Product-wet integrity testing: Cytiva risk-based strategies

Integrity testing of sterilizing-grade final filters is a cornerstone of a robust contamination control strategy (CCS) in aseptic processing. While standard-fluid testing with water remains the default, there are legitimate cases where testing using drug product itself, product-wet integrity testing (PWIT), is advantageous—particularly to avoid large volume post-use flushes, prevent dilution of product, or when product-membrane interactions alter standard-fluid test outcomes. In many facilities, pre-use, post-sterilization integrity testing (PUPSIT) is assessed within a risk-based contamination-control strategy. Cytiva’s position—supported by internal studies—is that, when performed under cGMP conditions, PUPSIT introduces a very low risk of bacterial penetration. This perspective is independent of PWIT, which serves a different purpose: establishing scientifically sound integrity-test limits when the product is used as the wetting fluid. PWIT complements, rather than replaces, PUPSIT by ensuring that integrity testing remains accurate and representative when product-specific wetting effects are relevant.

Across published industry approaches, we see two dominant schools of thought for developing product-wet limits: (1) ratio/correction-factor approaches that translate water-wet specifications to product-wet specifications via bubble-point or diffusive-flow ratios; and (2) cartridge-scale, multi-point forward-flow curve approaches that characterize the product and reference fluid behaviors and derive test pressure, forward-flow/pressure-hold, and bubble-point limits in one integrated workflow.

In this article, we show how Cytiva applies a rigorous, science-driven framework that verifies product-wet integrity results through multiple cross-checks, ensures data quality through clear acceptance criteria, and distinguishes between retention-correlated tests and those suited for broader system checks.

What is product-wet integrity testing, and why does it matter in filter validation?

PWIT assesses filter integrity when wetted with the drug product. It is typically part of a filter validation package, which includes a series of tests for Cytiva sterilizing-grade filters and single-use systems that minimize product consumption while meeting regulatory expectations. PWIT can be appropriate when the product is difficult to remove from the membrane without excessive flushing, dilution with a standard wetting fluid is undesirable, or the product’s surface-active components (e.g., detergents, proteins) change integrity measurements when compared with standard fluids.

This article provides our recommendations for performing PWIT in a way that is: risk-based and fact-grounded, complementary to PUPSIT and post-use IT risk assessments and process-specific bacterial challenges, and practical for manufacturing science and technology (MSAT) and validation engineers to adopt within modern CCS programs.

Filter sterilization: how process-wet integrity testing relates to PUPSIT

PWIT and PUPSIT address different steps in the sterilizing filtration life cycle. PUPSIT demonstrates filter integrity immediately after sterilization and assembly; PWIT defines integrity test limits and test conditions when the product is the wetting medium (commonly applied post-use, but also sometimes pre-use when product pre-wetting is a process requirement). PWIT, therefore, informs how a filter integrity test is performed with product; it does not mandate PUPSIT itself.

Our position—based on contaminant load calculations and process-specific challenge simulations—is that PUPSIT performed with water for injection (WFI) carries negligible risk of bacterial penetration given low bioburden limits and the high bacterial retention margins of qualified sterilizing‑grade filters. When PUPSIT is performed with product, the sterilizing-grade filter would again only be exposed to a very small bioburden load, like the example given for WFI. Cytiva also does not consider this a significant risk for bacterial penetration through the sterilizing-grade filter. However, as part of the risk assessment, users should consider:

• Properties of the process fluid (bactericidal vs non-bactericidal)
• Maximum allowable bioburden (presumably ≤10 colony-forming units [CFU]/100 mL)
• Qualitative assessment of prefiltration bioburden
• Volume of process fluid for filter wetting/integrity testing

PWIT is a prerequisite to the process-specific bacterial challenge study if an end user wants to evaluate the potential risk of bacterial penetration when using process fluid to perform PUPSIT.

Industry approaches to bubble point, forward flow, and cartridge filter integrity tests

We summarize below the prevailing (generalized) approaches discussed in widely cited references from major validation service providers and technical publications.

Bubble-point-ratio (BPR) framework

Essence: Determine a ratio between product-wet and water-wet (or other standard-fluid) bubble points at lab-scale (multiple discs, multiple lots), then confirm (or revalidate) the ratio at process-scale, typically with multiple devices and statistical control (e.g., coefficient of variation thresholds).

Strengths: Clear statistical thresholds (e.g., coefficient of variation), procedural templates for process qualification, and pragmatic fallbacks (validated rinse or lower surface-tension reference solvent) when PWIT is unstable.

Limitations: Disc-scale methods can under-represent full-scale exposure volume/surface and contact-time effects.

Correction-factor approach

Essence: Use correction factors from multi-lot water-to-product tests for bubble point, diffusive (forward) flow, and test pressures, with detailed “points to consider” on wetting, environment, automation, and above all volume-area scaling and contact time.

Strengths: Strong treatment of surface chemistry and scale-down fidelity; demonstrative case studies show that insufficient product volume/contact time can underestimate the product’s effect on integrity test results.

Limitations: Less prescriptive on multi-point forward-flow curve analysis and decision logic for issuing final limits.

