The study raises an important point about proposed Medicare for All plans: they would be significantly more generous than the benefits under the current system. "Current Medicare-for-all proposals in Congress and from presidential candidates would largely eliminate out-of-pocket costs for premiums and patient cost sharing, including for people now covered under Medicare," the report notes.

The additional cost of expanding both benefits and coverage as part of Medicare for All is a big part of why the proposals are controversial. As candidates are urged to show their math, the sticker shock that may come with a truly universal, single-payer system has been used to argue that voters are not ready for change on such a large scale. On the other hand, proponents argue that once the current system is replaced, overall health care costs will be lower.

Spending varies by type of beneficiary

The KFF report outlines how out-of-pocket costs can vary significantly for different types of beneficiaries. Besides the question of age, differences arose in other areas as well:

Out-of-pocket spending by women in traditional Medicare was higher than out-of-pocket spending by men ($5,748 versus $5,104).

Beneficiaries with poorer health, those with multiple chronic conditions, and those with any inpatient hospital use faced higher out-of-pocket costs than the average traditional Medicare beneficiary. "Beneficiaries with at least one inpatient stay in 2016 spent $7,613 out of pocket, on average, compared to $5,044 among those without an inpatient stay," the report said.

Medicare supplemental plans made a difference: those with no supplemental insurance spent more out of pocket than beneficiaries with some type of supplemental coverage. "In 2016, nearly one in five (6.1 million) Medicare beneficiaries did not have any source of supplemental coverage, which placed them at greater risk of incurring high medical expenses. People without any source of supplemental coverage were also more likely to have modest incomes and be ages 85 or older," the report said.

The unsettled future

The KFF study may lend ammunition to those who argue that switching to a Medicare for All system would simply bring too much change and too much uncertainty around funding. A recent study by the Committee for a Responsible Federal Budget painted a picture of Medicare for All being potentially unaffordable under several scenarios.

In addition, a new Health Affairs report also raises questions about the affordability of the Medicare system currently in place. That study found that more than half (54 percent) of seriously ill Medicare beneficiaries said they'd experienced financial hardships due to high costs of prescription drugs and hospital services.

The KFF analysis noted that fixing the current affordability issues with Medicare will require more government spending, aside from any expansion that may happen in the future.

"The fact that traditional Medicare does not have an annual out-of-pocket limit and does not cover certain services that older adults are more likely to need may undermine the financial security that Medicare provides, especially for people with significant needs and limited income," the study said. "Addressing these gaps would help to alleviate the financial burden of health care for people with Medicare, although doing so would also increase federal spending and taxes."

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