A companion diagnostic (CDx)[1] is a diagnostic test used as a companion to a therapeutic drug to determine its applicability to a specific person.
[2] Companion diagnostics are co-developed with drugs to aid in selecting or excluding patient groups for treatment with that particular drug based on their biological characteristics that determine responders and non-responders to the therapy.
[5] For example, there is an automatic immunohistochemistry staining machine for microscopy slides to highlight HER2 expression.
This machine is classified by the U.S. Food and Drug Administration (FDA) as a companion diagnostic device for the drug trastuzumab which works on tumors that over-express HER2.
[6] In Europe the regulation on in vitro diagnostics (IVDR) defines companion diagnostics as devices that are essential for the safe and effective use of corresponding medicinal products to identify, before and/or during treatment, patients who are most likely to benefit from the corresponding medicinal product; or to identify, before and/or during treatment, patients likely to be at increased risk of serious adverse reactions as a result of treatment with the corresponding medicinal products.