What is balance billing?

The definition

Balance billing is when a healthcare provider bills you for the difference between what they charged and what your insurance paid, after the insurer applied its negotiated rate.

For in-network providers, balance billing is normally prohibited by the contract between the provider and the insurer — they agreed to accept the negotiated rate as payment in full. Out-of-network providers have no such agreement, so historically they could charge whatever they wanted and send you the balance.

A concrete example

You go to an in-network hospital for surgery. The anesthesiologist on call happens to be out-of-network.

• Anesthesiologist's charge: $3,500
• Insurer's allowed amount: $800
• Insurer pays: $640 (after 20% coinsurance)
• Pre-NSA: anesthesiologist bills you $2,860 (the balance)
• Post-NSA: you owe only your in-network cost-share (roughly the $160 coinsurance). The rest gets resolved between the anesthesiologist and the insurer through arbitration.

When balance billing is illegal under federal law

The federal No Surprises Act bans balance billing for:

• Emergency services at any facility — including ER visits, stabilisation, and post-stabilisation care
• Non-emergency services from out-of-network providers at in-network facilities (anesthesia, radiology, pathology, hospitalist, emergency medicine, lab services, etc.)
• Air ambulance services

Where the gap still exists

• Ground ambulance services — the federal NSA does not cover them. Some states have closed this gap; many have not.
• Non-emergency, planned care where you voluntarily chose an out-of-network provider and were not at an in-network facility.
• Situations where you signed a valid notice-and-consent waiver at least 72 hours before scheduled care.

How to spot a balance-billing violation

1. Compare the provider's bill to the EOB. If the provider is charging more than the “patient responsibility” line on the EOB, that's a red flag.
2. Check whether the facility was in-network on the date of service.
3. Check whether the services were emergency, or were ancillary services at an in-network facility.
4. If the facility was in-network and the service falls under NSA categories, the extra charge is almost certainly illegal.

What to do

Send the provider a written dispute citing the No Surprises Act and the specific service. If they don't correct the bill within 30 days, file a complaint at cms.gov/nosurprises or by phone at 1-800-985-3059. Providers face civil penalties of up to $10,000 per violation, so most correct the bill rather than fight.

This guide is a definitional explainer, not legal advice. For complex cases or large bills, consult a healthcare-billing attorney or your state's Consumer Assistance Program.