# Companion diagnostic

Source: https://onco.cc/terms/companion-diagnostic-term/  
OnCo record `companion-diagnostic-term` (Term). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

A companion diagnostic is a test the FDA requires before a particular drug can be prescribed.

## Summary

A companion diagnostic is a test that the FDA requires before a particular drug can be prescribed, tying the medicine to the assay that identifies who should receive it. Examples include the 22C3 PD-L1 assay for pembrolizumab, FoundationOne CDx for many targeted drugs and myChoice CDx for niraparib and olaparib; the fuller account is in the Companion diagnostics technology entry. The term features in the bottlenecks on unvalidated biomarkers, fragmented care and slow knowledge diffusion, and in the LUNGevity Foundation collection. Ideas that address it include mutual recognition of companion diagnostic approvals between regulators, a reliance pathway so the test arrives with the drug, locking the biomarker cut-off before phase 3 and a fund for prospective validation of academic biomarkers.

## Fields

- Kind: Term
- Last checked: 2026-09-04

## Sources

- Wikipedia: https://en.wikipedia.org/wiki/Companion_diagnostic
- Wikipedia: https://en.wikipedia.org/wiki/Companion_diagnostic

## Connected records

- technologies: [Companion diagnostics](https://onco.cc/technologies/companion-diagnostic/)
- bottlenecks: [Biomarkers are not validated or standardised](https://onco.cc/bottlenecks/b-biomarker-validation/), [Fragmented care and guideline gaps](https://onco.cc/bottlenecks/b-care-fragmentation/), [Knowledge reaches practice too slowly](https://onco.cc/bottlenecks/b-knowledge-diffusion/)
- ideas: [A formal regulatory route for validating a biomarker on archived trial samples](https://onco.cc/ideas/idea-tr2-prospective-retrospective-path/), [A fund for prospective validation of academic biomarkers and companion diagnostics](https://onco.cc/ideas/idea-fund-biomarker-validation-fund/), [A reliance pathway for companion diagnostics so the test arrives with the drug](https://onco.cc/ideas/idea-reg-diagnostic-reliance-pathway/), [Drug labels must state which biomarker assays were validated and how they compare](https://onco.cc/ideas/idea-tr2-label-assay-concordance/), [Lock the biomarker cut-off before phase 3, and publish it](https://onco.cc/ideas/idea-tr2-cutpoint-lock/), [Pay for new biomarker tests only while evidence of clinical utility is being collected](https://onco.cc/ideas/idea-tr2-biomarker-cwe/), [Regulators recognise each other's companion diagnostic approvals](https://onco.cc/ideas/idea-tr2-cdx-mutual-recognition/), [Test the drug in biomarker-negative patients too, so the biomarker can be validated](https://onco.cc/ideas/idea-tr2-marker-stratified-default/)
- collections: [LUNGevity Foundation (and GO2 for Lung Cancer)](https://onco.cc/collections/lungevity/)
- terms: [Association for Molecular Pathology v. Myriad Genetics (2013)](https://onco.cc/terms/myriad-ruling/), [Biomarker](https://onco.cc/terms/biomarker/), [Genomic profiling](https://onco.cc/terms/genomic-profiling/), [Label, indication and label expansion](https://onco.cc/terms/label-indication/), [Medicare coverage with evidence development](https://onco.cc/terms/medicare-ced/), [Off-label](https://onco.cc/terms/off-label/), [State biomarker testing coverage laws](https://onco.cc/terms/state-biomarker-testing-laws/), [Targeted therapy](https://onco.cc/terms/targeted-therapy-term/)
- institutions: [FDA Oncology Center of Excellence](https://onco.cc/institutions/fda-oce/)
- drugs: [cobas EGFR Mutation Test v2](https://onco.cc/drugs/cobas-egfr-mutation-test/), [Guardant360 CDx](https://onco.cc/drugs/guardant360-cdx/), [Oncomine Dx Target Test](https://onco.cc/drugs/oncomine-dx-target-test/), [therascreen companion diagnostic kits (KRAS, EGFR, PIK3CA, FGFR, BRAF)](https://onco.cc/drugs/therascreen-cdx/)

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