Cartridge-scale, multi-point forward-flow spectrum methods

Essence: Execute multi-pressure forward-flow (knee limit, KL) spectra on a full-area pleated element first with reference fluid, then with product—deriving product-wet test pressure, forward-flow/pressure-decay (FF/PD) limit, and bubble-point limit from robust curve analytics and fluid-property cross-checks.

Strengths: Closer to real-world behavior (full-area element), physics-based cross-validation (product/reference), and transparent quality gates on data; integrates FF/PD and bubble point determination in one workflow.

Limitations: Requires more instrumentation discipline (temperature control, stable curve regions), and clear decision rules to manage divergence between property-based expectations and empirical curves.

Table 1. Summary of ratio/correction and cartridge-scale, multi-point approaches.

Our recommendations for implementing product-wet integrity testing

Our recommendations for implementing PWIT aim to establish product-wet integrity test limits that are scientifically defensible, representative of the process, and practical for routine use.

Use a cartridge-scale, multi-point approach to derive product-wet limits.

• Run a multi-pressure forward-flow spectrum first in a reference fluid (e.g., water for hydrophilic membranes), then with product under representative temperature and wetting conditions. Ensure robust temperature control and stable curve regions prior to analysis.
• From these datasets derive product-wet test pressure, forward-flow (or pressure-decay) limit, and bubble-point limit in an integrated manner.

Cross-validate empirical curves with fluid-property expectations.

• Compare product/reference behavior using both surface-tension-based expectations and curve-derived (KL) relationships. If they materially diverge, apply conservative decision logic and repeat testing if indicated. This dual track reduces the risk of false passes or false failures caused by adsorption, surfactants, or incomplete wetting.

Enforce clear curve-quality and applicability gates prior to issuance.

• Require a stable region (multiple points with low variability), clear transition (KL) into bulk flow, and appropriate goodness-of-fit. Treat curves showing non-wetting, no clear stable region, or foaming as invalid; re-wet and repeat.
• If the product-wet limits cannot be correlated back to the reference correlation for bacterial retention (e.g., because normalized flows are too elevated), classify the PWIT outcome as a gross integrity check rather than a critical, retention-correlated limit.

When to perform PWIT

• Post-use: Recommended where large volume water flushes are impractical, where post-use standard-fluid testing yields inconsistent results due to product residues, or to minimize product loss. The integrity test fluid used post-use should be diverted to waste.
• Pre-use: Consider pre-use PWIT only if the process design requires a product pre-wet (e.g., to avoid dilution), WFI sources contain waterborne bacteria, and where validated PWIT limits already exist.

How PWIT interacts with PUPSIT and post-use IT

• If a site’s CCS calls for PUPSIT with process fluid, first perform a PWIT study to determine product-wet forward flow test pressures and/or minimum expected bubble point values to inform the bacterial challenge study.
• The risk to batch sterility by performing post-use IT with process fluid is only a concern if the process fluid used during integrity testing is directed to final fill. While not common occurrence, it could be simulated as part of the process-specific bacterial challenge study and thus would require PWIT to be performed as a prerequisite for the reasons listed above. If required, the PWIT would be simulated at the end of the process-specific bacterial challenge test. However, careful consideration is needed to assess if simulating a post-use PWIT on a filter that has been challenged at ≥ 1.0 x 10⁷ CFU/cm² may add an excessive safety factor compared to what a process filter would be exposed to in the actual manufacturing process.

Life cycle and change control

• During PQ and routine monitoring, compare observed pre-/post-use integrity results with the issued product-wet windows. If field experience indicates the issued limits are overly stringent (e.g., recurrent false failures), adjust limits via a controlled update using the conservative branch of the decision logic.

Practical notes for MSAT and validation teams

• Representativeness matters. Literature demonstrates that exposure volume to membrane area and contact time can depress bubble-point values significantly; therefore, replicate process-like exposure during product-wet characterization.
• Measure what matters. Record temperature closely during tests; where relevant, measure surface tension (and when justified, viscosity/density) at the test temperature to support interpretation.
• Define critical vs gross leak checks. Product-wet limits that cannot be tied back to reference-based retention correlations should be clearly labeled as gross leak checks—they still provide value in detecting significant damage but should not be over-interpreted.
• Maintain transparency. Document curve-quality assessments, decision logic, and the rationale for the final issued limits; this ensures auditability and facilitates future life cycle updates.

Conclusion: product-wet integrity testing is key to robust CCS and PUPSIT strategies

PWIT is a valuable tool—when executed with rigorous, process-representative methods—to ensure integrity testing aligns with actual product behavior while supporting efficient operations (e.g., reduced flushing, minimized dilution). The cartridge-scale, multi-point approach recommended here integrates test-pressure, forward-flow/pressure-decay, and bubble-point determination in a single workflow, strengthened by dual derivations (fluid-property and curve-based) and explicit curve-quality gates. Within a modern CCS and in concert with risk-based PUPSIT strategies, this provides MSAT and validation engineers with a robust, auditable path to product-wet limits that are both scientifically defensible and operationally practical.

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