State opt-out
When a state opts out, its governor removes Medicare's requirement that CRNAs be supervised by a physician. CRNAs in that state can still bill Medicare. It only changes the supervision rule. See the 27 opt-out states.
Opting out of Medicare
An individual provider can choose to leave Medicare altogether. They then sign private contracts with Medicare patients and bill them directly. This lasts at least two years and has strict rules. It's rarely a good idea for anesthesia, because surgical patients often don't know who their anesthesia provider will be in advance.
The takeaway
Most CRNA groups want to be in an opt-out state and enrolled in Medicare. Very few should opt out of Medicare themselves. Talk to a healthcare lawyer before considering it.
This article is general information, not billing, legal or tax advice. Payer rules change; always check current guidance.