Companion diagnostics are medical tests designed to help healthcare professionals determine whether a particular treatment is appropriate for a patient. These tests identify specific biological characteristics, often called biomarkers, that can influence how a disease responds to a medicine.
They are closely connected to personalized medicine, also called precision medicine. Instead of relying only on a general diagnosis, healthcare professionals can use information about a patient's disease biology to support treatment decisions.
For example, some cancers contain genetic changes that make them more likely to respond to certain targeted medicines. A laboratory test can identify those changes and help clinicians determine whether the related treatment may be suitable.
Companion diagnostics do not independently diagnose every aspect of a disease or guarantee a successful outcome. They provide specific information that must be considered alongside a patient's medical history, overall condition, and other clinical findings.
Why Were These Tests Developed?
Traditional treatment approaches often rely on clinical guidelines, disease type, and established treatment protocols. Although these approaches remain important, patients with the same diagnosis may respond differently to the same medicine.
Researchers have found that genetic variations, protein expression, and other biological characteristics can influence treatment response. Companion diagnostics were developed to help identify these differences.
Their main purposes include:
Identifying patients who may benefit from a particular medicine.
Detecting biomarkers associated with treatment response.
Supporting decisions about targeted therapies.
Helping clinicians understand certain risks related to treatment.
Reducing uncertainty when selecting among available treatment options.
These tests are especially important in oncology, where the biological characteristics of a tumor can influence which medicines are considered appropriate.
Why Companion Diagnostics Matter Today
Supporting Personalized Treatment Decisions
Modern healthcare increasingly recognizes that diseases can behave differently in different people. Companion diagnostics help clinicians identify biological differences that may influence treatment selection.
In cancer care, for example, a tumor may contain a mutation or protein pattern that makes a specific targeted medicine more suitable. Testing can help identify whether that marker is present before a treatment begins.
The results may also help doctors understand whether a particular treatment is less likely to work for a patient. This information can support discussions about alternative approaches, although the final decision depends on several clinical factors.
Who Benefits From Companion Diagnostic Testing?
Several groups are involved in this process:
Patients: Receive treatment decisions informed by relevant biological test results.
Healthcare professionals: Use laboratory findings alongside clinical evidence to guide care.
Pathology laboratories: Analyze tissue, blood, and other appropriate samples using validated methods.
Clinical researchers: Study relationships between biomarkers and treatment outcomes.
Medicine developers: Investigate which patient groups may benefit from a particular therapy.
Companion diagnostics are especially relevant to oncology, but biomarker-guided testing also has applications in selected infectious diseases and other medical fields. The exact role depends on the disease, the medicine, and the evidence supporting the test.
Common Types of Companion Diagnostic Tests
Different laboratory methods detect different biological characteristics. The most suitable approach depends on the biomarker being investigated and the approved purpose of the test.
Testing method | What it examines | Typical application |
|---|---|---|
Polymerase chain reaction (PCR) | Selected DNA or RNA sequences | Detecting specific genetic changes |
Next-generation sequencing (NGS) | Multiple genetic regions at once | Identifying relevant mutations in tumors |
Immunohistochemistry (IHC) | Specific proteins in tissue | Measuring protein expression |
In situ hybridization (ISH) | Selected genetic material in cells | Identifying gene amplification or rearrangements |
Blood-based molecular testing | Genetic material circulating in blood | Detecting certain tumor-related changes |
Not every method is appropriate for every biomarker. A laboratory must use a test that has been validated for the intended purpose and sample type.
Problems These Tests Help Address
Companion diagnostics can help address several challenges in treatment planning:
Biological differences: Patients with the same disease may have different biomarkers.
Treatment uncertainty: Laboratory evidence can help narrow appropriate treatment options.
Unnecessary exposure: In certain settings, testing may help identify patients unlikely to benefit from a specific medicine.
Treatment safety: Some tests identify markers associated with a higher risk of particular adverse effects.
Research precision: Biomarker results help researchers evaluate treatment responses in defined patient groups.
However, these tests have limitations. A negative result does not always mean that a medicine cannot work, and a positive result does not guarantee that it will. Test accuracy, sample quality, disease changes, and other medical factors all matter.
Recent Developments in Companion Diagnostics
Advances in Genomic Testing
During 2025 and 2026, precision medicine continued to develop through broader genomic testing, improvements in molecular laboratory methods, and closer coordination between diagnostic tests and targeted medicines.
Next-generation sequencing is an important part of this trend. It can examine multiple genetic changes in one analysis, potentially helping clinicians identify biomarkers relevant to different treatment options. However, not every finding from a broad genetic panel qualifies a patient for a specific medicine.
Liquid biopsy is another developing area. It examines biological material, such as circulating tumor DNA, in blood. Depending on the clinical context, it may provide useful information when tumor tissue is limited or when clinicians need to investigate genetic changes over time. A negative blood result may still require tissue testing if the relevant biomarker has not been detected.
Regulatory Updates During 2025 and 2026
Regulatory agencies continued updating their guidance and information about companion diagnostics.
