--- title: "CDx, Companion Diagnostic | MedTech Terms" description: "IVD that identifies patients who will benefit from a specific drug. Plain-English Market Segments definition for MedTech teams, with examples and related terms." lang: en json-ld: | { "@context": "https://schema.org", "@graph": [ { "@type": "DefinedTerm", "@id": "https://medtechterms.com/terms/companion-diagnostic#term", "name": "Companion Diagnostic", "alternateName": "CDx", "description": "A companion diagnostic is an IVD device co-developed with a therapeutic and required (per the drug label) to select patients eligible for treatment - for example, HER2 testing for trastuzumab or PD-L1 testing for pembrolizumab.", "url": "https://medtechterms.com/terms/companion-diagnostic", "termCode": "companion-diagnostic", "inDefinedTermSet": { "@type": "DefinedTermSet", "name": "MedTech Terms", "url": "https://medtechterms.com/terms" } }, { "@type": "Article", "@id": "https://medtechterms.com/terms/companion-diagnostic#article", "headline": "CDx, Companion Diagnostic", "description": "IVD that identifies patients who will benefit from a specific drug.", "url": "https://medtechterms.com/terms/companion-diagnostic", "mainEntityOfPage": { "@type": "WebPage", "@id": "https://medtechterms.com/terms/companion-diagnostic" }, "about": { "@id": "https://medtechterms.com/terms/companion-diagnostic#term" }, "articleSection": "Market Segments", "inLanguage": "en", "keywords": "Companion Diagnostic, CDx, Market Segments, medical device, MedTech", "author": { "@type": "Person", "name": "Christian Espinosa", "jobTitle": "Founder, Blue Goat Cyber", "url": "https://bluegoatcyber.com" }, "publisher": { "@type": "Organization", "name": "MedTech Terms", "url": "https://medtechterms.com" }, "isPartOf": { "@type": "WebSite", "name": "MedTech Terms", "url": "https://medtechterms.com" }, "datePublished": "2026-05-05", "dateModified": "2026-05-05", "citation": [ { "@type": "CreativeWork", "name": "FDA CDx list", "url": "https://www.fda.gov/medical-devices/in-vitro-diagnostics/list-cleared-or-approved-companion-diagnostic-devices-in-vitro-and-imaging-tools", "publisher": { "@type": "Organization", "name": "FDA" } }, { "@type": "CreativeWork", "name": "MedTech Europe - Facts & Figures", "url": "https://www.medtecheurope.org/datahub/", "publisher": { "@type": "Organization", "name": "MedTech Europe" } }, { "@type": "CreativeWork", "name": "MedTech Dive", "url": "https://www.medtechdive.com/", "publisher": { "@type": "Organization", "name": "MedTech Dive" } } ], "mentions": [ { "@type": "DefinedTerm", "@id": "https://medtechterms.com/terms/ivd#term", "name": "In Vitro Diagnostics", "alternateName": "IVD", "url": "https://medtechterms.com/terms/ivd" }, { "@type": "DefinedTerm", "@id": "https://medtechterms.com/terms/molecular-diagnostics#term", "name": "Molecular Diagnostics", "url": "https://medtechterms.com/terms/molecular-diagnostics" } ] }, { "@type": "BreadcrumbList", "itemListElement": [ { "@type": "ListItem", "position": 1, "name": "Home", "item": "https://medtechterms.com/" }, { "@type": "ListItem", "position": 2, "name": "Terms", "item": "https://medtechterms.com/terms" }, { "@type": "ListItem", "position": 3, "name": "Market Segments", "item": "https://medtechterms.com/terms?cat=Market%20Segments" }, { "@type": "ListItem", "position": 4, "name": "Companion Diagnostic", "item": "https://medtechterms.com/terms/companion-diagnostic" } ] }, { "@type": "FAQPage", "@id": "https://medtechterms.com/terms/companion-diagnostic#faq", "mainEntity": [ { "@type": "Question", "name": "How does a Companion Diagnostic differ from a complementary diagnostic?", "acceptedAnswer": { "@type": "Answer", "text": "A companion diagnostic is essential for the safe and effective use of a drug, as specified in the drug's label, implying a mandatory test. 