Why Would Medicare Deny a Claim? Reasons and How to Appeal

Medicare denies claims for a short list of predictable reasons: the service was ruled not medically necessary, it falls into a category Medicare never covers, the bill contained a coding or filing error, required prior authorization was skipped, you were not enrolled or the provider was not enrolled on the date of service, or another insurer should have paid first. So if you’re asking why Medicare would deny a claim, the answer almost always sits in one of those buckets, and most denials can be appealed. Read the reason printed on your Medicare Summary Notice; that phrase points you to which bucket applies and to the appeal path that fits.

The Service Was Ruled Not Medically Necessary

Federal law limits Medicare payments to items and services that are “reasonable and necessary” for diagnosing or treating an illness or injury.¹ A service can be legitimate medicine and still be denied if the review concludes it was not appropriate for your diagnosis, exceeded what your condition required, or could have been provided in a less costly setting, such as at home instead of in a hospital.²

What counts as medically necessary is not left entirely to your doctor. CMS publishes National Coverage Determinations that apply program-wide, and where no national rule exists, the Medicare Administrative Contractor for your region can set a Local Coverage Determination that applies in its area only.³ A service ordered without issue in one part of the country can be denied in yours because a different contractor handles claims there. If you get a medical-necessity denial, ask your provider which specific coverage determination triggered it. That gives you a concrete document to argue against.

Services Medicare Does Not Cover

Some categories are excluded from coverage no matter how necessary they are:

Routine physical exams. Medicare pays for the one-time “Welcome to Medicare” preventive visit and an annual wellness visit, but those are risk assessments and prevention plans, not head-to-toe physicals. Extra tests done at a wellness visit can generate a bill.⁴
Most dental care. Cleanings, fillings, extractions, and dentures are generally excluded. Dental work directly tied to certain covered procedures, like clearing an oral infection before a heart valve replacement, organ transplant, chemotherapy, or dialysis, can be covered.⁵
Eyeglasses, contact lenses, routine eye exams, hearing exams for fitting hearing aids, and hearing aids themselves.⁶
Cosmetic surgery performed for appearance rather than to treat illness or injury.
Long-term custodial care, meaning help with bathing, dressing, and eating when you do not also need skilled medical care.²
Care received outside the United States, with narrow exceptions.

Preventive screenings also carry frequency limits. A mammogram, colonoscopy, or prostate screening done more often than the schedule allows will be denied, even though the same test at the correct interval would be covered.¹ Before a screening, confirm with the provider’s office that enough time has passed since your last one.

Experimental or investigational treatments are also excluded. If you join a qualifying clinical trial, Medicare may pay the routine costs of your care, such as office visits, labs, and hospital stays, but not the experimental drug, device, or procedure itself.⁷

Billing, Coding, and Filing Errors

A large share of denials have nothing to do with your care. If the diagnosis code and the procedure code don’t line up, if a code is wrong, if bundled services are billed separately, or if a more expensive service is billed than the one actually performed, the claim gets rejected. Missing information does the same work: an incomplete Medicare number, the wrong date of birth, a missing date of service. Submitting a claim twice produces a duplicate denial on the second try.

These are mostly the provider’s mistakes, but they show up on your Medicare Summary Notice and you are the one who needs to catch them. A call to the billing department fixes many of them without any formal appeal.

Claims also have a hard deadline. Anything submitted more than 12 months after the date of service is denied as untimely, with very limited exceptions.⁸ The clock usually matters to the provider, but if you’re submitting your own claim for a non-participating provider’s services, it applies to you too.

Prior Authorization Was Not Obtained

Some items and services under Original Medicare require prior authorization before they’re delivered. CMS applies this to certain durable medical equipment categories, specific hospital outpatient procedures, and other services with a history of improper payments.⁹ If the provider doesn’t get approval first, Medicare can deny the claim.

Prior authorization is far more common in Medicare Advantage plans, which are run by private insurers. These plans routinely require advance approval for specialist visits, imaging, surgeries, and prescription drugs. Roughly one in six initial Medicare Advantage claims is denied, and a large share of those denials are overturned when challenged. If you’re in Medicare Advantage, checking whether a service needs prior authorization before scheduling it is one of the most effective ways to prevent a surprise bill.

Eligibility and Provider Enrollment Problems

A claim is denied if you were not actually enrolled in Medicare on the date of service. This happens when Part B has not yet started, when there was a gap in coverage, or when enrollment wasn’t processed on time. It also happens when the claim goes to the wrong plan. If you have a Medicare Advantage plan and see an out-of-network provider for non-emergency care, Original Medicare will not cover the service because your coverage runs through the private plan.

The provider’s status matters too. If your doctor, hospital, or supplier isn’t enrolled in Medicare or has let enrollment lapse, the claim is denied. Providers who have formally opted out of Medicare can charge you directly, and Medicare will not reimburse either side.⁶

Another Insurer Should Have Paid First

Medicare is not always the primary payer. Sending a claim to Medicare before the primary insurer has processed it produces a denial. Medicare typically pays second when:

You’re 65 or older and covered by a group health plan through your own or a spouse’s current employment at an employer with 20 or more employees; for beneficiaries under 65 with a disability, the threshold is 100 or more employees.¹⁰
The care relates to a job injury or illness covered by workers’ compensation.
The care stems from an auto accident or other event covered by no-fault or liability insurance.

If you have both Medicare and Medicaid, Medicare pays first and Medicaid picks up remaining eligible costs.¹⁰ Give every provider your complete insurance information so claims go out in the right order.

