Custom Assays for Companion Diagnostic Development
Companion diagnostics now function as structural components of therapeutic development rather than ancillary tools. The U.S. Food and Drug Administration (FDA) defines a companion diagnostic as a medical device, often an in vitro diagnostic (IVD), that provides information essential for the safe and effective use of a corresponding therapeutic product. These diagnostics may identify patients most likely to benefit from a therapy, identify patients at increased risk for serious adverse reactions, or monitor treatment response.
As a result, assay performance cannot be considered solely in terms of analytical sensitivity or specificity. The assay must also function reliably within real clinical workflows across sites, operators, specimens, and patient populations. FDA’s In Vitro Companion Diagnostic Devices guidance emphasizes the importance of considering companion diagnostics alongside therapeutic development rather than treating the diagnostic as an independent, late-stage component.
Off-the-shelf assays may struggle to meet these demands. Assays optimized under controlled laboratory conditions must ultimately contend with heterogeneous specimens, variable handling practices, and real-world clinical constraints. Custom assay development provides an opportunity to address those variables during assay design rather than attempting to retrofit performance later.
Why Companion Diagnostics Demand Purpose-Built Assays
Companion diagnostics must tolerate biological, operational, and temporal variability while producing consistent clinical classifications. Unlike exploratory assays, they may ultimately be expected to deliver stable performance across geographically distributed sites and evolving study phases.
This makes early coordination between therapeutic and diagnostic development particularly important. FDA’s Principles for Codevelopment of an In Vitro Companion Diagnostic Device with a Therapeutic Product provides a framework for developing a therapeutic product and its accompanying companion diagnostic together.
Biological variability further complicates assay development. Tumor heterogeneity, ischemia time, fixation conditions, sample age, and other pre-analytical variables can influence biomarker measurements. A purpose-built assay strategy can account for these variables using specimens and workflows that more closely represent the conditions expected during clinical development.
Fit-for-purpose design also extends to analytical thresholds. Cutoffs established using highly controlled reference materials may not perform identically when applied to heterogeneous clinical specimens. Designing and validating assays around representative sample types and intended-use conditions helps ensure that performance characteristics reflect the environment in which the assay will actually be used.
Integrating Assay Design With Specimen Strategy
Successful companion diagnostic development requires close coordination between assay development, specimen procurement, and sample processing. An assay optimized independently of the specimens it will ultimately evaluate may encounter unexpected variability when introduced into clinical workflows.
Representative specimen types, handling conditions, processing methods, and storage histories can therefore be incorporated during assay optimization. This approach helps identify vulnerabilities while modifications to the assay or workflow remain feasible.
The importance of rigorous clinical assay validation is illustrated by work published in Nature Biotechnology, where Frampton and colleagues described the development and validation of a clinical cancer genomic profiling assay using massively parallel DNA sequencing. The study demonstrates the extensive analytical characterization required to translate a complex molecular assay into a clinical testing environment.
Lifecycle considerations further reinforce the need for integration. Companion diagnostics may evolve alongside therapeutic indications, creating the need for retrospective studies, additional validation, label expansion, or longitudinal analysis. An assay and specimen strategy designed with these future requirements in mind can provide greater flexibility as a program develops.
Regulatory Durability and Long-Term Viability
Companion diagnostic development does not end with initial assay validation. Assays must maintain reproducible performance as clinical programs advance and may need to accommodate additional therapeutic products, indications, biomarkers, or patient populations.
FDA’s 2020 guidance on developing and labeling companion diagnostics for groups of oncology therapeutic products illustrates this broader lifecycle perspective. The guidance addresses circumstances in which a companion diagnostic may support a specific group of oncology therapeutic products rather than a single individual therapy.
Long-term specimen stewardship is also important to this durability. Archived samples may become valuable for retrospective validation, additional biomarker studies, assay refinement, or future program expansion. Maintaining controlled storage conditions, chain of custody, and associated specimen metadata helps preserve those samples as usable research assets.
Supporting Companion Diagnostics Through Integrated Infrastructure
Avrok supports companion diagnostic development by aligning custom assay design with biospecimen procurement, specialty lab processing, and long-term biostorage under a single operational framework. Assays can be developed using clinically representative specimens rather than relying exclusively on idealized controls, helping performance evaluation more closely reflect real-world conditions.
Centralized specimen processing, testing, documentation, and storage can also reduce unnecessary handoffs while maintaining continuity throughout the development lifecycle. Samples used during assay development and validation can remain accessible for additional testing, retrospective analysis, or future program expansion.
By integrating custom assay development with specimen strategy, laboratory infrastructure, and long-term sample stewardship, Avrok helps research teams build companion diagnostic programs designed to remain useful from early development through validation and future clinical applications.