FAQs
Companion Diagnostics & Biomarker Strategy

Q14: What is the difference between a companion diagnostic and a complementary diagnostic?​

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A companion diagnostic (CDx) is essential for the safe and effective use of the associated therapeutic. This means the drug labeling states that the diagnostic test must be used to select patients for treatment. 

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A complementary diagnostic supports the use of a therapeutic product but is not required. The drug can be prescribed without it. Complementary diagnostics typically have a lower evidentiary bar for FDA clearance than companion diagnostics and are often cleared via 510(k) rather than PMA.

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The practical distinction matters significantly for regulatory strategy for FDA approval. A biomarker considered complementary rather than companion faces different clinical evidence requirements and a different regulatory pathway. Landrich Group helps CDx and complementary diagnostic developers choose the right regulatory positioning early in development.

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