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However, their co-development with a drug introduces unique submission and review considerations." } } ] } ] } --- [ MedTech Terms The authoritative reference ](/) Browse Learn [Latest](/latest) About 1. [Home](/) 2. / 3. [Terms](/terms) 4. / 5. [Market Segments](/terms?cat=Market%20Segments) 6. / 7. Companion Diagnostic [All terms](/terms) Market Segments [Strategic Landscape](/ecosystems/strategic-landscape)CDx # Companion Diagnostic IVD that identifies patients who will benefit from a specific drug. Reviewed by [Christian Espinosa, Founder, Blue Goat Cyber](/authors/christian-espinosa) Last reviewed May 5, 2026 ## Definition A companion diagnostic is an [IVD](/terms/ivd) device co-developed with a therapeutic and required (per the drug label) to select patients eligible for treatment - for example, HER2 testing for trastuzumab or PD-L1 testing for pembrolizumab. What the regulation says The FDA regulates companion diagnostics ( [CDx](/terms/ivdr-companion-diagnostic)) as medical devices, typically in conjunction with the therapeutic product. Per 21 CFR 860.3, a CDx is an in vitro diagnostic device that provides information essential for the [safe](/terms/safe-note) and effective use of a corresponding therapeutic product. The EU [MDR](/terms/mdr-reporting) 2017/745 also covers companion diagnostics, emphasizing their role in patient selection and the need for robust clinical evidence to support their [intended use](/terms/intended-use), as detailed in Annex I, [General Safety and Performance Requirements](/terms/gspr). ## What this means in practice Co-developed with pharma; FDA reviews drug and [CDx](/terms/ivdr-companion-diagnostic) approvals together. Growing tail of pan-tumor and signature-based CDx (e.g., MSI-H, TMB-high) tied to immunotherapy. ## Examples - A pharmaceutical company develops a new oncology drug and concurrently designs a companion diagnostic to identify patients who overexpress a specific biomarker, required for the drug's efficacy. - A MedTech firm develops an IVD test for a specific genetic mutation, and a drug manufacturer later develops a therapy whose label specifies this test for patient selection. - A diagnostic company updates an existing IVD device to become a companion diagnostic for a newly approved targeted therapy, necessitating a new regulatory submission to reflect the expanded intended use. Common pitfalls - • Misinterpreting the "companion" aspect as merely associated, rather than medically essential for the drug's safe and effective use. - • Failing to establish a clear analytical and clinical validity bridge between the CDx and the therapeutic product. - • Neglecting to consider the global regulatory variations in CDx approval pathways, which can impact market access strategies. - • Underestimating the complexities of co-developing a medical device and a drug, including differing review timelines and evidentiary requirements. - • Not adequately addressing cybersecurity considerations for digital companion diagnostics, especially concerning patient data privacy and diagnostic result integrity. ## Frequently asked questions How does a Companion Diagnostic differ from a complementary diagnostic? A companion diagnostic is essential for the [safe](/terms/safe-note) and effective use of a drug, as specified in the drug's label, implying a mandatory test. A complementary diagnostic, conversely, provides additional information that can inform treatment decisions but is not strictly required per the drug label. What is the role of the FDA in the approval of companion diagnostics? Are companion diagnostics subject to the same regulatory requirements as other IVDs? ## Cross-references ### See also Closely related context worth reading. - [ IVDR Risk Classes A–D ](/terms/ivdr-classes) ## Related terms Grouped by theme ### Editor's picks · Hand-selected related concepts [ Market Segments In Vitro Diagnostics(IVD) Tests performed on biological samples outside the body. ](/terms/ivd)[ Market Segments Molecular Diagnostics IVDs that detect nucleic acids - DNA or RNA - to diagnose disease. ](/terms/molecular-diagnostics) ### IVDR Essentials · From this learning path [ Regulatory CE Mark Mandatory European conformity mark for medical devices placed on the EU market. Adjacent lesson ](/terms/ce-mark?from=ivdr-essentials)[ Regulatory IVDR Risk Classes A–D EU IVDR's risk-based classification: Class A (lowest) to Class D (highest). Adjacent lesson ](/terms/ivdr-classes?from=ivdr-essentials)[ Clinical & Trials Clinical Evaluation Plan(CEP) EU MDR document that defines the scope, methods, and acceptance criteria for the clinical evaluation that will be reported in the Clinical Evaluation Report (CER). ](/terms/clinical-evaluation-plan?from=ivdr-essentials)[ Regulatory EU In Vitro Diagnostic Regulation(IVDR) Regulation (EU) 2017/746 governing in vitro diagnostic medical devices in the EU. ](/terms/ivdr?from=ivdr-essentials) ### More in Market Segments · Same category [ Market Segments Aesthetic & Energy-Based Devices Lasers, RF, ultrasound, cryolipolysis, and IPL systems used for medical and cosmetic indications, body contouring, hair removal, skin tightening, vascular lesions. ](/terms/aesthetics-energy-devices)[ Market Segments AI Imaging Software Software that analyzes medical images to triage, detect, or quantify disease. ](/terms/ai-imaging) Cited by Where this term appears across MedTech Terms. Learning paths (1) - [IVDR Essentials](/paths/ivdr-essentials)Lesson 3 of 13 Ecosystems (1) - [Strategic Landscape](/ecosystems/strategic-landscape) ## Mentioned in recent activity 1 entry Recalls (1) [Sequencing Agent SEQ0067, component of FoundationOne CDx (F1CDx) ](https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfRES/res.cfm?id=Z-1927-2026) Class II · 2026-03-20 ## Latest in MedTech [View all](/latest?term=companion-diagnostic) Recent regulatory actions, publications, and trials matched to this term. Updated daily. - Recall 2026-03-20 [Sequencing Agent SEQ0067, component of FoundationOne CDx (F1CDx) ](https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfRES/res.cfm?id=Z-1927-2026) Class II · Foundation Medicine, Inc. ## Primary references 3 sources Link health: 3 verified · last checked 2026-06-20 FDA· 1 MedTech Europe· 1 MedTech Dive· 1 1. [1 FDA CDx list Verified FDA · fda.gov ](https://www.fda.gov/medical-devices/in-vitro-diagnostics/list-cleared-or-approved-companion-diagnostic-devices-in-vitro-and-imaging-tools) 2. [2 MedTech Europe - Facts & Figures Verified MedTech Europe · medtecheurope.org ](https://www.medtecheurope.org/datahub/) 3. [3 MedTech Dive Verified MedTech Dive · medtechdive.com ](https://www.medtechdive.com/) Inline markers like \[1\] jump to the matching reference above. Sponsor note ### Building a connected device in this segment? Blue Goat Cyber specializes in MedTech cybersecurity across imaging, cardiac, IVD, surgical, and other connected segments. [Book a 30-minute discovery session](https://go.bluegoatcyber.com/meetings/blue-goat-cyber/discovery-session) - No obligation - Expert-led from minute one - NDA available on request MedTech Terms is a community resource sponsored by [Blue Goat Cyber](https://bluegoatcyber.com). Definitions are independent of any vendor. On this term Category Market Segments Acronym CDx Sources 3 Updated 5/5/2026 [Compare with another term](/compare?a=companion-diagnostic) Learn in 60 seconds Card Lesson Quiz IVD that identifies patients who will benefit from a specific drug. - · Co-developed with pharma; FDA reviews drug and CDx approvals together. - · Growing tail of pan-tumor and signature-based CDx (e.g., MSI-H, TMB-high) tied to immunotherapy. Remember this Watch out: Misinterpreting the "companion" aspect as merely associated, rather than medically essential for the drug's safe and effective use. Related terms - [In Vitro Diagnostics(IVD) ](/terms/ivd) - [Molecular Diagnostics ](/terms/molecular-diagnostics) You may also need Auto-suggested from Market Segments and shared keywords. - [Companion Diagnostic (IVDR)(CDx) ](/terms/ivdr-companion-diagnostic) - [Ophthalmology Devices ](/terms/ophthalmology-devices) - [Point-of-Care Diagnostics(POC) ](/terms/point-of-care) - [Performance Study Application (IVDR) ](/terms/ivdr-performance-study-application) - [Aesthetic & Energy-Based Devices ](/terms/aesthetics-energy-devices) - [Clinical Laboratory Improvement Amendments(CLIA) ](/terms/clia) [All Market Segments terms](/terms?cat=Market%20Segments) MedTech Terms An authoritative, plain-language reference for the regulatory, quality, cybersecurity, and software terms that shape modern medical devices. 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