How to Write Biocompatibility Justifications That Survive Regulatory Review

Every time you exclude an endpoint, rely on existing data, claim biological equivalence, or choose which reprocessing timepoints to evaluate, you are making a judgment call that needs to be documented. Weak justifications are among the most common causes of deficiency findings in biological evaluation documentation. The problem is rarely that the decision was wrong. It is that the documented rationale is too vague or too generic to be evaluated by a reviewer with no prior knowledge of your device.

This article covers the anatomy of a strong justification, the most common deficiency patterns, and where FDA and EU notified body expectations differ.

What a justification actually needs to do

A justification explains why a specific decision is scientifically supportable for this specific device. A justification that would be equally valid for any device, regardless of its materials or clinical use, is not specific enough. ISO 10993-1:2025 requires the BER to document rationale for decisions at each stage, aligning with ISO 14971's traceability requirements.¹ The EU MDR requires manufacturers to demonstrate through their GSPR documentation that biological risks are minimized, with notified bodies expecting traceable reasoning for every significant decision.² The FDA similarly emphasizes that deviations from the evaluation framework should be justified with clear conclusions and identified gaps.³

The four components of a strong justification

1. What decision was made

State the specific decision clearly. "Genotoxicity testing was not performed." "Chemical characterization data from the predicate device was used in lieu of new extractable testing." "Biocompatibility evaluation was performed at T-end only, without baseline testing." The reviewer needs to know exactly what you decided before they can evaluate whether the reasoning supports it.

2. Why the decision is appropriate for this device

This is where most justifications fail. The rationale must connect the decision to the specific characteristics of the device under review: its materials, its contact type and duration, its reprocessing method, its clinical use, and its patient population. A statement that "the material has a long history of safe use" is not a justification because it does not identify which material, in what configuration, under what conditions, or reference any evidence.

3. What evidence supports the decision

Every justification must be traceable to specific evidence. This could be a test report, a published study, chemical characterization data, a toxicological risk assessment, a biological equivalence evaluation, or a documented risk assessment. The reviewer should be able to locate and evaluate the referenced evidence. If the evidence is "internal knowledge" or "industry practice," it is not traceable and will not hold up.

4. Why the evidence is still relevant

If the evidence comes from a prior evaluation cycle, a different device variant, or a predicate device, the justification must address whether anything has changed that would affect the relevance of that evidence. Material changes, manufacturing process changes, sterilization method changes, or changes in the clinical use of the device can all invalidate previously adequate evidence.

The most common justification failures

Generic material safety claims

The weak version makes a claim about a general material class. The strong version identifies the specific grade, references specific test reports, and connects each conclusion to documented evidence. A reviewer can verify every claim in the strong version. They cannot verify anything in the weak version.

Circular reasoning

The weak version uses the conclusion as its own justification. The strong version shows the work: it identifies the evaluation method, references the data, and explains how the conclusion was reached.

Undocumented precedent

Prior acceptance is not inherently invalid as supporting evidence, but it must be documented with specifics. Which submission? Which device? What was the same, and what has changed? Reviewers change, expectations evolve, and standards are revised. An approach accepted in 2019 may not be accepted in 2026, especially following the 2025 revision of ISO 10993-1.

Missing rationale for excluded effects

One of the most frequent deficiency patterns is a BEP that lists which biological effects were evaluated and which were not, without documenting why the excluded effects were excluded. The 2025 standard's restructured evaluation tables make this more visible: if an effect is indicated for your device category, its absence from your evaluation requires a documented justification. For detailed guidance on this specific topic, see how to justify excluding biocompatibility endpoints in your BEP.

Where FDA and EU MDR expectations actually differ

Both sides want documented, evidence-based justifications. But the mechanics are different in ways that affect how you write.

The FDA gives you a checklist to work from. The FDA-modified biocompatibility matrix tells you which endpoints to evaluate based on your device's contact type and duration. If your evaluation follows the matrix, the justification burden is straightforward. If you deviate, you justify the deviation against a published agency position. The FDA has also published specific methodological expectations for chemical characterization, AET calculations, and test article preparation, so reviewers are checking your work against concrete guidance documents.³

EU notified bodies do not have an equivalent matrix. They assess your BEP and BER as part of the full technical documentation review and judge whether your evaluation is adequate based on the ISO standard, MDCG guidance, and their own interpretation of current "state of the art." This gives you more flexibility in how you structure your evaluation, but less certainty about what will be accepted. Notified body reviewers also tend to scrutinize cross-document traceability more heavily: a justification that is scientifically adequate but contradicts a statement elsewhere in your technical file will generate a finding.²

The EU requires CMR substance justifications the FDA does not. Under MDR Annex I, Section 10.4.1, devices containing carcinogenic, mutagenic, or reproductive toxic substances above 0.1% w/w require a documented justification that no suitable alternative exists and that the benefit-risk ratio supports their use. This is an entire category of justification that does not have a direct equivalent in the FDA framework.⁵

Equivalence is harder in the EU. The FDA's 510(k) substantial equivalence typically focuses on demonstrating that your device is similar enough to a predicate. EU notified bodies apply a three-phase assessment covering technical, biological, and clinical equivalence (MDCG 2020-5), and specifically scrutinize whether manufacturing differences could affect the release characteristics of substances from the device.⁴

For a detailed breakdown of these differences with specific examples and deficiency finding patterns from each side, see FDA vs. EU MDR expectations for biological evaluation: where they actually differ.

Justifications across different decision types

Endpoint exclusions: Every excluded biological effect requires device-specific rationale. See the detailed endpoint exclusion guide.

Use of existing data: Confirm the test was performed on a representative article, conditions remain relevant, and no device changes have occurred since testing. If the test used a prior standard revision, acknowledge this and explain why the data remains adequate.

Biological equivalence: Identify the comparator, document the comparison across all relevant dimensions, and address any differences. Under the 2025 standard, focus on whether the biological risk profile is sufficiently comparable, not just whether inputs match. Under the EU MDR, additionally address MDCG 2020-5's three-phase framework.⁴

Test method deviations: Document what was different, why the deviation was necessary, and why results remain valid.

Reprocessing timepoints: Document why the selected timepoints adequately characterize the device across its reuse life. See the detailed reprocessing timepoint guide.

A self-check before submission

For every justification in your BEP and BER, confirm that the decision is stated clearly, the rationale is device-specific, the supporting evidence is referenced and traceable, and continued relevance is addressed for any prior data. Then check the one thing most teams miss: cross-document consistency. Does the rationale contradict anything stated in the BEP, BER, chemical characterization, TRA, or risk management file?

Cross-document consistency is one of the hardest things to maintain and one of the most common sources of deficiency findings. BioEvalPro scores every justification in your package for specificity, traceability, and internal consistency, and flags the ones that would not survive the scrutiny described in this article. request a gap analysis for early access, or reach out to discuss your documentation needs.