In May 2025, Australia's Therapeutic Goods Administration updated its guidance on in vitro companion diagnostic devices. The revision clarified topics including clinical performance requirements and planning for companion testing during medicine development.
These developments do not mean that every new technology is approved for routine clinical use. Regulatory status, evidence quality, and the intended use of each test must be considered separately.
Laws and Policies Governing Companion Diagnostics
Regulatory Oversight in the United States
In the United States, the Food and Drug Administration (FDA) regulates medical devices, including many in vitro diagnostic tests. A companion diagnostic provides information considered essential for the safe and effective use of a corresponding medicine or biological product.
When a medicine's approved labeling requires a particular diagnostic test, healthcare professionals should follow the applicable labeling and regulatory requirements. The FDA also publishes a list of authorized companion diagnostic devices, including their associated biomarkers and therapeutic products.
Rules in the European Union
In the European Union, companion diagnostic tests are covered by the In Vitro Diagnostic Medical Devices Regulation, commonly known as IVDR. The framework establishes requirements for the safety, performance, and assessment of in vitro diagnostic medical devices.
For certain companion diagnostics associated with centrally authorized medicines, notified bodies must seek a scientific opinion from the European Medicines Agency or follow the applicable regulatory consultation process.
Companion Diagnostics in India
In India, in vitro diagnostic devices are regulated within the medical-device framework, including the Medical Devices Rules, 2017, under the broader Drugs and Cosmetics Act, 1940. Applicable requirements depend on the device classification, intended use, and relevant regulatory provisions.
In March 2025, the Indian Council of Medical Research and the Central Drugs Standard Control Organization invited comments on standard evaluation protocols for in vitro diagnostics. Related protocols published during 2025 demonstrate efforts to support consistent performance evaluation of diagnostic tests. These documents do not automatically approve every companion diagnostic.
Patient Privacy and Laboratory Quality
Alongside device regulations, laboratories must consider patient confidentiality, appropriate sample handling, accurate reporting, and quality management.
Genetic information can be sensitive because it may reveal details about a person's biology and, in some cases, inherited health risks. Healthcare organizations should follow applicable privacy laws and establish appropriate access controls.
Patients can ask their healthcare team how results will be stored, who may access them, and whether genetic counseling is appropriate.
Tools and Resources for Companion Diagnostic Testing
Laboratory and Data Analysis Tools
Companion diagnostic testing depends on reliable laboratory equipment, validated procedures, and accurate interpretation of results. Common resources include:
Molecular testing systems: Detect selected genetic sequences and mutations.
Genomic analysis software: Helps process sequencing data and identify potentially relevant genetic variants.
Pathology review systems: Support the examination and documentation of tissue samples.
Laboratory information management systems: Organize sample records, test workflows, quality checks, and reporting.
Quality control materials: Help laboratories monitor analytical consistency and identify testing problems.
These tools must be appropriate for the test being performed. Software findings should be reviewed by qualified professionals rather than treated as automatic treatment recommendations.
Educational and Regulatory Resources
Useful resources include official regulatory databases, laboratory practice guidelines, clinical research publications, and educational materials about precision medicine.
Healthcare professionals can consult approved diagnostic labeling, relevant medicine information, biomarker interpretation guidance, and laboratory validation documents. Patients may benefit from educational materials explaining genetic testing, informed consent, and the meaning of positive, negative, and uncertain results.
A practical checklist for understanding a companion diagnostic test includes:
What biomarker does the test examine?
Which sample type is required?
Is the test validated for its intended purpose?
Which medicine or treatment decision is associated with the result?
Are additional tests or specialist consultations needed?
How will the result affect the next clinical discussion?
These questions can help patients and healthcare professionals understand the role and limitations of laboratory findings.
Frequently Asked Questions
What is the main purpose of companion diagnostics?
The main purpose is to provide information that helps healthcare professionals determine whether a particular treatment is appropriate for a patient. These tests often detect biomarkers linked to treatment response or safety.
Are companion diagnostics the same as genetic testing?
No. Genetic testing is a broad category that examines genetic material for different purposes. Companion diagnostics are designed for a specific treatment-related purpose and may examine genetic changes, proteins, or other biomarkers.
Are companion diagnostic tests used only for cancer?
No, although oncology is a major area of application. Biomarker-guided diagnostics may also be relevant in selected other diseases. Their role depends on the evidence available and the intended use of the specific test.
Does a positive test result guarantee that treatment will work?
No. A positive result indicates that the relevant biomarker has been detected according to the test's criteria. It does not guarantee a successful outcome because treatment response also depends on other biological and clinical factors.
What happens if a test does not identify the expected biomarker?
A healthcare professional may consider the test's limitations, sample quality, the patient's clinical findings, and whether additional testing is appropriate. A negative result should be interpreted in the context of the specific test and treatment.
Conclusion
Companion diagnostics are an important part of personalized medicine because they connect laboratory testing with treatment decisions. By identifying relevant biomarkers, these tests can help healthcare professionals understand which patients may be suitable for particular medicines and which factors could influence treatment safety.