Medicare patients receiving care at the nation’s 1,300 “critical access” hospitals pay far more out-of-pocket than those who are treated at other acute care facilities, according to a recent analysis by the Wall Street Journal.

Read: Are health care providers setting costs based on coverage?

The analysis found astonishing differences in co-pays charged to beneficiaries at regular hospitals and critical care facilities, which are located in rural areas at least 35 miles from the nearest hospital, are supposed to have no more than 25 beds for inpatient care, and are not supposed to keep patients for more than 96 hours at a time.

Some of the big differences the analysis found:

• Medicare patients at critical access hospitals were hit with an average copay of $1,926 for outpatient hernia surgery in 2013, compared to $562 at other hospitals.

• Medicare patients at critical access hospitals paid an average of $2,272 for bunion surgery, compared to an average of $780 elsewhere.

• Medicare patients at critical access hospitals paid an average of $840 for colonoscopies, compared to $270 elsewhere.

What’s up with the big differences?

Medicare requires patients at critical care facilities to pay 20 percent of the total hospital cost, while elsewhere it simply mandates they pay a certain percentage of the fee that it has negotiated with hospitals.

Those fees, which are set in stone, are typically far lower than what critical access facilities charge for services.

While Medicare pledges to pay 101 percent of “reasonable costs” charged by critical access facilities, the definition of “reasonable” is obviously somewhat elastic.

Indeed, according to a recent study by the Medicare Payment Advisory Commission found that non-critical access hospitals’ margins on Medicare patients was -5.4 percent. But when critical access hospitals were included in the overall sample, average hospital margins rose to 1.2 percent.

According to the Wall Street Journal, many critical access facilities are found in the Midwest, while some densely populated states, such as New Jersey, Connecticut, Rhode Island, Delaware and Maryland, have none.