Four specialties that are often out-of-network — anesthesiologists, pathologists, radiologists and assistant surgeons — raise employer insurance spending by 3.4%, according to a new study in Health Affairs.

Why it matters: Surprise medical bills are not only unaffordable for the patients who receive them, but also inflate everyone else's premiums.

Between the lines: Providers are more likely to be out-of-network at for-profit hospitals and those located in concentrated markets, the study found.

These four specialties are providers that patients don't choose. They ultimately get paid several times more than they'd make from treating a Medicare patient.

In contrast, orthopedists performing knee surgeries — which patients do have the ability to choose — were paid 164% of Medicare rates, on average.

The study's authors argue that these four specialties can use the threat of billing patients directly to gain leverage in negotiations with insurers.

By the numbers: If the payment rates for these four specialties were reduced to 164% of Medicare, total physician spending among privately insured patients would be reduced by 13.4%, or $40 billion a year for people with employer coverage, the study found.

That's exactly why providers are fighting so hard against Congress' efforts to include a benchmark payment rate for out-of-network care as part of a surprise billing solution.

Whatever they spend in lobbying is, comparatively, chump change.

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