Reference book using a five-digit numerical system to identify codes and describe medical, surgical, and diagnostic procedures and services established, maintained, and copyrighted by the American Medical Association. It has been selected for use under the Health Insurance Portability and Accountability Act (HIPAA) for noninstitutional and nondental professional transactions. This code system is used by physicians and outpatient facilities to identify the type and level of service given to each patient when submitting insurance claims to insurance companies for payment. In the Medicare program, this system is referred to as Level 1 codes .