Insured patients are getting more surprise medical bills, and more expensive surprise medical bills, even as Congress attempts to tackle this problem.

According to a new study, 42.8 percent of emergency department patients now receive surprise medical bills for out-of-network services, up from 32.3 percent in 2010, with those surprise bills rising from a mean of $220 in 2010 to $628 in 2016.

Patients experience similar frustrations with inpatient visits, with surprise bills for out-of-network services arriving in the mailboxes of 42 percent of patients in 2016, up from 26.3 percent in 2010.  Those surprise bills rose from a mean of $804 in 2010 to $2040 in 2016.

The most common reason for a surprise medical bill?  Ambulance service, with more than 85 percent of patients who arrive at a hospital emergency department via ambulance receiving a surprise bill and more than 81 percent of patients served on an inpatient basis who use ambulances receiving such bills.

Learn more about how the surprise medical bill problem continues to grow, despite state and federal efforts to address it, in the JAMA Internal Medicine study “Assessment of Out-of-Network Billing for Privately Insured Patients Receiving Care in In-Network Hospitals.”