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The test that decides whether a specific drug is right for you, approved together with the drug.
A companion diagnostic is an analytically and clinically validated assay tied to a drug label, so the test result decides whether that specific drug is indicated. Examples include PD-L1 immunohistochemistry (22C3, SP142), HER2 IHC and ISH, FoundationOne CDx, Guardant360 CDx, myChoice CDx for HRD and Oncomine Dx. The model enriches for responders and gives regulatory clarity about who should receive a drug. Its weaknesses are threshold effects, where patients just below a cut-off are excluded, and the fact that different assays for the same biomarker are not interchangeable. The 'HER2-low' and 'HER2-ultralow' labels for trastuzumab deruxtecan showed that an existing test can acquire a new decision threshold overnight, turning a long-standing negative result into eligibility. It is the test approved alongside a drug that decides whether the drug is right for you.
Analytically and clinically validated assay linked to a drug label.
The inherited BRCA test that decides who can have PARP inhibitor pills for ovarian, breast, pancreatic and prostate cancer.
The lung cancer gene test that became the first blood-based companion diagnostic the FDA ever approved.
The FDA-approved tissue (324 genes) and blood genomic tests that serve as companion diagnostics for dozens of drugs.
A blood test that reads a tumour's mutations without a tissue biopsy and is the FDA-approved gateway to several targeted drugs.
The tests that grade a breast or stomach cancer's HER2 level, from the original trastuzumab test in 1998 to the new 'HER2-low' and 'ultralow' cut-offs.
A 70-gene test that tells whether an early breast cancer is genomically low or high risk, used to decide who can skip chemotherapy.
The first FDA-approved test that sequences all of a tumour's genes and gene activity at once, used to match patients to several targeted drugs.
A tumour test that measures DNA-repair scarring so that women with ovarian cancer beyond BRCA carriers can benefit from PARP inhibitors.
The first FDA-approved gene panel that could match one biopsy to several different lung cancer drugs at once.
A 21-gene test that tells most women with early hormone-positive breast cancer whether they can safely skip chemotherapy.
The 22C3 pharmDx assay is the PD-L1 stain tied to pembrolizumab since 2015 and the source of the combined positive score (CPS). The cut-off differs by cancer: 1% of tumour cells in lung cancer, CPS 1 in gastric, cervical and head and neck cancer, CPS 10 in oesophageal and triple-negative breast cancer, so one stain is read differently per disease.
A blood test approved with adagrasib to find the KRAS G12C mutation in lung cancer when a tissue biopsy is not possible.
Tempus's tumour-and-normal gene panel, FDA-approved in 2023 as a companion test for EGFR antibodies in bowel cancer.
A family of quick single-gene tests that decide who can have several bowel, lung, breast and bladder cancer drugs.
A large gene panel hospitals can run themselves, approved by the FDA in 2024 as a companion diagnostic for the tumour-agnostic drug larotrectinib.
A four-stain test that shows whether a womb cancer has lost its DNA spell-checker, which makes immunotherapy likely to work.
The PD-L1 stain that decides who can have atezolizumab, scored on immune cells rather than tumour cells in breast cancer.
The VENTANA SP263 assay is an immunohistochemistry stain that measures PD-L1 on tumour cells. Approved in 2017 alongside durvalumab in bladder cancer, it became the companion test in 2021 for adjuvant atezolizumab in resected lung cancer with PD-L1 on at least 1% of tumour cells, and it agrees closely with 22C3, so laboratories often validate it as a single platform.
Zolbetuximab is the first drug against Claudin 18.2, a protein exposed on stomach cancer cells. Added to chemotherapy it extends survival by two to three months; nausea is the price.
Patients with newly diagnosed advanced stomach cancer should now have Claudin 18.2 tested alongside HER2, PD-L1 and mismatch repair, because roughly a third will be eligible for zolbetuximab, which adds about three months of median survival. The main practical problem is nausea and vomiting during infusions, which needs aggressive prophylaxis. How to sequence or combine it with immunotherapy in PD-L1-positive tumours is unresolved.
Patients whose breast cancer was previously called HER2-negative may now be eligible for an effective HER2-directed drug if their tumour has even low-level HER2 staining, so pathology reports must now distinguish HER2-low (1+ or 2+/ISH-negative) from HER2-zero. This applies to metastatic disease after at least one line of chemotherapy; it does not mean these patients benefit from trastuzumab or other older HER2 drugs.
It is the reference for what a colorectal molecular report must contain, and the document that turned extended RAS from a trial finding into a requirement.
Query for this technology: (TITLE:"companion diagnostic" OR ABSTRACT:"companion diagnostic" OR TITLE:"companion diagnostics" OR ABSTRACT:"companion diagnostics"). Results are unfiltered search hits about Companion diagnostics, not a curated reading list.
Shares Agilent Technologies (Dako), Ibex Medical Analytics, Publish each laboratory's biomarker proficiency results, Histology automation and IHC autostainers.
Shares Labcorp, Strata Oncology, Michael F. Berger, Funda Meric-Bernstam.
Shares Turn the map of immune cells inside a tumour into a standardised test, IMpower110, KEYNOTE-010, CheckMate 026.
Shares Regulators recognise each other's companion diagnostic approvals, Every tumour sequencing report lists open, nearby, matched trials pulled live, HRD genomic scar scores (GIS, LOH, HRDetect), FoundationOne CDx / Liquid CDx.
Shares Myriad Genetics, myChoice CDx, BRACAnalysis CDx, RAD51 foci assay (functional HRD test).
Shares Pathologists order genomic profiling automatically at diagnosis of advanced cancer, Every tumour sequencing report lists open, nearby, matched trials pulled live, TruSight Oncology Comprehensive, Tempus xT CDx.
Shares Nonagen Bioscience, OncoHost, Acrivon Therapeutics, A commons for leftover trial biospecimens with standard access for approved research.
Shares TruSight Oncology Comprehensive, IMpower110, Tumour mutational burden testing, Biomarker.