Therapeutic drug monitoring (TDM)6 of certain drugs with a narrow therapeutic index significantly improves patient outcome. For example, the quantification of immunosuppressive drugs in samples from patients after organ transplantation is an essential prerequisite for the prevention of both adverse drug reactions and rejection events. The need for accurate, precise, and standardized measurement of drugs presents a major challenge for clinical laboratories and the diagnostics industry. A plethora of different techniques have been developed in the past to meet these requirements. Currently, liquid chromatography–tandem mass spectrometry (LC-MS/MS)-based methods and immunoassays appear to be the most prevalent approaches in clinical laboratories. Since these techniques differ in many aspects, in making the choice the laboratory must consider the technical, clinical, and economic criteria, as well as staff qualifications. In this Q&A, 4 leading experts in the field of immunosuppressive drug monitoring present their opinions on the advantages and disadvantages of the analytical methods and provide a general guide for the optimization of analytical strategies in the field of TDM. From your perspective, what are the advantages and disadvantages of mass spectrometry–based assays and immunoassays in the context of TDM? Gregory Maine: One of the main advantages of mass spectrometry is that users can develop their own assays. Mass spectrometry–based assays can be analytically sensitive, specific, and capable of measuring several compounds in a single run. The disadvantages of this technique include that the instrument is not a “turnkey” analyzer, it requires large capital investment and service costs, it is subject to matrix effects and ion suppression, and the results from different laboratories are not necessarily comparable. The advantages of drug immunoassays include their easy integration into the core laboratory and the ability to measure them along with other immunoassays, such as those for serologic factors and hormones. In addition, TDM immunoassays …