What is medical necessity? A Medicare glossary

Short answer

Medical necessity is Medicare’s rule that services must be “reasonable and necessary” for diagnosing or treating your medical condition — and must meet accepted standards of care. If a service doesn’t meet that bar, Medicare won’t pay for it. The rule applies to every Original Medicare claim and, in a modified form, to Medicare Advantage plans too. It’s the single most common reason for a Medicare denial.

What Medicare means by “medical necessity”

Medicare’s official definition comes from a single line in the Social Security Act, §1862(a)(1)(A). It says Medicare will not pay for items or services that aren’t “reasonable and necessary for the diagnosis or treatment of illness or injury or to improve the functioning of a malformed body member.”

Translated into plain English: Medicare pays for medically appropriate care that meets the accepted standards of medicine for your specific condition. It won’t pay for care that is experimental, unproven, cosmetic, or delivered in a more expensive setting than the situation requires.

The federal Medicare glossary uses simpler language. Medically necessary means “health-care services or supplies needed to diagnose or treat an illness, injury, condition, disease, or its symptoms and that meet accepted standards of medicine.”

Both definitions point to the same idea. The care has to fit your situation, and it has to fit what mainstream medicine considers appropriate for that situation.

The four questions Medicare asks

Every time Medicare reviews a claim for coverage, it’s really answering four questions. If any one of them lands on “no,” the service can be denied.

1. Is this for a condition Medicare covers?

Medicare covers medical conditions — not lifestyle preferences, not cosmetic issues, and not services provided for someone else’s convenience. A hospital bed to help you recover from a stroke qualifies. The same bed because it’s more comfortable does not.

2. Does the treatment meet accepted standards of care?

Medicare looks at whether medical professionals in your specialty would consider this treatment appropriate for your diagnosis. Two tools do most of the work here: National Coverage Determinations (NCDs) are Medicare’s national rules for whether a service is covered at all. Local Coverage Determinations (LCDs) are rules from the Medicare contractor for your region, and they usually spell out the exact clinical criteria — the diagnosis codes, prior treatments, and lab values that make a service coverable.

3. Is it the right setting, frequency, and duration?

Medicare covers care at the least intensive level that will work. If a wound can be treated safely at home, Medicare won’t cover an inpatient hospital admission for the same wound. The same logic applies to how often a service is delivered — physical therapy three times a week for four weeks might qualify; the same therapy every day for six months usually would not, unless the record shows why.

4. Is it supported by the medical record?

This is where most people are surprised. Medicare will deny a service that was genuinely needed if the doctor’s chart doesn’t say so. The medical record has to describe your condition, why the specific treatment was chosen, what the alternatives were, and how the care fits the LCD or NCD criteria.

How doctors document medical necessity

The paperwork that turns a doctor’s clinical judgment into a Medicare-covered service usually includes several pieces.

A standing written order (SWO) replaces the older “Certificate of Medical Necessity” form for most durable medical equipment. Since Medicare retired the CMN process on January 1, 2023, the SWO plus a face-to-face encounter and the physician’s clinic note is what Medicare wants to see for equipment like wheelchairs, oxygen, and hospital beds.

Clinic notes are the day-to-day documentation from every visit. For a Medicare claim, the note needs to describe your diagnosis, the reasoning behind the recommended treatment, and — for equipment — how the item will improve your condition. A one-line “patient needs walker” almost never survives Medicare review.

Diagnosis codes (ICD-10) must match the LCD or NCD list for the service being billed. If the code doesn’t match, the claim can be denied even when the care was appropriate.

A Letter of Medical Necessity (LMN) is sometimes requested for higher-cost equipment, non-standard therapies, or when a Medicare Advantage plan asks for extra documentation. In the Medicare context, an LMN is the doctor’s written explanation of why a specific item or service is needed for the patient’s condition — how long they’ve had it, what’s been tried, and what the item will do. (This is different from the tax-related LMNs used for health savings accounts. In Medicare, the LMN is about coverage, not tax deductions.)

Weak documentation is one of the most common reasons Medicare denies a service that was clinically appropriate. The care was needed; the paperwork just didn’t prove it.

Where you’ll see “medical necessity” in your Medicare paperwork

Three documents will bring the phrase into your life at different moments.

Advance Beneficiary Notice of Noncoverage (ABN — CMS-R-131). If a provider believes Medicare will deny a service as not medically necessary, they must give you an ABN before the service. It’s a two-page federal form that names the service, explains why the provider thinks Medicare won’t cover it, and asks you to choose whether to receive the service anyway. If you sign and receive the service, you agree to be financially responsible if Medicare denies the claim. A provider cannot bill you for a denied service without an ABN signed in advance. If you get a bill for a service you never received an ABN for, that’s a strong signal to appeal.

Medicare Summary Notice (MSN). Original Medicare mails you a summary every three months (or you can view it online through your MyMedicare account). If a claim was denied for medical necessity, the MSN will use language like “not medically reasonable and necessary” and give a denial code. The MSN also tells you the deadline to appeal — usually 120 days from the date on the notice.

