CPT Code 99355: Prolonged office service, each additional 30 min

When to Use CPT 99355

Use CPT code 99355 when billing for prolonged office service, each additional 30 min. This code is used by licensed therapists (LCSW, LPC, MFT, Psychologist) for outpatient sessions.

Adding CPT 99355 to a Superbill

When generating a superbill with CPT 99355, include the ICD-10 diagnosis code, date of service, fee charged, amount paid, and your NPI number. Superbilled pre-fills common therapy CPT codes automatically.

Common ICD-10 Pairings

These diagnosis codes are frequently used alongside CPT 99355:

Documentation Requirements

Document each additional 30-minute increment of prolonged time with clinical justification for the extended encounter.

Common Denial Reasons

• ✗ Billed without 99354 as the primary prolonged service code
• ✗ Additional time not clearly documented
• ✗ Total prolonged time exceeds clinical justification

Modifier Notes

Add-on code — must be billed with 99354.

Reimbursement Expectations

OON reimbursement typically 45-60% of UCR. Expect $23-$60 per additional 30 minutes.