While in-network providers agree with the plan on how much they're going to charge for services, all bets are off when out-of-network providers enter the picture. (Photo: Getty)

One factor that drives up the cost of care for those who have health insurance—even employer-provided coverage—is the presence of care from out-of-network providers. And according to a new Kaiser Family Foundation analysis, almost one in five inpatient hospital admissions—18 percent—comes with bills from providers not in the health plan's networks, which can expose patients to higher cost-sharing and perhaps even more bills from providers.

According to the analysis, it doesn't necessarily matter that workers choose in-network facilities, since 15 percent of admissions include a bill from an out-of-network provider such as a surgeon or anesthesiologist.

And since these providers can charge enrollees more than their plans are willing to cover, that means those patients could be on the hook for high out-of-pocket expenses, thanks to a practice known as “balance billing,” and can bring “surprise” medical bills if a patient didn't intend to get care from someone out of network. In addition, says the report, health insurance plans also hold the patient responsible for a greater portion of out-of-network claims.