When a therapist treats multiple patients simultaneously (who are not related), the correct code is CPT 90853 (Group Psychotherapy).
Untimed Code Rules
Unlike individual therapy, 90853 is an untimed code. Whether the group session lasts 45 minutes or 90 minutes, you only bill 1 unit of 90853 per patient.
Medicare typically expects group sizes to be reasonable (usually between 4 to 12 patients). If a group exceeds 12 patients, it often borders on a "psychoeducational class" rather than active group therapy, which may not be covered.
Medical Necessity
Each patient in the group must have their own individualized medical record entry documenting their specific participation, reaction, and progress toward their own treatment plan goals during that group session.
NCCI Edits and Modifier 59
If a patient attends group therapy (90853) in the morning, and has a severe breakdown requiring an individual therapy session (90834) in the afternoon, you must append Modifier 59 to the group therapy code to bypass the mutually exclusive edit.