A companion diagnostic is an IVD device co-developed with a therapeutic and required (per the drug label) to select patients eligible for treatment - for example, HER2 testing for trastuzumab or PD-L1 testing for pembrolizumab. A companion diagnostic is essential for safe and effective use of a specific drug and is named in the drug's labeling. In the US it usually needs PMA approval, coordinated with the drug review. Under the IVDR it is Class C and requires consultation with a medicines authority.

Why it matters

Co-developed with pharma; FDA reviews drug and CDx approvals together. Growing tail of pan-tumor and signature-based CDx (e.g., MSI-H, TMB-high) tied to immunotherapy.

Common pitfalls

•Misinterpreting the "companion" aspect as merely associated, rather than medically essential for the drug's safe and effective use.
•Failing to establish a clear analytical and clinical validity bridge between the CDx and the therapeutic product.
•Neglecting to consider the global regulatory variations in CDx approval pathways, which can impact market access strategies.
•Underestimating the complexities of co-developing a medical device and a drug, including differing review timelines and evidentiary requirements.
•Not adequately addressing cybersecurity considerations for digital companion diagnostics, especially concerning patient data privacy and diagnostic result integrity.

Sources

FDA CDx list · FDA
MedTech Europe - Facts & Figures · MedTech Europe
MedTech Dive · MedTech Dive

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Common comparison patterns

Regulatory pathway choice - 510(k) vs PMA, 510(k) vs De Novo, Abbreviated vs Special 510(k).
Quality systems - QSR vs ISO 13485, QMSR vs QSR.
Software classification - SaMD vs SiMD, Locked vs adaptive AI.
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Cybersecurity artifacts - SPDX vs CycloneDX SBOM, CVE vs CVSS.
Post-market actions - Recall vs field action, Removal vs correction.

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