance company.

If the external reviewer decides in your favor, the decision is generally binding on the plan, which must then provide the coverage. The review is free or low-cost to you; some state programs may charge a small filing fee. Which external review process applies depends on your state and plan type. See our guide to external independent medical review.

A Hypothetical Example of the Process

The following example is hypothetical. A patient’s outpatient procedure, with a $4,200 allowed amount, is denied as “not medically necessary.” The denial letter gives a reason code and says the plan’s criteria were not met.

1. Week 1: The patient asks the provider’s billing office to hold the account, then requests the claim file. The file shows the reviewer found “no record of conservative treatment.”
2. Week 3: The treating doctor writes a letter of medical necessity and attaches notes showing three months of physical therapy and medication that did not work.
3. Week 4: The patient files the internal appeal with the letter, the notes, and a copy of the denial, and keeps proof of delivery.
4. Later: If the plan upholds the denial, the patient requests external review using the instructions in the final denial letter.

The outcome is not guaranteed at any step, but the appeal now answers the exact reason the plan gave instead of simply repeating that the care was needed.

Deadlines to Watch

Appeal rights come with time limits, and missing one can end the appeal. Under the federal rules that apply to most private health plans, you generally have at least 180 days from a denial notice to file an internal appeal and four months from a final internal denial to request external review. Urgent cases can qualify for expedited review. Some plans have two internal levels, and Medicare, Medicaid, and some other plans follow different rules.

Always use the dates printed in your own denial letter and plan documents. Our guide to deadlines at each stage of an insurance appeal lays out the stages in more detail.

Appeal Checklist

• Denial letter or EOB with the reason code and appeal instructions
• Claim number, member ID, date of service, and provider name
• Your written appeal letter, signed and dated
• Letter of medical necessity from your treating doctor
• Relevant medical records, test results, and treatment history
• Clinical guidelines or studies your doctor recommends including
• Proof of delivery (certified mail receipt, fax confirmation, or portal confirmation)
• A log of every call: date, name, and reference number

Questions to Ask Your Insurer and Your Doctor’s Office

• What is the exact reason for the denial, and which plan provision or clinical policy was applied?
• Is this a coding or missing-information issue that the provider can correct and resubmit?
• What is my deadline to appeal, and where do I send it?
• How many levels of internal appeal does my plan have?
• Is my plan fully insured or self-funded, and which external review process applies?
• Will the provider hold my account from collections while the appeal is pending?
• Can my doctor request a peer-to-peer review?

Common Mistakes to Avoid

• Paying the bill before the appeal is decided. Ask the provider to wait. See why you should not pay a denied claim right away.
• Appealing by phone only. A written appeal creates a record and starts the clock.
• Arguing in general terms. Respond to the specific reason and criteria the plan cited.
• Sending originals. Send copies and keep a complete set.
• Stopping after the first “no.” External review exists because internal reviews are not always right.
• Missing the deadline. File on time even if some documents will follow later, and say so in the letter.

Frequently Asked Questions

Is a denial the same as a rejected claim?

Not exactly. A rejected claim usually has an error, such as a wrong ID number, and can be corrected and resubmitted by the provider. A denied claim was processed and found not payable, and it is challenged through the appeal process.

Can my doctor file the appeal for me?

Often yes. Many plans let a provider appeal on your behalf or let you name an authorized representative. Ask the office whether they will, and stay involved so deadlines are not missed.

How long does an appeal take?

It depends on the plan type and whether the care has already been provided. Federal rules set maximum timeframes for many plans, with faster handling for urgent situations. Your denial letter should explain what applies to you.

Do I need a lawyer?

Most people handle internal appeals and external review themselves or with their doctor’s help. For large amounts or complex disputes, a patient advocate, your state’s consumer assistance program, or an attorney may be worth contacting.

Who can I complain to if the insurer does not follow the rules?

For fully insured plans, your state insurance department. For self-funded employer plans, the U.S. Department of Labor’s Employee Benefits Security Administration. For Marketplace plans, the Marketplace can also direct you.

The Bottom Line

A denial notice can look final, but it is often the start of a process and not the end of one. By finding out the exact reason, gathering clinical evidence with your doctor, using your internal appeal rights, and requesting external review when it is available, you give a valid claim a fair chance of being paid.

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