CDx
A diagnostic test required to identify patients who will benefit from a specific drug.
Definition
A companion diagnostic (CDx) is an in vitro diagnostic test, approved alongside its therapeutic, that identifies patients likely to benefit from (or be harmed by) a specific drug. Key examples: PD-L1 IHC for pembrolizumab, HER2 IHC/FISH for trastuzumab, EGFR mutation testing for osimertinib, BRCA testing for olaparib.