The Observation Status Trap

One denial pattern is worth calling out on its own because the dollars are so large. If a hospital classifies you as an outpatient receiving “observation services” rather than admitting you as an inpatient, you are technically never admitted, even after multiple nights in a hospital bed. Skilled nursing facility coverage requires a qualifying inpatient stay of at least three consecutive days, and time spent under observation does not count.¹¹

Beneficiaries who spent days in a hospital under observation often assume they’ve met the three-day requirement, then find out at the nursing facility that Medicare won’t cover the stay. The resulting bills can run into the tens of thousands. If a nursing facility stay seems likely, ask directly whether you have been formally admitted as an inpatient. You have the right to appeal a decision classifying a hospital stay as observation, and inpatient-status appeals can reach back to stays as early as January 2009.¹¹

When an Advance Beneficiary Notice Changes Who Owes

If a provider expects Medicare to deny a service that would normally be covered, the provider must give you an Advance Beneficiary Notice of Non-coverage (ABN) before performing it. The ABN explains why Medicare may not pay and asks you to decide: proceed and accept financial responsibility if the claim is denied, or decline the service.¹²

The ABN acts as your financial shield in these situations. If the provider was required to give you one and didn’t, the provider, not you, can be held liable for the denied service. Providers do not have to issue an ABN for services that are never covered, such as routine dental cleanings or cosmetic surgery; for those, you can be billed directly with no advance Medicare notice.¹² The ABN applies only to Original Medicare; Medicare Advantage and Part D use their own notice processes.

How to Appeal a Denied Claim

You have the right to appeal any Medicare coverage or payment denial.¹³ Start with your Medicare Summary Notice. It lists the services billed, what Medicare paid, the specific reason for the denial, and instructions for filing.

The first level is called a redetermination. Circle the denied item on a copy of your MSN, write a short explanation of why you disagree, attach any supporting medical records, and mail it to the Medicare Administrative Contractor that processed the claim. The deadline is 120 days from the date you received the MSN, and there is no minimum dollar amount. If the redetermination goes against you, you have 180 days to request reconsideration by a Qualified Independent Contractor.¹⁴ Most denials that get overturned are resolved at one of these two levels.

Three further levels exist if you need them: a hearing before an Administrative Law Judge (60-day deadline, at least $200 in dispute for 2026), review by the Medicare Appeals Council, and finally federal district court (at least $1,960 in dispute for 2026).¹⁵¹⁶¹⁷ You can authorize a family member, advocate, or attorney to act on your behalf at any level by completing CMS Form 1696.¹⁸

Fast-Track Appeals When Coverage Is Ending

If a hospital, skilled nursing facility, home health agency, or hospice tells you that your Medicare-covered services are ending and you disagree, there’s a faster track through an independent reviewer called a Beneficiary and Family Centered Care Quality Improvement Organization (BFCC-QIO).¹⁹

For a hospital discharge, contact the BFCC-QIO no later than the day you are scheduled to leave. Request the review in time and you can stay in the hospital without cost while you wait, and the BFCC-QIO must decide within one day of getting the necessary information. For a skilled nursing facility, home health, or hospice discharge, act by noon the day before the coverage end date listed on your Notice of Medicare Non-Coverage; the BFCC-QIO decides by the close of business the following day.¹⁹ The hospital must give you a document titled “Important Message from Medicare” that explains these rights and lists the BFCC-QIO phone number. If you didn’t receive it, ask for it immediately.

How to Reduce the Chance of a Denial

Check coverage before the service, not after. Call the number on your Medicare card or sign in to your Medicare account to confirm that a planned procedure, test, or piece of equipment is covered, and ask whether prior authorization is required. If a provider hands you an ABN, read it carefully and ask which option preserves your appeal rights.

Make sure every provider is enrolled in Medicare, and if possible, accepts assignment, which caps what they can charge at the Medicare-approved amount. If you have other insurance alongside Medicare, give every provider the full picture so claims go to the right payer in the right order.

Read each Medicare Summary Notice against your own record of what you received and when. Catching a billing error early often means a phone call to the provider’s billing office resolves it. If the office can’t fix it, a Level 1 appeal within 120 days is free and has a reasonable chance of success.

1
Office of the Law Revision Counsel. 42 U.S. Code 1395y – Exclusions From Coverage and Medicare as Secondary Payer
2
Centers for Medicare & Medicaid Services. Items and Services Not Covered Under Medicare
3
Centers for Medicare & Medicaid Services. Medicare Coverage Determination Process
4
Medicare. Yearly “Wellness” Visits
5
Centers for Medicare & Medicaid Services. Medicare Dental Coverage
6
Medicare. What’s Not Covered?
7
Novitas Solutions. Billing/Coding of Routine Costs
8
Centers for Medicare & Medicaid Services. Transmittal 2140 – Changes to the Time Limits for Filing Medicare Fee-For-Service Claims
9
Centers for Medicare & Medicaid Services. Prior Authorization and Pre-Claim Review Initiatives
10
Centers for Medicare & Medicaid Services. Medicare Secondary Payer
11
Medicare. Skilled Nursing Facility Care
12
Centers for Medicare & Medicaid Services. Advance Beneficiary Notice of Non-coverage Tutorial
13
Medicare. Filing an Appeal
14
U.S. Department of Health & Human Services. Level 2 Appeals: Original Medicare (Parts A & B)
15
eCFR. 42 CFR Part 405 Subpart I – ALJ Hearings
16
eCFR. 42 CFR Part 405 Subpart I – Medicare Appeals Council Review
17
Federal Register. Medicare Program; Medicare Appeals; Adjustment to the Amount in Controversy Threshold Amounts for Calendar Year 2026
18
Centers for Medicare & Medicaid Services. Appointment of Representative
19
Medicare. Fast Appeals