Does Medicare Cover It? A California Guide to What Medicare Pays For

“Does Medicare cover this?” is the single most common question I hear, and it almost always comes up at an inconvenient moment — after a doctor recommends a procedure, or after a bill arrives that nobody expected. The answer is rarely a flat yes or no. It usually depends on why the service is being done.

This page is the starting point for that question. Below you will find the rules Medicare uses to decide what it pays for, followed by detailed guides on the specific services people ask about most. It is educational information only, not medical or financial advice.

The three questions Medicare asks

Almost every coverage decision under Original Medicare comes down to three things:

1. Is it medically necessary? Medicare pays for services needed to diagnose or treat a condition. Care that is cosmetic, elective, or done purely for convenience is generally excluded — LASIK is a classic example.
2. Is it on the preventive list? Medicare covers a defined set of screenings — colonoscopies, mammograms, diabetes screenings, and others. Many of these cost you nothing when the provider accepts assignment.
3. Is it routine care Medicare simply does not include? Routine dental, routine eye exams for glasses, and hearing aids fall outside Original Medicare no matter how necessary they feel.

Preventive vs. diagnostic: the distinction that causes surprise bills

This is worth understanding before any procedure. A preventive service is a screening done when you have no symptoms; many are covered at no cost to you. A diagnostic service investigates or treats something already found, and it carries the normal Part B cost-sharing — typically 20% coinsurance after the deductible.

The catch is that a single appointment can start as one and end as the other. A screening colonoscopy where the doctor finds and removes a polyp is reclassified as diagnostic mid-procedure, which is why people receive a bill for a screening they were told was free. Asking the provider’s office ahead of time what happens in that scenario is the simplest way to avoid the surprise.

What’s covered, at a glance

How your coverage choice changes what you pay

What Medicare covers is a federal benefit and is identical in every state. What differs is the coverage sitting alongside it — and that is where your decisions matter, especially in California, where plan lineups and networks vary county by county.

• With Original Medicare plus a Medicare Supplement (Medigap) policy, your supplement may pick up some or all of the coinsurance, depending on which lettered plan you have.
• With a Medicare Advantage plan, the plan defines your copays, your network, and any referral or prior-authorization steps.

Our comparison of Medicare Advantage and Medicare Supplement lays out the trade-offs, and our overview of how Medicare costs work explains deductibles and coinsurance across Parts A, B, and D. If you are new to all of this, start with the Turning 65 in Southern California guide.

When you can change plans

If reviewing your coverage makes you want to switch, timing matters. Most people can make changes during the Annual Enrollment Period (October 15 – December 7), with changes effective January 1. Medicare Advantage members get one more change during the Medicare Advantage Open Enrollment Period (January 1 – March 31). A new diagnosis or a recommended procedure does not on its own open a special enrollment window, so it pays to think about plan fit ahead of time.

Frequently asked questions

How does Medicare decide what it covers?

Original Medicare generally pays for services that are medically necessary to diagnose or treat a condition, plus a defined list of preventive screenings. Routine care that is not tied to a medical condition — such as routine dental cleanings, eyeglasses for everyday vision, or hearing aids — and elective procedures are generally excluded, no matter how helpful they may be.

What is the difference between a preventive and a diagnostic service?

A preventive service is a screening done when you have no symptoms, and many preventive services are covered at no cost to you when the provider accepts assignment. A diagnostic service investigates or treats a known symptom or finding, and it normally carries the standard Part B cost-sharing. A screening can be reclassified as diagnostic during the visit — for example, when a polyp is removed during a screening colonoscopy.

Does Medicare Advantage cover the same services as Original Medicare?

Yes. Medicare Advantage plans must cover everything Original Medicare covers. What differs is the cost-sharing, the provider network, and any prior-authorization or referral rules the plan sets. Many plans also add limited extra benefits, such as dental or vision allowances, that Original Medicare does not include.

Are Medicare coverage rules different in California?

No. What Medicare covers is a federal benefit and is the same in every state. What varies by California county is which Medicare Advantage and Part D plans are offered, which providers are in network, and Medigap pricing and rules — including California’s Birthday Rule for existing Medigap policyholders.

Sources & official references

• Medicare.gov — Is my test, item, or service covered?
• Medicare.gov — Medicare costs
• California Health Advocates (HICAP) — free Medicare counseling

This is a non-government resource and is not affiliated with or endorsed by Medicare.gov or the federal Medicare program. An independent broker may not offer every plan available in your area. For all of your options, contact Medicare.gov, 1-800-MEDICARE, or your local State Health Insurance Assistance Program (SHIP) — in California, the Health Insurance Counseling and Advocacy Program (HICAP). Last reviewed: August 2026. Confirm current-year deductibles, coinsurance, and approved amounts with Medicare.gov or your plan. Educational information only, not medical, financial, or individualized advice.