Abbott entered the Therapeutic Drug Monitoring Market in 1981 with the introduction of the TDx. Still in use today, this system uses as technology known as FPIA (Fluorescence Polarization Immunoassay) to measure serum drug concentration in a patient. Therapeutic Drugs, used in the treatment of different diseases, often exhibit a narrow therapeutic range requiring close monitoring so the patient receives the optimal amount of drug to be effective. By virtue of its design and engineering, the TDx has revolutionized the TDM market, setting the standard for ease of use, quality of results and reliability. During the 1980's, the TDx menu was expanded beyond therapeutic drugs to include tests for endocrine function, hormones, toxicology, abused drugs and immunosupressants allowing for consolidation of testing on one automated system.