Companion and Complementary Diagnostics: Clinical and Regulatory Perspectives.

Nearly 20 years ago, the US Food and Drug Administration (FDA) approved the first companion diagnostic assay and, today, this type of test governs the use of 18 different drugs. With the appearance of PD-L1 immunohistochemistry (IHC) assays linked to the use of different PD-1/PD-L1 immune checkpoint inhibitors, a new class of predictive biomarker assays has emerged; the complementary diagnostics. These are predictive biomarker assays that aid the therapeutic decision process but are not a prerequisite for receiving a specific drug, as is the case with companion diagnostics. Both types of assay have the individual patient as a point of reference and they will be decisive for the move toward a more individualized pharmacotherapy. They are also considered important elements in the realization of precision medicine. Here, I discuss both companion and complementary diagnostics.