Improving Access to Mental Health Act of 2018 This bill increases the Medicare reimbursement rate for clinical social worker services. The bill excludes clinical social worker services from the prospective payment system in which predetermined amounts form the basis for payment under Medicare. Additionally, the bill alters the definition of "clinical social worker services" as it relates to Medicare. Under current law, "clinical social worker services": (1) include services performed for the diagnosis and treatment of mental illnesses, and (2) exclude services furnished to an inpatient of a skilled nursing facility as a condition of the facility's participation in the Medicare program. The bill repeals these provisions and instead specifies that such services include certain types of health behavior assessment and intervention.