How to Appeal a Health Insurance Denial: A Step-by-Step Guide

You opened the letter and your stomach dropped: the medication, scan, or procedure your doctor ordered has been denied. It feels final. It usually is not. The hardest part of appealing is not the writing — it is knowing the steps and not missing the deadline. This guide walks through both.

First: many denials are a documentation problem, not a verdict

A denial rarely means your doctor was wrong. In ACA marketplace plans in 2024, only about 5% of in-network denials were for “lack of medical necessity”; far more were administrative or for a missing prior authorization or referral. Many denials, in other words, come down to information the plan simply did not have in front of it — a missing record, an unmet documentation requirement, a coding mismatch, or a step the plan wanted tried first.

That is also why appeals succeed as often as they do: the appeal is your chance to supply the documentation the first reviewer never saw. A denial decided on an incomplete file often reverses once the file is complete — the clinician's notes, the record of treatments already tried, the guideline that supports the request. Some denials are genuine benefit exclusions or clinically appropriate decisions, and an appeal will not change those. But you cannot tell which kind you are holding until you read the reason and answer it with the right documentation.

Does appealing actually work?

Often enough to be worth the effort — though the odds depend on your plan and which stage you reach. Here is the honest picture, by plan type:

The thread running through all of it: fewer than 1 in 100 denied marketplace claims are ever appealed. The opportunity is real precisely because so few people use it.

Step 1: Read the denial letter — and the deadline

Your denial notice is also your roadmap. Find three things:

• The reason. A code or short phrase such as “not medically necessary,” “step therapy required,” or “non-formulary.” Your appeal has to answer this specific reason. (Not sure what the reason code means? The EOB Explainer and our guide to EOB codes decode it.)
• The deadline to file. This is the date that quietly kills most appeals. Note that it is different from the insurer's deadline to decide — this is your window to submit.
• Where to send it. Usually a fax number, mailing address, or member-portal upload.

Mark the deadline on your calendar the day the notice arrives. For reference, once you file, the insurer generally must decide a commercial internal appeal within 30 days before-service, 60 days after-service, or 72 hours if it is urgent.

Step 2: File the internal appeal

The internal appeal asks your insurer to look again — and federal rules require the reviewer to be someone who was not involved in the original denial, with appropriate clinical training when medical judgment is at issue. You (or your clinician's office) submit a written appeal that answers the denial reason directly. If a delay could seriously jeopardize your health, ask for an expedited appeal, decided as fast as your condition requires and generally within 72 hours — and you can run the internal appeal and external review at the same time in urgent cases.

Step 3: Build your evidence

Strong appeals are built on documentation. Gather:

• A copy of the denial notice.
• Your clinician's visit notes from when the treatment was ordered.
• A record of treatments already tried and why they did not work (this answers “step therapy” denials).
• Relevant labs, imaging, and the clinical guidelines that support the treatment — your clinician's office can supply these.

Two pieces carry the most weight, and both come from your clinician's office:

• A Letter of Medical Necessity — the clinical narrative explaining why this treatment is right for you. It is often the single most powerful document in an appeal. (See how a Letter of Medical Necessity works.)
• A peer-to-peer review — a direct call between your clinician and the insurer's medical reviewer, often the fastest way to clear a denial that came down to missing context. (See the Peer-to-Peer Review Coach.)

Step 4: External (independent) review

If the internal appeal is upheld, you have the right to an external review by an independent review organization (IRO) that is not employed by or affiliated with your insurer — and, as the numbers above show, external reviewers overturn roughly half of the denials that reach them.

• Deadline: generally at least four months after the final internal denial to request it.
• Timeline: a standard decision within about 45 days; an expedited decision within 72 hours for urgent cases.
• Who runs it: many states run their own process; where a state does not, the federal process administered by MAXIMUS Federal Services applies.
• Cost: the federal process is free; some state or IRO processes charge a small fee capped at $25, refunded if you win.

The denial letter that upholds your internal appeal must explain how to request external review.

Step 5: If you are still denied

Appeals are a ladder, not a single shot. If both an internal appeal and an external review uphold the denial, more doors remain:

Many state attorneys general also accept consumer health-insurance complaints. And under federal rules, denial and appeal notices must be available in non-English languages spoken by 10% or more of the population in your county — ask for a translation at no cost if you need one.