If your therapy only works in a biomarker-defined subset of patients, then your companion diagnostic is not a supporting detail; it is the mechanism by which your drug finds the people it can help. A diagnostic that arrives late can delay your launch, and one that is approved but unavailable or unreimbursed can blunt your uptake badly. The right companion diagnostics partner has a proven regulatory track record, a platform genuinely suited to your biomarker, real commercial reach, and the willingness to co-develop on your timeline. Here is how to choose one.
Engage far earlier than feels necessary
The single most important thing to understand is timing. The diagnostic needs to be ready to identify patients for your pivotal trial, and its regulatory approval ideally arrives alongside your drug’s. That means the partnership has to be established early in clinical development, not negotiated once your trial is designed. Companies that treat the diagnostic as a late-stage procurement exercise routinely find themselves with a drug approaching approval and no validated, approved test to select patients, which delays launch or forces compromises. Bring the diagnostic into your development plan as a parallel program from the start.
What to evaluate in a partner
- Regulatory track record. Has this partner actually taken companion diagnostics through approval alongside therapies? This is where partnerships most often stumble, and demonstrated experience is far more valuable than technical capability alone.
- Platform fit for your biomarker. Does your biomarker require tissue-based testing, sequencing-based genomic profiling, or blood-based liquid biopsy? The right platform follows from the biology, and a partner strong in one may be weak in another.
- Co-development experience and flexibility. Can they work to your timeline, coordinate with your trial, and adapt as your program evolves? Co-development is a genuine collaboration, not a vendor transaction.
- Commercial reach and market access. After approval, can the test actually reach the labs and clinicians who need to run it, and will it be reimbursed? A test nobody can access cannot find your patients.
- Analytical and clinical validation capability. Can they generate the rigorous validation data regulators require?
The questions that matter
Ask specifically about companion diagnostics they have taken through approval with a drug partner, and what the timeline actually looked like versus the plan. Ask how they would approach validation for your particular biomarker and what the hard problems are, since a partner who identifies real challenges is more credible than one who does not. Ask how the regulatory submissions would be coordinated with yours. Ask concretely about commercial availability and reimbursement after approval, because this is routinely underexplored and routinely painful. And ask who will be assigned to your program and how they will coordinate with your clinical team.
The pitfalls
The failures here are predictable. Engaging too late is the most common and the most damaging. Underestimating validation is next, since generating the analytical and clinical evidence regulators require takes real time and can surprise teams focused on the drug. Misaligned timelines between the drug and the diagnostic cause launch delays. Neglecting market access means a technically successful test that clinicians cannot obtain or get paid for. And treating the diagnostic as a vendor relationship rather than a co-development partnership tends to produce exactly the lack of coordination that causes the other problems.
The bottom line
Your companion diagnostic determines whether your therapy can find its patients, which makes the partner choice a strategic decision with direct consequences for your launch. Engage early, prioritize demonstrated regulatory success in co-development over raw technical capability, match the platform to your biomarker’s biology, and take commercial availability and reimbursement as seriously as approval. Treat the diagnostic and the drug as one coordinated program rather than two, and you avoid the delays and access problems that quietly undermine so many precision-medicine launches.
The regulatory coordination problem
The mechanics of coordinating two regulatory submissions, one for a drug and one for a diagnostic, are more demanding than most first-time teams anticipate, and understanding the shape of the problem helps you avoid the delays it causes. The drug and the test are reviewed under different frameworks, often by different parts of the regulatory agency, with different requirements, timelines, and evidentiary standards, yet they must arrive at approval together for the therapy to launch with its patient-selection tool intact. That coordination has to be managed deliberately. The diagnostic’s analytical and clinical validation must be planned so it is complete when needed, which typically means the test used in your pivotal trial should be, or closely resemble, the one you intend to commercialize, since using an unvalidated assay in the trial and hoping to bridge to a commercial version later creates real risk. Regulatory interactions should address the drug and the diagnostic together, so the agency understands the linked strategy and any concerns surface early rather than late. And the timelines must be actively managed as one program, since a delay on either side delays the launch of both. The teams that handle this well typically engage regulatory expertise experienced specifically in companion diagnostic co-development, involve the diagnostic partner in regulatory planning rather than treating them as a supplier, and build the coordination into their program timeline explicitly rather than assuming it will work itself out. Treating the two submissions as one coordinated effort, planned from the beginning, is what separates the programs that launch on time from those that arrive with an approved drug and no way to identify the patients it was designed for.
The bottom line, restated
The companion diagnostic is how your therapy finds its patients, which makes it a strategic program rather than a procurement item. Engage early, prioritize demonstrated regulatory co-development success over technical capability alone, match the platform to your biomarker, coordinate the two regulatory submissions as one effort, and treat commercial availability and reimbursement as seriously as approval itself. Do that, and the diagnostic becomes the enabler it is meant to be. Neglect it, and you may find yourself with an approved drug, an approved test, and no practical way for clinicians to connect the two.
Budget the diagnostic properly
A recurring and avoidable error is underfunding the diagnostic relative to the drug. The validation work, the regulatory effort, and the commercial launch of a companion diagnostic all cost real money, and teams that treat the test as a minor line item find themselves unable to execute it properly at exactly the moment it becomes critical. Build the diagnostic’s full cost into your program budget from the start, including its regulatory work and its market access effort, because a therapy that cannot identify its patients is not a cheaper program, it is a failed one.
Confirm the trial assay is the commercial assay
One technical point causes disproportionate pain: the test used to select patients in your pivotal trial should be, or closely bridge to, the version you intend to commercialize. Teams that run their trial on a research-grade or interim assay and plan to bridge to a commercial version later frequently find the bridging study harder, slower, and riskier than anticipated, and sometimes find that regulators are unpersuaded. Sort this out at trial design, not afterward, because the cost of getting it wrong lands squarely on your approval timeline.
Assign one owner
The organizational fix that prevents most companion diagnostic failures is to give one person clear accountability for the combined drug-and-diagnostic program, with authority across both workstreams. When responsibility is split, the diagnostic reliably becomes the poor relation, under-resourced and behind schedule, until it becomes a crisis. A single accountable owner, with visibility into both regulatory paths and both timelines, is the simplest and most effective safeguard against the coordination failures that delay so many precision-medicine launches.
To find and compare diagnostics partners, browse the BioMed Nexus precision medicine and diagnostics directory, and see our overview of the top companion diagnostics companies and our explainer on precision versus personalized medicine.
Frequently asked questions
When should I engage a companion diagnostics partner?
Far earlier than most teams expect, in early clinical development rather than as a late-stage procurement exercise. The diagnostic must be ready to identify patients for your pivotal trial, and its approval ideally arrives alongside your drug's. Companies that engage late often reach approval with no validated test to select patients, delaying launch or forcing compromises.
How do I choose a companion diagnostics partner?
Prioritize a demonstrated regulatory track record taking companion diagnostics through approval alongside therapies, since this is where partnerships most often stumble. Match the platform to your biomarker's biology (tissue, sequencing or liquid biopsy), assess co-development experience and flexibility, and take commercial reach and reimbursement seriously, because a test nobody can access cannot find your patients.
What are the common pitfalls in companion diagnostic development?
Engaging too late is the most damaging and most common. Others include underestimating the time required for analytical and clinical validation, misaligned timelines between the drug and the diagnostic causing launch delays, neglecting market access so clinicians cannot obtain or get reimbursed for the test, and treating the diagnostic as a vendor relationship rather than a co-development partnership.
