FDA Companion Diagnostic Approval Process: A Step-by-Step Regulatory Guide

The FDA Companion Diagnostic Approval Process: A Regulatory Guide for Clinical Development and RA Teams

For clinical development and regulatory affairs professionals working at the intersection of targeted therapies and precision diagnostics, the FDA companion diagnostic approval process carries consequences that extend well into a drug program’s commercial timeline.

A misaligned companion diagnostic strategy can delay drug approval by two years or more. In tissue-agnostic indications, the mean lag between drug approval and the corresponding IVD companion diagnostic has historically approached 707 days. That is not an abstract statistic: it represents patients waiting for access to a therapy that is already approved.

Key Takeaways

• A companion diagnostic is legally required for patient access to the associated drug, and misaligning CDx strategy with drug development timelines has added over 700 days to approval in tissue-agnostic indications.
• Three FDA centers (CDRH, CDER, and CBER) must coordinate on every CDx program, and each requires a separate marketing application regardless of whether a single sponsor develops both products.
• Most companion diagnostics go through the Class III PMA pathway, and the choice of assay used in the pivotal trial directly determines whether a bridging study and up to 24 months of additional review time will be needed.
• FDA’s November 2025 proposed rule would move nucleic acid-based oncology CDx from PMA to 510(k) clearance, but the final rule has not been published and programs should not restructure submissions around it yet.

What “Essential” Actually Means in the Legal Definition

The foundational definition comes directly from FDA’s 2014 guidance on In Vitro Companion Diagnostic Devices. An IVD companion diagnostic device provides information essential for the safe and effective use of a corresponding therapeutic product.

That single word carries legal weight. When the FDA determines that a diagnostic result is necessary for any of the following purposes, the labeling of both the drug and the diagnostic must reflect that requirement:

• Patient selection: identifying patients most likely to benefit from the therapy
• Safety exclusion: identifying patients at elevated risk for serious adverse reactions
• Dose optimization: monitoring therapeutic or toxic effects to guide dose adjustment
• Response monitoring: tracking treatment response after initiation

The applicable regulations are 21 CFR 201.57 for therapeutic product labeling and 21 CFR 809.10 for IVD device labeling.

How CDRH, CDER, and CBER Each Play a Distinct Role in CDx Review

Every IVD companion diagnostic program touches three FDA centers simultaneously. Regulatory affairs teams that treat this as a single-center process tend to discover the coordination gap at the worst possible time.

A CDx program requires two parallel submission tracks with different evidentiary standards, different review timelines, and different internal review divisions that must actively coordinate. FDA’s codevelopment guidance makes clear that separate marketing applications are always required for the therapeutic product and the IVD companion diagnostic, even when a single sponsor is developing both.

Letters of Authorization allow each application to cross-reference the other’s proprietary data without duplicating confidential submissions.

FDA strongly encourages sponsors to request early joint meetings involving all relevant centers before an IND is filed. Teams that wait until the NDA stage to engage CDRH consistently face longer review timelines and more deficiency letters.

How the PMA Pathway for In Vitro Diagnostic Devices Actually Works in Practice

Premarket Approval under section 515 of the FD&C Act is the predominant regulatory route for companion diagnostics in the United States. FDA classifies most companion diagnostics as Class III devices on the basis that a misclassified result directly influences whether a patient receives or is withheld from a targeted therapy. Of the 78-plus drug/CDx combinations approved by early 2025, the overwhelming majority obtained marketing authorization through the PMA pathway.

What a PMA Submission Must Contain

A PMA submission must demonstrate reasonable assurance of safety and effectiveness. FDA has expressed a clear preference for the modular PMA format, in which sponsors submit four sequential modules as data become available rather than waiting for a single complete package.

Engaging CDRH through a pre-submission before filing Module 1 is considered standard practice. That pre-submission should align the table of contents, content expectations for each module, and review timelines.

When a Supplemental PMA Is the Right Mechanism

For a drug sponsor adding a companion diagnostic indication to an already-approved PMA device, the correct filing mechanism is a Supplemental PMA (sPMA). The sPMA is a narrower submission focused on the analytical and clinical validation data supporting the specific new drug indication. It does not require re-justifying the device’s general safety profile, which shortens the evidentiary package considerably.

What the PMA Pathway Costs in Time and Fees

The PMA pathway is resource-intensive. User fees for a PMA submission run into the hundreds of thousands of dollars. The statutory review standard is 180 days under normal circumstances. Advisory panel meetings may be convened for novel device types or complex clinical performance questions, which can extend that timeline.

FDA’s Proposed Reclassification of Nucleic Acid-Based Oncology CDx from Class III to Class II

In November 2025, FDA published a proposed order in the Federal Register that would materially change the submission pathway for a major category of companion diagnostics. The proposed rule, docketed as FDA-2025-N-4622, would create a new Class II device type under 21 CFR 866.6075.

If finalized, NGS panels, PCR-based assays, and NAAT-based oncology companion diagnostics would move from the Class III PMA pathway to 510(k) clearance with special controls.

The Regulatory Logic Behind the Proposed Downclassification

FDA’s rationale is grounded in retrospective analysis of 17 PMAs across a decade of oncology CDx approvals. The agency concluded that the risk profile of these tests is now fully characterizable through defined special controls covering:

• Analytical validity requirements (accuracy, precision, limit of detection, reportable range)
• Clinical validity expectations linking the biomarker to the therapeutic indication
• Design and labeling specifications
• Post-market controls

The evidentiary bar remains rigorous. What changes is the submission architecture. Substantial equivalence to a well-characterized archetype would, in principle, eliminate the need for a full clinical trial per new test when analytical comparability can be demonstrated.

