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    Companion Diagnostic

    IVD that identifies patients who will benefit from a specific drug.

    Reviewed by Christian Espinosa, Founder, Blue Goat CyberLast reviewed May 5, 2026

    Definition

    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.
    What the regulation says
    The FDA regulates companion diagnostics (CDx) as medical devices, typically in conjunction with the therapeutic product. Per 21 CFR 860.3, a CDx is an in vitro diagnostic device that provides information essential for the safe and effective use of a corresponding therapeutic product. The EU MDR 2017/745 also covers companion diagnostics, emphasizing their role in patient selection and the need for robust clinical evidence to support their intended use, as detailed in Annex I, General Safety and Performance Requirements.

    What this means in practice

    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.

    Examples

    • A pharmaceutical company develops a new oncology drug and concurrently designs a companion diagnostic to identify patients who overexpress a specific biomarker, required for the drug's efficacy.
    • A MedTech firm develops an IVD test for a specific genetic mutation, and a drug manufacturer later develops a therapy whose label specifies this test for patient selection.
    • A diagnostic company updates an existing IVD device to become a companion diagnostic for a newly approved targeted therapy, necessitating a new regulatory submission to reflect the expanded intended use.
    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.

    Frequently asked questions

    A companion diagnostic is essential for the safe and effective use of a drug, as specified in the drug's label, implying a mandatory test. A complementary diagnostic, conversely, provides additional information that can inform treatment decisions but is not strictly required per the drug label.

    Cross-references

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    Primary references

    3 sources
    Link health: 3 verified· last checked 2026-06-20
    FDA·1MedTech Europe·1MedTech Dive·1
    1. 1
      FDA CDx list
      Verified
      FDAfda.gov
    2. 2
      MedTech Europe - Facts & Figures
      Verified
      MedTech Europemedtecheurope.org
    3. 3
      MedTech Dive
      Verified
      MedTech Divemedtechdive.com

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