Why it matters: These bills can be devastating to patients, even those who have some savings and a steady source of income.

The big picture: The study used an Optum database to look at patients covered by a large commercial insurer who received either inpatient or emergency care at hospitals covered by their insurance, and got a bill for out-of-network care.

These bills were probably unexpected. Patients often assume that the care they receive at an in-network hospital will all be covered by their insurer.

Yes, but: Most hospitals don’t produce out-of-network bills this often. The results are skewed by serial offenders.