CompanionDiagnostics

Companion diagnostics are in vitro diagnostic tests developed for use with a specific therapeutic product. These tests analyze biomarkers such as genetic mutations, DNA alterations, or protein expression to determine whether a patient is likely to benefit from the therapy, and may also be used to monitor treatment response or safety during therapy.

Common biomarker categories include somatic mutations (for example EGFR or KRAS), gene amplifications (such as HER2), and protein-expression biomarkers (like PD-L1). Notable examples include HER2 testing to guide trastuzumab therapy in breast cancer, KRAS testing to select colorectal cancer patients for anti-EGFR antibodies, and PD-L1 expression testing to inform several immune checkpoint inhibitors.

Regulatory framework: In the United States, the FDA defines a companion diagnostic as an in vitro diagnostic device that provides information essential for the safe and effective use of a corresponding drug or biologic. A companion diagnostic is typically developed in parallel with its drug and is authorized or approved as part of the drug’s labeling, which specifies the required test and interpretation. In the European Union, companion diagnostics are recognized within the broader in vitro diagnostic regulatory framework and are subject to regulatory oversight designed to ensure alignment with the associated therapeutic product.

Development and market: Co-development with a drug presents scientific and regulatory challenges, including evidence generation that links biomarker status to treatment benefit, manufacturing of a diagnostic device, and payer reimbursement considerations. Adoption is driven by collaboration among pharmaceutical companies, diagnostic developers, regulators, and clinicians, and is central to precision medicine initiatives.

Impact and considerations: Companion diagnostics help tailor therapy to likely responders, potentially improving outcomes and reducing unnecessary exposure. They raise considerations around access, cost, data privacy, and the need for ongoing validation as therapies and biomarkers evolve.