Companion diagnostic
A companion diagnostic is a test which is developed specifically to aid decision-making in relation to the prescribing of a particular drug.
The companion diagnostic may form part of the product label or data-sheet such as the need to perform a test for over-expression of the human epidermal growth factor 2 (HER-2) in a tumour prior to prescribing the anticancer drug, trastuzumab.
Other examples (not necessarily required by the drug licence) include:
(i) Testing for HLA-B*5701 in patients with HIV to avoid the hypersensitivity reaction associated with treatment by abacavir
(ii) Testing for whether a breast cancer is oestrogen receptor positive or negative prior to prescribing treatment with tamoxifen (which binds to the oestrogen receptor) -recurrences are lower where the breast cancer is oestrogen receptor positive.
