About 1 in 6 hospital stays for patients enrolled in large employer health plans results in out-of-network bills, which tend to be costly and not fully covered by insurance, according to an analysis released Monday. 

The Kaiser Family Foundation (KFF) study of medical bills from large employer plans found that 18 percent of inpatient admissions result in out-of-network claims. 

Even when enrollees choose in-network facilities, KFF found, 15 percent of admissions include a bill from an out-of-network provider. 

The bills arise when a patient goes to an in-network hospital, but receives care from a medical professional who is out-of-network.

Doctors or facilities that don’t have contracts with certain insurers are considered out-of-network. 

Patients who go out-of-network for care — sometimes because their insurer does not contract with a needed doctor or facility — might not have their services covered by insurance.

However, in-network visits and services are typically covered by insurers to some extent. 

Researchers found that the problem is more common with anesthesia or pathology claims. 

In these instances, it is “doubtful” that enrollees could anticipate or control their use of out-of-network providers, the analysis says. 

The analysis found that both inpatient and outpatient services that include an emergency room claim have a higher chance of receiving an out-of-network bill, even when they use in-network facilities. 

“Even when the facility is in-network, the professional services may not be and the patient often is not in a position to direct their care,” the study says.

The analysis also found that inpatient treatments for mental health and substance abuse — such as an overnight stay in a psychiatric facility — have a much higher chance of including claims from out-of-network providers.

Of the inpatient admissions that included a claim from an out-of-network provider, 33.5 percent were for psychological and substance abuse services, according to KFF’s review. 

“These high rates raise questions about the availability of services within plan networks, and perhaps of the quality of the network providers,” the study says. 

“In some cases, services may be urgent and patients may not have in-network options available.”

Those receiving outpatient mental health service — such as a session with a therapist — are also more likely to have a claim from an out-of-network provider, the analysis shows. 

Several states have taken action to limit out-of-network costs to consumers. 

In California, KFF notes, a recently adopted law protects some insured patients from high medical bills from out-of-network providers if they receive care at an in-network facility.