Companion Diagnostics: Tests Tied to Specific Treatments

The Definition

A companion diagnostic is a test that is essential for the safe and effective use of a specific drug. It identifies patients likely to benefit, or those at higher risk of serious harm, and the drug's label refers to it.

The test and drug are typically co-developed so that the exact assay used in the registration trial becomes the approved companion test.

Companion vs Complementary

A complementary diagnostic informs treatment but is not mandatory. For example, a PD-L1 result may enrich for benefit from an immunotherapy without being an absolute requirement for its use.

The distinction matters: a companion test result gates access to the drug, while a complementary result is one factor among several.

One Biomarker, Several Tests

The same biomarker can have multiple approved companion tests, sometimes on different platforms, and results are not always perfectly concordant between them. Different PD-L1 antibody clones and scoring systems are a well-known example.

Laboratories may also use a validated laboratory-developed test rather than the exact kit from the trial, which is generally acceptable if performance is demonstrated to be equivalent.

Off-Label and Broad Panel Results

Broad sequencing panels report many alterations, including some with an approved drug in a different tumour type. Acting on such a result is off-label use and rests on weaker evidence, sometimes a basket trial or case series.

A panel is not itself the companion diagnostic for every drug it might implicate; formal companion status is drug- and often assay-specific.

Interpretation Notes

When a targeted drug is being considered, it is worth knowing whether the biomarker was assessed with an appropriate, validated method for that purpose.

A biomarker found on a broad panel may support treatment, but the strength of evidence depends on tumour type and whether use is on- or off-label.

Key Takeaways

• ·A companion diagnostic is required for safe, effective use of a specific drug.
• ·A complementary diagnostic informs but does not gate treatment.
• ·One biomarker can have several non-identical approved tests.
• ·Alterations found on broad panels may imply off-label use with weaker evidence.