{"entity":{"id":"companion-diagnostic","kind":"technology","name":"Companion diagnostics","aka":[],"tldr":"The test that decides whether a specific drug is right for you, approved together with the drug.","summary":"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.","status":"standard-of-care","asOf":"2026-09-04","wikipedia":"https://en.wikipedia.org/wiki/Companion_diagnostic","links":[{"label":"Wikipedia","url":"https://en.wikipedia.org/wiki/Companion_diagnostic"}],"tags":[],"related":["hrd-genomic-scar-scores","rad51-foci-assay","dpyd-genotyping","fes-pet","cellsearch-ctc-count","serum-tumour-markers"],"cancers":[],"sections":["diagnostics"],"technologies":[],"targets":[],"drugs":[],"companies":["acrivon-therapeutics","ataraxis-ai","cofactor-genomics","ibex-medical-analytics","imagene-ai","insight-molecular-diagnostics","nonagen-bioscience","oncohost","signatur-biosciences","valar-labs"],"institutions":[],"pathways":[],"terms":["cps","her2-low"],"trials":[],"people":[],"bottlenecks":[],"keyPapers":[],"journals":[],"dependsOn":[],"notes":[],"principle":"Analytically and clinically validated assay linked to a drug label.","strengths":["Enriches responders","Regulatory clarity"],"limitations":["Threshold effects","Different assays for the same biomarker are not interchangeable"]},"route":"/technologies/companion-diagnostic/","neighbours":{"technology":[{"id":"cellsearch-ctc-count","kind":"technology","name":"CellSearch circulating tumour cell count","route":"/technologies/cellsearch-ctc-count/"},{"id":"dpyd-genotyping","kind":"technology","name":"DPYD genotyping and DPD phenotyping before fluoropyrimidines","route":"/technologies/dpyd-genotyping/"},{"id":"fes-pet","kind":"technology","name":"FES PET (oestrogen receptor imaging)","route":"/technologies/fes-pet/"},{"id":"ihc-autostainers-histology-automation","kind":"technology","name":"Histology automation and IHC autostainers","route":"/technologies/ihc-autostainers-histology-automation/"},{"id":"hrd-genomic-scar-scores","kind":"technology","name":"HRD genomic scar scores (GIS, LOH, HRDetect)","route":"/technologies/hrd-genomic-scar-scores/"},{"id":"msi-mmr-testing","kind":"technology","name":"MSI and mismatch-repair testing","route":"/technologies/msi-mmr-testing/"},{"id":"rad51-foci-assay","kind":"technology","name":"RAD51 foci assay (functional HRD test)","route":"/technologies/rad51-foci-assay/"},{"id":"reference-laboratories","kind":"technology","name":"Reference laboratories and companion-diagnostic testing","route":"/technologies/reference-laboratories/"},{"id":"serum-tumour-markers","kind":"technology","name":"Serum tumour markers: proper use and misuse","route":"/technologies/serum-tumour-markers/"},{"id":"tmb-testing","kind":"technology","name":"Tumour mutational burden testing","route":"/technologies/tmb-testing/"}],"section":[{"id":"diagnostics","kind":"section","name":"Diagnostics & Biomarkers","route":"/fronts/diagnostics/"}],"company":[{"id":"acrivon-therapeutics","kind":"company","name":"Acrivon 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Genetics","route":"/companies/myriad-genetics/"},{"id":"nonagen-bioscience","kind":"company","name":"Nonagen Bioscience","route":"/companies/nonagen-bioscience/"},{"id":"oncohost","kind":"company","name":"OncoHost","route":"/companies/oncohost/"},{"id":"qiagen","kind":"company","name":"QIAGEN","route":"/companies/qiagen/"},{"id":"signatur-biosciences","kind":"company","name":"Signatur Biosciences","route":"/companies/signatur-biosciences/"},{"id":"strata-oncology","kind":"company","name":"Strata Oncology","route":"/companies/strata-oncology/"},{"id":"thermo-fisher","kind":"company","name":"Thermo Fisher Scientific","route":"/companies/thermo-fisher/"},{"id":"valar-labs","kind":"company","name":"Valar Labs","route":"/companies/valar-labs/"}],"term":[{"id":"biomarker","kind":"term","name":"Biomarker","route":"/terms/biomarker/"},{"id":"cps","kind":"term","name":"Combined positive score (CPS)","route":"/terms/cps/"},{"id":"companion-diagnostic-term","kind":"term","name":"Companion diagnostic","route":"/terms/companion-diagnostic-term/"},{"id":"driver-passenger-model","kind":"term","name":"Driver and passenger mutations: the refined somatic mutation theory","route":"/terms/driver-passenger-model/"},{"id":"driver-mutation","kind":"term","name":"Driver mutation","route":"/terms/driver-mutation/"},{"id":"fda-ldt-rule","kind":"term","name":"FDA laboratory-developed test (LDT) rule","route":"/terms/fda-ldt-rule/"},{"id":"her2-low","kind":"term","name":"HER2-low and HER2-ultralow","route":"/terms/her2-low/"},{"id":"ivdr","kind":"term","name":"IVDR (EU In Vitro Diagnostic Regulation 2017/746)","route":"/terms/ivdr/"},{"id":"pd-l1-testing","kind":"term","name":"PD-L1 expression testing (22C3, SP142, SP263)","route":"/terms/pd-l1-testing/"},{"id":"state-biomarker-testing-laws","kind":"term","name":"State biomarker testing coverage laws","route":"/terms/state-biomarker-testing-laws/"}],"cancer":[{"id":"colorectal","kind":"cancer","name":"Colorectal 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BRAF)","route":"/drugs/therascreen-cdx/"},{"id":"trusight-oncology-comprehensive","kind":"drug","name":"TruSight Oncology Comprehensive","route":"/drugs/trusight-oncology-comprehensive/"},{"id":"ventana-mmr-rxdx","kind":"drug","name":"VENTANA MMR RxDx Panel","route":"/drugs/ventana-mmr-rxdx/"},{"id":"ventana-pd-l1-sp142","kind":"drug","name":"VENTANA PD-L1 (SP142) Assay","route":"/drugs/ventana-pd-l1-sp142/"},{"id":"ventana-pd-l1-sp263","kind":"drug","name":"VENTANA PD-L1 (SP263) Assay","route":"/drugs/ventana-pd-l1-sp263/"},{"id":"zolbetuximab","kind":"drug","name":"Zolbetuximab","route":"/drugs/zolbetuximab/"}],"paper":[{"id":"paper-destiny-breast04-nejm-2022","kind":"paper","name":"DESTINY-Breast04: trastuzumab deruxtecan works in HER2-low breast cancer, creating a new treatable group","route":"/key-papers/paper-destiny-breast04-nejm-2022/"},{"id":"paper-sepulveda-molecular-biomarkers-colorectal-guideline-jco-2017","kind":"paper","name":"Molecular biomarkers for the evaluation of 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