If you're thinking about having basic Medicare without any supplemental coverage, experts have a message for you: Don't. With deductibles, copays, coinsurance and — this is a biggie — no out-of-pocket maximum, the program has a variety of costs that make having no backup insurance a huge financial risk. "All it takes is one big hospital stay and you could be out tens of thousands of dollars," said certified financial planner Ken Waltzer, co-founder and managing partner of KCS Wealth Advisory in Los Angeles. "Even if you don't face a big event like that, the smaller ongoing expenses can really add up."

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Roughly 51 million older adults are enrolled in Medicare. Beneficiaries can stick with just Part A (hospital coverage) and Part B (outpatient care) or get their benefits delivered through an Advantage Plan. Part A typically costs nothing, and Part B has a monthly standard premium of $135.50 for 2019. About one-third of beneficiaries use Advantage Plans, which offer out-of-pocket maximums and often include dental and vision coverage or other benefits. Those plans also typically provide Part D prescription drug coverage. The average premium for an Advantage Plan is $29, according to the Kaiser Family Foundation. Some, however, come with no premium. The other two-thirds of recipients choose to go with original Medicare (and pair it with a standalone Part D prescription plan).

Supplemental coverage among beneficiaries with basic Medicare Employer-sponsored insurance 30% Medigap 29% Medicaid 22% Other coverage 1% No supplemental coverage 19%

In that situation, unless you have some type of employer-sponsored insurance or you get extra coverage from Medicaid, the option for mitigating your out-of-pocket costs is a Medigap policy. Those supplemental policies, which are sold by private insurance companies, either fully or partially cover cost-sharing aspects of Parts A and B, including deductibles, copays and coinsurance. They also limit what you'll pay out of pocket each year. Yet about 19%, or 6.1 million, who stick with basic Medicare have no extra coverage, according to a 2018 study from the Henry J. Kaiser Family Foundation. That's risky, experts say. While Part A is free for most beneficiaries, it comes with a $1,364 deductible per benefit period. And although Part B comes with a low $185 per-year deductible, you typically pay 20% of the remainder for most doctor services — including while you're a hospital inpatient — as well as outpatient therapy and durable medical equipment such as wheelchairs or walkers.

"That 20% is after your deductible, and there's no limit to how much you'd pay out of pocket," said Elizabeth Gavino, founder of Lewin & Gavino in New York and an independent broker and general agent for Medicare plans. "If you have a heart attack, need multiple surgeries and hospital visits, you could literally end up bankrupt," she said. Heart bypass surgery can cost more than $100,000, according to Statista. Heart-valve replacement can run upwards of $170,000. For illustration purposes only: If all those charges were delivered through Part B, your 20% share would be at least $20,000 for the bypass and $34,000 for the valve replacement. A 65-year-old male will pay anywhere from $126 to $464 monthly for a Medigap policy, according to the American Association for Medicare Supplement Insurance. For 65-year-old women, the range is $118 to $464. More from Personal Finance:

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Here's how your income stacks up against your neighbor's When you first enroll in Medicare Part B, you get six months to purchase a Medigap policy without an insurance company nosing through your health history and deciding whether to insure you. After that, unless your state allows special exceptions, you have to go through medical underwriting. While a number of companies offer Medigap insurance, they can only offer policies from a list of about 10 standardized plans. Each is simply assigned a letter: A, B, C, D, F, G, K, L, M and N. Some states also offer a high-deductible version of Plan F. This standardization means that, say, Plan A at one insurance company is the same as Plan A at another. Be aware, however, not every plan is available in all states. They also do not cover any costs associated with Part D prescription drug coverage.

Everyone should at least consult with a Medicare agent to understand what the results of their choices could be. Carolyn McClanahan CFP and founder of Life Planning Partners

The plans differ on what is covered. For instance, Plan F pays your Medicare deductibles while Plan A does not. Or, some plans cover 100 percent of your deductibles and co-insurance, while others might only pay a portion of those costs. The Centers for Medicare and Medicaid Services has a chart on its website that shows the differences. You also can use the agency's search tool to find available plans in your ZIP code. It's worth noting that beginning Jan. 1, 2020, Medigap plans that are newly sold won't be allowed to cover the Part B deductible. If you already have a policy that provides that coverage, though, it will continue.