Denial letter or Explanation of Benefits (EOB). Medicare Advantage plans send an EOB after each service. If the plan denied a service or reduced payment for lack of medical necessity, the EOB will name the coverage rule (often a specific LCD or plan medical policy) and describe your appeal rights.

When Medicare denies for lack of medical necessity

A “not medically necessary” denial can be appealed. The Medicare appeals process for Part A and Part B claims has five levels.

Level 1 — Redetermination (by the Medicare Administrative Contractor). You file within 120 days of the denial. No dollar minimum applies. The contractor re-reviews the claim, usually with the same medical policy but sometimes new evidence you add.

Level 2 — Reconsideration (by a Qualified Independent Contractor). You file within 180 days of the Level 1 decision. This is your first look by a reviewer independent of the original decision-maker. Adding a Letter of Medical Necessity or an updated clinic note at this level often changes the outcome.

Level 3 — Administrative Law Judge (ALJ) hearing at the Office of Medicare Hearings and Appeals. You file within 60 days of the Level 2 decision. The 2026 minimum amount in controversy (AIC) is $200 — the total denied charges have to reach at least that much to qualify for an ALJ hearing. Hearings are usually by phone or video.

Level 4 — Medicare Appeals Council review by the Departmental Appeals Board. You file within 60 days of the ALJ decision. No dollar minimum.

Level 5 — Federal district court. You file within 60 days of the Appeals Council decision. The 2026 AIC for federal court review is $1,960. Most people never reach this level.

Medicare Advantage plans and Part D prescription drug plans have their own appeal ladders that mirror this structure, with different deadlines (60 days at each level for Part D, for example).

You can appeal on your own, but a Medicare patient advocate can help you gather the right clinical documentation, write the medical-necessity argument, and track the deadlines. Baba’s team has walked hundreds of families through denied claims — often the difference between a denial and an approval is a two-paragraph letter that connects the doctor’s clinical judgment to the LCD language Medicare uses.

How Baba helps

• Translates coverage rules into plain English. Our advocates read the LCD or plan medical policy that triggered the denial and explain — in language you can act on — what Medicare wanted the record to show.
• Reviews your paperwork before you sign an ABN. If you’re being asked to sign an Advance Beneficiary Notice, we’ll help you weigh whether the service is likely to be covered before you accept financial risk.
• Builds the medical-necessity argument for your appeal. We work with your doctor to get the clinic note, diagnosis codes, and Letter of Medical Necessity aligned with Medicare’s specific standard for your service.
• Tracks every deadline. Missing a 120-day redetermination window is one of the most common reasons appeals fail. We keep the calendar so you don’t have to.
• Escalates through the five levels. Most denials resolve at Level 1 or Level 2. If yours doesn’t, we prepare the file for ALJ hearing, including the AIC calculation and the clinical exhibits.
• Speaks the acronyms. LCD, NCD, ABN, MSN, MAC, QIC, ALJ — our team lives in this vocabulary so you don’t have to learn it in a crisis.

Call (855) 765-9011 or schedule a call to talk to a Baba advocate about a Medicare denial or a pending service.

Frequently asked questions

Is “medically necessary” the same as “reasonable and necessary”?

Yes. “Reasonable and necessary” is the statutory phrase from the Social Security Act. “Medically necessary” is the everyday translation used across Medicare’s glossary, patient notices, and denial letters. They mean the same thing.

Who decides whether a service is medically necessary?

The Medicare Administrative Contractor (MAC) for your region makes the first decision on Original Medicare claims, using national NCDs and local LCDs. Medicare Advantage plans decide for their own members using CMS rules plus the plan’s medical policies. If you disagree, the appeals process moves the decision to independent reviewers.

What does medical necessity mean if I have Medicare Advantage?

Medicare Advantage plans must cover everything Original Medicare covers and follow the same medical-necessity standard for Part A and Part B services. Plans can add extra rules — prior authorization, in-network requirements, step-therapy — but they can’t be stricter about what counts as medically necessary than Original Medicare.

Can Medicare deny a service my doctor said I needed?

Yes. A doctor’s recommendation is essential but not sufficient. Medicare also needs the service to fit the coverage rule, the clinical criteria in the LCD or NCD, and the documentation in your record. If any of those pieces are missing, Medicare can deny even a service your doctor believes is necessary.

How do I prove medical necessity for a service that was denied?

The strongest evidence is a Letter of Medical Necessity from your treating physician that ties your specific diagnosis to the exact coverage criteria in the applicable LCD or NCD. Add updated clinic notes, test results that support the diagnosis, and — where relevant — records of prior treatments that didn’t work. A Medicare patient advocate can help assemble the package.

Do I have to sign the ABN?

You can decline to sign, but the provider may then choose not to perform the service. If you sign and check the option to receive the service, you accept that you’ll pay if Medicare denies the claim. Signing does not waive your right to appeal — you can still challenge the denial after the service.

This information is for educational purposes and should not substitute for professional guidance. Healthcare coverage details vary by individual plan. Consult with Baba’s support team or your insurance provider for information specific to your situation.