What This Reclassification Does Not Cover

Regulatory affairs professionals should note that this reclassification applies exclusively to nucleic acid-based oncology tests. IHC-based, FISH-based, imaging, and other modality companion diagnostics are not covered by the proposal.

The comment period closed in January 2026. A final rule is anticipated but has not yet been published. CDx programs in active development should monitor the docket closely rather than planning around assumed finalization.

Structuring an IVD Companion Diagnostic Pharma Partnership to Withstand FDA Scrutiny

The IVD companion diagnostic pharma partnership is frequently where CDx programs encounter their most consequential operational challenges. Regulatory affairs teams need to exercise oversight that extends well beyond submission management.

When the drug sponsor and the diagnostic manufacturer are separate legal entities (the most common configuration in oncology CDx), the partnership must resolve several foundational questions before clinical trials begin.

Data Sharing and Letters of Authorization

The diagnostic sponsor needs access to clinical outcome data from the drug trial to support clinical validation. The drug sponsor needs the diagnostic sponsor’s performance data to complete its NDA or BLA. Neither party can finalize its FDA submission without the other.

Data sharing rights and the flow of clinical specimens between institutions require contractual agreements that anticipate FDA’s requirement for Letters of Authorization. These letters allow each sponsor’s submission to cross-reference the other’s proprietary data without transferring confidential information directly.

Timing Alignment and the Contemporaneous Approval Goal

FDA’s preferred model is contemporaneous approval, meaning the CDx PMA and the drug NDA or BLA are approved on the same day. Achieving that requires:

• Joint regulatory strategy sessions from the pre-IND stage
• Coordinated pre-submission meetings with CDRH and the drug review center
• Contract provisions that prevent either party from advancing or withdrawing an FDA submission unilaterally without the other’s awareness

The Bridging Study Decision and Its Downstream Consequences

The choice of which assay to use in the clinical trial determines whether a bridging study will be required after the pivotal study closes.

Bridging studies must demonstrate that patients selected by the clinical trial assay show equivalent clinical outcomes when retested on the final CDx. Both biomarker-positive and biomarker-negative samples from all screened subjects must be banked with confirmed analyte stability. Patient consent for retesting must be obtained at enrollment. Retroactive consent collection is nearly impossible in practice.

Sponsorship Opportunity: ImmunoMark Summit

The partnerships shaping CDx co-development are built in rooms where pharma, biotech, and diagnostics leaders meet — not in inboxes. ImmunoMark Summit is a dedicated conference series at the intersection of immuno-oncology and biomarker science, running annually across London and Boston. The programme includes a dedicated Biomarkers and CDx track, with 400+ expected attendees per edition and 45–50% of the audience at VP level or above.

If your organisation works in companion diagnostic development, regulatory strategy, or CDx commercialisation and wants direct access to that decision-making audience, sponsorship positions are available for the Boston 2026 edition (7–8 October).

Enquire about sponsorship at ImmunoMark Summit

The Two Types of Validation Evidence FDA Requires in Every CDx PMA

FDA’s validation requirements for an IVD companion diagnostic operate on two distinct axes. Conflating them is a common source of deficiency letters.

Analytical Validation: Proving the Assay Measures What It Claims To Measure

Analytical validation establishes that the assay accurately and reliably detects or measures the analyte it is designed to detect. The parameters FDA examines include:

• Accuracy and precision
• Analytical sensitivity and specificity
• Limit of detection (LoD)
• Reproducibility across sites and lot numbers
• Analytical cutoff justification
• Linearity
• Interference testing

All analytical validation must be conducted to CLSI standards. For molecular assays, FDA expects documentation covering every element of the testing chain: the nucleic acid extraction kit, the assay reagents, the detection platform, and the bioinformatics software used to interpret results.

Clinical Validation: Proving the Test Result Predicts the Right Clinical Outcome

Clinical validation establishes that the test result predicts the clinical outcome for which the drug is indicated. Patients identified as biomarker-positive by the CDx should benefit from the therapy; patients identified as biomarker-negative should not (or should face a documented elevated safety risk if treated).

FDA generally expects clinical validation data to come from the pivotal clinical trial, using samples from the intended-use population.

For rare biomarkers where sufficient trial samples are not available, the FDA has permitted alternative sample sources, including archival specimens, retrospective samples, and commercially acquired specimens. Sponsors pursuing this route should engage the FDA early through pre-IDE meetings or Q-Submissions to align on the justification before committing to a validation design.

What Happens When Contemporaneous CDx and Drug Approval Cannot Be Achieved

For serious or life-threatening conditions with no satisfactory alternative therapy, FDA may approve the therapeutic product before the companion diagnostic is cleared. The drug label will reference an appropriate diagnostic test generically until the CDx PMA is complete, at which point the label must be updated.

Regulatory affairs teams should treat this as a contingency, not a planning assumption. The decisions that create approval lag are made years before their consequences become visible.

The CDx strategies that prevent these delays get built through the right conversations, at the right level. ImmunoMark Summit brings together 400+ pharma, biotech, and diagnostics leaders with a dedicated CDx track. Sponsorship for Boston 2026 is open.

Submit a sponsorship enquiry