Your insurance company denied your claim. Before you accept that answer, know two numbers from KFF’s analysis of federal transparency data: insurers on HealthCare.gov denied 19% of in-network claims in 2024 — and consumers appealed fewer than 1% of the claims they denied.
The denial letter is not the final word. In many cases it is the opening position. This guide walks through what your denial letter actually means, the deadlines that matter, the federal laws that force your insurer to take your appeal seriously, and how to write an appeal that gets read. If you would rather not draft it yourself, our insurance appeal tool builds one from your denial documents.
Why claims get denied in the first place
Knowing which category you are in determines your entire strategy. The federal data is genuinely surprising here, and it should shape your expectations.
Of in-network denials in 2024, KFF found the most common reason insurers reported was “Other” — reason not listed — at 36%, followed by administrative reasons at 25%. Only 5% were for lack of medical necessity. 13% were for an excluded service, and 9% for lack of prior authorization or referral.
Read that again, because it inverts the common assumption. Most denials are not clinical judgments about your care. They are paperwork.
- Administrative and clerical problems. A wrong CPT code, a transposed digit, a mismatched diagnosis code, a missing modifier, a claim filed after a timely-filing window. These collapse quickly when someone challenges them — they are errors, not decisions.
- “Other.” The single largest bucket, and one insurers are not required to explain further in this reporting. If your denial letter is vague, this is the category you are probably in, and vagueness is itself something you can push back on.
- Excluded service. The plan says it does not cover this. Worth checking against your actual plan documents rather than the denial letter’s characterization.
- Prior authorization missing. Often fixable, especially where the situation was urgent or the requirement was not disclosed.
- “Not medically necessary.” Only 5% of in-network denials, but disproportionately the ones that matter most — high-cost treatment for serious conditions. Your treating physician’s documented judgment carries real weight here.
- Out-of-network. If this happened at an in-network facility or in an emergency, the No Surprises Act may prohibit the charge outright rather than merely making it appealable — see our guide to surprise medical bills.
The laws that give your appeal teeth
You are not asking for a favor. Federal law requires your insurer to give you a full and fair review.
ERISA § 503 (29 U.S.C. § 1133). If you get insurance through a private employer, your plan must “provide adequate notice in writing” setting forth “the specific reasons for such denial, written in a manner calculated to be understood by the participant,” and must afford “a reasonable opportunity … for a full and fair review.” Both of those phrases are enforceable — a denial that does not give specific reasons is not doing what the statute requires.
Your deadline: at least 180 days. The Department of Labor states you have “at least 180 days to file an appeal,” and directs you to check your Summary Plan Description in case your plan allows longer.
The insurer’s deadlines to decide. Per DOL: 72 hours for urgent care appeals, 30 days for pre-service appeals, 60 days for post-service appeals. A plan may extend these only with your consent.
Your right to the file, free. DOL again: “The plan must provide you copies of documents, records, and other information relevant to your claim for free if you request them. You can also request the identity of any medical or vocational expert whose advice was obtained by the plan.” This is the most underused right in the entire process.
A genuinely fresh reviewer. On appeal your claim “must be reviewed by someone new,” who “cannot be the same person who made the initial decision or that person’s subordinate,” and who “must give no consideration to the initial decision.”
External review. Non-grandfathered plans must generally offer an independent external review by a reviewer who does not work for your insurer, and that decision binds the insurer.
If your coverage comes through a private employer, our ERISA appeal letter guide covers these mechanics in more depth — particularly the claim-file request in Step 2, which is where most of the leverage is.
Step 1: Read the denial letter and find your deadline
The denial letter — or the Explanation of Benefits — must state the reason for denial and how to appeal.
Find three things and write them down: the denial code and stated reason, the appeal deadline, and the address or portal for appeals. Then check whether your plan is an ERISA plan (private employer), a government or church plan, an individual policy, or Marketplace coverage, because that determines which rules apply.
Note whether the claim is pre-service (care not yet received) or post-service (care given, payment refused). That sets the insurer’s decision clock and affects urgency.
Do not use the full 180 days. You will need time to get records from the insurer and a letter from your doctor, and neither arrives quickly.
Step 2: Request the claim file before you write anything
Send a short written request, separate from and before your appeal, asking for everything the insurer relied on:
- The complete claim file.
- The specific plan provisions the denial rests on.
