Companion Diagnostic (CDx)

What is a companion diagnostic?

A companion diagnostic provides information essential for the safe and effective use of a corresponding therapeutic product — for example, identifying patients most likely to benefit or those at increased risk of a serious adverse reaction. The FDA's companion diagnostics pages explain the category and list authorized devices with their linked therapeutics. Oncology examples include HER2, EGFR, PD-L1 and genomic assays, but the definition depends on the test's essential role, not the disease area.

How are companion diagnostics regulated in the United States?

They are regulated as medical devices under the classification and pathway applicable to the device type. Many currently authorized companion diagnostics are Class III and were approved through PMA, but that is not an immutable rule for every future CDx; the FDA has discussed De Novo classification and reclassification where general and special controls could support Class II. Development is often coordinated with the therapeutic so the evidence, intended use and labeling remain aligned.

How does the EU IVDR treat CDx?

The IVDR defines companion diagnostics and generally places them in Class C. Conformity assessment involves a notified body, which consults a medicines competent authority or the EMA, as applicable, on the suitability of the device in relation to the medicinal product concerned. The consultation supports the notified body's conformity assessment; it does not transfer the full IVD certification decision to the medicines authority.

What makes CDx programs strategically distinct?

The diagnostic program must satisfy device requirements in the United States and European Union while supporting a therapeutic program's clinical and labeling timeline. Assay versions, cutoffs, specimen types and trial-use procedures need controlled bridging into the commercial device. Laboratory delivery can also intersect with the LDT boundary, so the intended commercial model should be defined before pivotal evidence is locked.