Wiki Individual therapy

The CPT books gives you the answer, in the notes under 90837.

"Use the appropriate prolonged service code (99354, 99355, 99356, 99357) for psychotherapy services not performed with an E&M service of 90 minutes or longer face-to-face with the patient."

Remember that psychotherapy is not routinely medically necessary for more than 45 minutes. In other words, sessions of longer than 45 minutes (with the extra 15 minutes for paperwork/clinician time) should be more of an exception than the norm.
 
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