- Any internal rule, guideline, protocol, or clinical criterion applied — including licensed third-party criteria sets.
- The identity and credentials of the clinician who reviewed the claim.
They must provide these free. This matters more than any other step: the denial letter gives you a conclusion, and the claim file gives you the reasoning. You cannot rebut criteria you have never seen — and given that 36% of denials are reported only as “Other,” the file is often the only way to learn what actually happened.
Also gather your Summary Plan Description, the relevant medical records, and a letter of medical necessity from your treating physician. Give your doctor the denial letter and the criteria so the letter can address them directly. A one-page letter that applies the plan’s own criteria to your documented history is worth far more than a general endorsement of the treatment.
Step 3: Write the appeal letter
A strong appeal is specific, cited, and unemotional. It should:
- Identify the claim — member ID, claim number, date of service, provider — in the first lines.
- State that you are exercising your appeal rights under ERISA § 503 and 29 C.F.R. § 2560.503-1, or your state’s equivalent for non-ERISA plans.
- Quote the stated denial reason, then rebut that specific reason — not denials in general.
- Apply the plan’s own criteria to your facts, using the guidelines from the claim file.
- Request the reviewing clinician’s credentials and, where the denial is clinical, ask for review by a relevant specialist.
- Note procedural failures — documents requested and not provided, a reviewer without relevant expertise.
- State the remedy and the deadline the insurer must meet.
Attach everything you want considered. For ERISA plans the administrative record generally becomes the record a court would later review, so evidence you leave out now may be evidence you cannot introduce later.
Mail it certified with return receipt. Appeal deadlines are strict, and you want dated proof of receipt.
Sample appeal letter excerpt
Re: Appeal of Claim No. 2026-4471-XX — Member ID 88-XXXXXXX Date of Service: January 22, 2026
I am filing a formal appeal of the above claim under ERISA § 503 (29 U.S.C. § 1133) and request a full and fair review by a reviewer with appropriate clinical expertise.
Your notice dated February 4, 2026 states the claim was denied as “not medically necessary under plan criteria.” The criteria you provided in response to my document request require documented failure of conservative therapy prior to authorization.
That criterion is satisfied. As documented in the attached records, I completed twelve weeks of physical therapy between October 2025 and January 2026 with no measurable improvement. The attached letter from Dr. A. Reyes, my treating orthopedic surgeon, states that continued conservative management is not clinically appropriate.
I further note that the reviewing clinician identified in the claim file is credentialed in family medicine, not orthopedics. I request review by a board-certified orthopedic specialist.
I request that the denial be reversed and the claim paid in full.
FightThis drafts the full version from your documents — preview free.
Step 4: If the internal appeal fails, escalate
A second denial is not the end — but be realistic about the odds. KFF found insurers upheld 66% of internal appeals in 2024. Roughly one in three appeals produced a different answer. That is meaningfully better than the zero you get by not appealing, and lower than the optimistic numbers that circulate.
Your escalation path:
External review. Independent, binding on the insurer, and free to you. It is strikingly underused: KFF found Marketplace enrollees filed at least 5,881 external appeals in 2024, about 4% of upheld internal appeals. The odds there appear better than at the internal stage — a 2025 Health Affairs study of external review decisions in four states, cited by KFF, found almost half overturned the initial denial.
Your state insurance commissioner, for fully insured and non-ERISA plans. States generally cannot regulate self-funded ERISA plans.
The Department of Labor, whose Employee Benefits Security Administration enforces ERISA and whose benefits advisors will speak with plan participants directly.
An ERISA § 502(a) lawsuit. You generally must exhaust internal appeals first, and your appeal letter becomes the core of the court record — another reason to build it carefully rather than quickly.
If what you are actually holding is a bill rather than a coverage denial, that is a different fight — one that starts with demanding an itemized statement from the provider rather than appealing to the insurer. See how to fight medical bills, or use our medical bill dispute tool directly.
The bottom line
Insurers denied about one in five in-network claims in 2024, and consumers challenged fewer than one in a hundred of those. The arithmetic of denial works because almost nobody answers it.
The process is genuinely winnable. Deadlines are generous, the law entitles you to the insurer’s own reasoning for free, and roughly a third of the people who push back at the internal stage get a different answer — with better odds still at external review.
This article is general information, not legal advice. For disputes involving large sums, ongoing litigation, or complex medical situations, consult a licensed attorney.