CO-50 — Not Medically Necessary

CO-50 is the most common and most appealable behavioral health denial. The payer says the service was not medically necessary under its criteria. Under MHPAEA (29 CFR § 2590.712), the medical necessity criteria applied to mental health services must be no more restrictive than those for analogous medical/surgical services. Request the payer's criteria in writing, include clinical notes supporting necessity, and invoke MHPAEA comparative analysis. CO-50 denials have the highest MHPAEA relevance of any code.