Question: Please verify the correct way to bill for a nurse practitioner. California Subscriber Answer: You should use HCPCS Level II modifier SA for supervised nurse practitioner services when the insurer requires the modifier. Some Medicaid programs such as Medi-Cal require you to apply Modifier SA to all NP services which are submitted under the physician provider number. Example: Use modifier SA for incident-to services that are billed under the supervising physician's NPI number. The modifier tells the insurer that the NP rather than the physician provided the service and the physician supervised the NP. Incident-to services are paid at 100 percent of the Medicare Physician Fee Schedule amount. Insurers may apply a reduction to services billed directly under the NP's NPI. Typical reduction is 15 to 20 percent. Never heard of modifier SA? HCPCS Level II created the HIPAA compliant modifier to standardize NP reporting. For instance, prior to February 2009, Medi-Cal previously used modifier YT (Nurse practitioner service [multiple modifiers]).