If you're counting on Medicare to cover your healthcare costs after you turn 65, you may be in for a nasty financial surprise that could derail your retirement.

Even for seniors who purchase optional Medicare supplementary plans, healthcare costs could be as high as $350,000 for a couple during their retirement years. And, that scary number doesn't even factor in long-term care costs and is based on the current benefits system. Costs could be much greater if Congress changes Medicare. 

Why does Medicare leave seniors on the hook for so much healthcare spending? Here are three big reasons you can't count on Medicare to keep you out of the poorhouse if you have costly health issues after age 65. 

Doctor and patient discussing healthcare

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1. Medicare typically won't pay for long-term care coverage

Around 70% of seniors will need long-term care during their lifetimes, and seniors will need care for an average of three years. Unfortunately, Medicare will likely pay nothing toward this care.

Medicare covers only skilled nursing care and provides this coverage only if you have a qualifying hospital stay . Skilled nursing care is care a medical professional must perform, such as changing surgical bandages. For most seniors, it is custodial care -- or routine help with activities of daily living like bathing or preparing food -- that necessitates long-term care. 

Custodial care isn't covered by Medicare, Medicare Advantage, or Medigap policies. And it costs an average of $4,099 monthly for a home health aide and $7,148 for a semi-private room in a nursing home, according to the 2017 Genworth Cost of Care survey. If you don't want to pay these costs out-of-pocket, your only options are to buy a long-term care insurance policy or to qualify for Medicaid coverage.

While Medicaid does pay for nursing home care, you'll have to impoverish yourself to qualify because Medicaid's benefits are means-tested -- or you'll have to create an asset protection plan, which usually requires help from an attorney. 

2. Medicare has high coinsurance costs

Medicare does not provide 100% coverage for healthcare services, even when it does cover your care.

If you require hospital or inpatient care -- which is covered for by Medicare Part A -- you'll need to pay a $1,316 deductible (as of 2017). If you're in the hospital for more than 60 days, you'll owe a coinsurance cost of $329 per day, or more depending on the length of your stay. 

Medicare Part B -- which covers routine care like doctor visits and durable medical equipment -- also charges coinsurance costs of 20% for services. And Medicare Part D plans -- which cover prescriptions -- require costly premiums, may have deductibles up to $400, and charge coinsurance costs based on what tier a covered drug falls into.  

While many Medicare beneficiaries shop for Medicare Advantage plans or Medigap plans to pay part of their coinsurance costs, this means paying extra premiums -- and most supplementary plans still require beneficiaries to pick up at least some of their care expenses. 

3. The future of Medicare benefits is uncertain

Medicare is a popular program, but it's financially troubled. A 2017 report from the Medicare Board of Trustees' revealed the Hospital Insurance Trust -- which covers Medicare part A expenses -- is expected to be depleted by 2029. 

And, while the trust funds for Medicare Parts B and D are expected to be adequately financed, these programs are financed by premium income and general revenue income. Financing will need to increase at a faster pace than economic growth to keep up with projected expenditures.

The Trustees warn Medicare expenditures are expected to grow at a faster pace than both the economy and aggregate workers' earnings, necessitating "substantial steps" to be taken to address financial challenges with the program.  

Unfortunately, recent budget proposals put forth by the Republican Congress do not include comprehensive plans to address shortfalls but instead cut projected Medicare spending by as much as $473 billion over the next decade. 

A substantial cut in funding to Medicare is likely to exacerbate problems and necessitate reforms that could result in a benefits cut to covered seniors. 

What can you do?

To make sure healthcare expenditures don't derail your retirement, your best option is to learn about Medicare coverage limitations as early as possible and make plans to overcome them.

Consider purchasing a long-term care insurance policy and supplementary Medicare coverage, invest in a health savings account to pay expenditures with tax-free funds, and work with an elder law attorney to determine if you should have an asset protection plan in place so you can qualify for Medicaid when you need costly care. 

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