After the Handover: A Comparability Protocol for Antibody Changes in UK IVD Development
Antibody changes are rarely announced as assay changes. A supplier discontinues a line, a recombinant expression system is scaled up, a conjugation process moves to a new contractor, and the procurement record shows a like-for-like replacement. For IVD developers, that framing is too narrow. A change in antibody source or manufacture can alter binding, glycosylation, aggregation, purity and conjugation efficiency. The assay may still run, but the evidence base has moved.
In the UK, the regulatory direction of travel is clear: UKCA readiness depends on a technical file that can withstand scrutiny. A bridging study is the mechanism for showing that an antibody change has not undermined the assay's performance. It should be planned before the change is adopted, not assembled afterwards from fragmentary bench notes.
Why antibody changes are not procurement events
A supplier change can affect epitope recognition, affinity and avidity. An expression system change can alter post-translational modification, folding and impurity profiles. A conjugation change can shift the dye-to-protein ratio, alter activity or introduce new lot-to-lot variation. These are not abstract quality attributes; they can change signal, background, dynamic range and, in some cases, clinical interpretation.
The risk is highest where the antibody is a critical component: capture or detection in a sandwich assay, the recognition element in a lateral flow device, or the primary reagent in an immunohistochemistry workflow. In those settings, a change should trigger a formal risk assessment and a bridging plan. The plan should state what changed, why it changed, what critical-to-quality attributes are affected and what evidence will be generated to demonstrate comparability.
A UK bridging checklist
A defensible bridging file usually has four layers.
1. Change description and risk assessment
Document the old and new antibody, including catalogue number, lot, supplier, expression system and conjugation method. Record the reason for the change and any supplier quality information, such as certificates of analysis, audit findings or change notifications. Assess the impact on intended use, assay format, sample matrix and regulatory status. This is also the point to check UK supply continuity: a change made for availability reasons still needs performance evidence.
2. Comparability data
Comparability data should be generated side by side, using the same antigen panel and, where possible, the same instrument and operator. A minimum package would include identity and purity data, aggregation analysis, functional binding by ELISA, surface plasmon resonance or biolayer interferometry, and a comparison of multiple lots where available. The question is not whether the new antibody binds, but whether it binds in the same way across the range that matters to the assay.
3. Assay performance evidence
The bridging study should then move from antibody characterisation to assay performance. Precision, limit of detection, linearity, interference and cross-reactivity should be repeated on the intended platform. Matrix comparison is essential where the assay is used in serum, plasma, tissue or another complex sample type. Where clinical samples are available, paired comparison between old and new antibody can provide useful concordance data; sample governance and consent arrangements should be confirmed before use. Stability and lot-to-lot data should also be considered, particularly if the change affects conjugation or storage conditions.
4. Documentation and UKCA readiness
The bridging file should sit within the quality management system and link to change control, risk management and validation records. It should include a protocol with predefined acceptance criteria, raw data, analysis, conclusions and approval. For UKCA readiness, the file should be traceable to the technical documentation and to the manufacturer's regulatory obligations. This is not legal advice, and developers should confirm current MHRA guidance, but the principle is straightforward: the evidence trail should allow a reviewer to see what changed, what was tested and why the conclusion is justified.
Designing a bridge that can fail
A bridging study that cannot fail is not a study. Acceptance criteria should be set before the experiment and should reflect the assay's intended use. For quantitative assays, a paired comparison with bias and agreement analysis may be appropriate; for qualitative assays, concordance and discordance review are more relevant. The sample size should be justified, not inherited from a previous project without thought.
If the data do not meet the criteria, the response should be proportionate. Options may include re-optimisation of the new antibody, a change to the reference standard, additional purification or conjugation controls, or a decision not to adopt the change. The bridging file should record that decision as well. A negative result, properly documented, is still evidence of control.
The handover is the assay's problem
Antibody changes are often managed as supply-chain events. For IVD developers, they are assay events. The handover from one antibody to another should be accompanied by a comparability protocol that connects reagent characterisation to assay performance and, ultimately, to regulatory evidence. In 2026, with UKCA readiness demanding a coherent technical file, that discipline is not optional. The antibody may change hands, but the burden of proof stays with the developer.
