In the field of sterile pharmaceutical preparations, product sterility is critical to safeguarding patient medication safety. Sterilizing‑grade filtration has been incorporated as a key process in the sterilization methods specified in the Chinese Pharmacopoeia, and its effectiveness directly determines pharmaceutical sterility. Pre‑Use and Post‑Sterilization Integrity Test (PUPSIT) is a focal point stipulated in international regulatory requirements and industrial standards, bearing great significance for pharmaceutical manufacturers. This article comprehensively analyzes key compliance inspection points for PUPSIT, helping pharmaceutical enterprises thoroughly understand and implement this practice to ensure production meets high‑level sterility requirements.
Part 01 Definition and Functions of PUPSIT
1 Confirmation of filter physical integrity
Sterilization processes such as high‑temperature steam autoclaving may damage filter membranes and component structures, resulting in filter deformation, perforation, cracking or melting. Such defects will severely compromise microbial retention capacity, enabling microbial penetration downstream and consequently causing contamination of sterile pharmaceutical products.
Accordingly, for final‑step sterilizing‑grade filtration processes, sterilizing filters shall undergo integrity testing in the pre‑use and post‑sterilization state. This mitigates the risk of product contamination stemming from filter damage incurred during sterilization that impairs microbial retention performance.
PUPSIT draws high industrial attention also due to a widely‑discussed phenomenon: Flawed Filter Masking. This describes a scenario where a sterilized sterilizing filter contains micro‑defects that allow microbial penetration yet still passes integrity testing. In 2017, PDA and the Biopharmaceutical Unit Operations Consortium jointly established an expert working group under the Sterilizing Filtration Quality Risk‑Management Alliance to address PUPSIT, whose research findings were subsequently published in professional journals. The study rested on two hypotheses:
① Filter membrane defects exist with pore sizes large enough for bacterial penetration yet small enough to become blocked during filtration;
② Process fluid can block such defective pores, allowing the filter to pass integrity testing.
Research demonstrated that masking of defective pores occurs when filter blockage exceeds 81%.
By performing PUPSIT, manufacturers can detect inherent manufacturing defects or operation‑induced damage prior to filter use, verifying that filters maintain satisfactory physical condition and proper filtration function before process deployment. This delivers proactive risk mitigation for the sterility of filtered pharmaceutical products.
2 Regulatory compliance requirements
Beyond the aforementioned international regulations and industrial standards, EU GMP Annex 1, WHO, PIC/S and other global regulatory frameworks explicitly mandate PUPSIT implementation for sterile pharmaceutical manufacturing. Within China, NMPA issued the Guidelines for the Technical Application of Sterilizing Filtration in 2018, setting forth requirements for pre‑use integrity testing of sterilizing filters under Section 6.3 Integrity Testing: “After use, sterilizing filters shall be subjected to timely integrity testing with appropriate methods and results shall be recorded. For pre‑use integrity testing of sterilizing filters, risk assessment shall be performed to determine whether testing is required, and whether testing shall be performed pre‑sterilization or post‑sterilization.”
Domestic regulations have therefore established clear expectations for relevant design and operational controls. Consequently, inspection of PUPSIT configuration and management represents an essential measure for pharmaceutical enterprises to comply with industrial specifications and achieve regulatory market authorization.
Part 02 Inspection Key Points for PUPSIT
1 Supplier Audit
2.1.1 Review of core validation documentation
Sterilizing‑grade filters are routinely deployed within sterilization workflows to protect downstream filtrates from microbial contamination. PUPSIT verifies that sterilizing‑grade filters retain intended performance post‑sterilization and prior to production use. Test methodologies including wetting solution type and concentration, test pressure and acceptance criteria are generally provided by filter suppliers. Supplier audits shall therefore be conducted from the holistic perspective of the sterilizing filtration process. Core supporting validation documents shall cover: filter bacterial retention test results, extractables data under representative simulated solvents and test conditions, filter compatibility with designated sterilization modalities (e.g. SIP or gamma irradiation), and safety evaluation data. For safety assessment, compliance with standards such as USP Class VI is typically verified, as most international suppliers complete these studies during product development. Filters meeting such criteria are expected not to compromise pharmaceutical quality or patient safety upon product contact. These datasets are normally compiled within supplier validation guides.
2.1.2 Review of ongoing validation records
Beyond core validation files, ongoing supplier documentation including batch‑specific filter certificates of conformance shall be reviewed. This confirms each filter batch has undergone rigorous in‑house quality control and integrity testing, supported by corresponding quality certificates, providing additional assurance through the supplier’s quality‑management system.
2.1.3 Review of process‑specific validation documentation
Internal process audits shall evaluate complete process‑specific validation reports generated using actual product or simulated process fluids under production‑representative conditions. Typical scope includes bacterial retention testing, chemical compatibility testing, extractables and/or leachables studies, safety assessment and adsorption evaluation.
Bacterial retention challenge studies expose filters to defined microbial challenges under simulated production conditions to demonstrate microbial retention performance consistent with sterility requirements. Compatibility validation confirms no adverse chemical or physical interactions between filter materials and product components that would impair filtration or pharmaceutical quality. Complementary extractables and leachables reports characterize substances released from filter materials; per international best practices, such data alone is insufficient to demonstrate product safety and must be supplemented by toxicological safety assessments.
Where manufacturers reuse sterilizing filters, process‑specific validation reports for repeated‑use conditions are mandatory. Bacterial retention and chemical compatibility studies are generally required. Other validation endpoints may be justified via scientific evaluation referencing single‑use filter datasets, with rationale documented in assessment reports in lieu of full replicate testing.
Supporting data on filter resistance to sterilization cycles are also critical to justify defined maximum repeated‑sterilization counts (specified numerical values rather than open‑ended ranges), confirming sustained bacterial retention without excessive filter degradation.
2 In‑House Process Audit
2.2.1 Filtration‑system design review
Certain product formulations such as liposomes present elevated downstream bacterial‑penetration risk. Consequently, sequential filtration architectures including redundant filtration systems have been widely adopted. A typical redundant filtration setup comprises primary and secondary (redundant) filter stages, substantially mitigating single‑point‑of‑failure risk. Should the primary filter sustain unexpected damage or failure, the upstream secondary filter provides an additional safeguard for product sterility. Redundant filtration systems, whether constructed from stainless‑steel components or single‑use technology (SUT), involve complex piping configurations and multi‑step PUPSIT workflows. Thorough upfront design of piping and component connections is therefore required to achieve intended functional performance.
For PUPSIT implementation within SUT‑based redundant sterilizing filtration systems, critical system functions and workflows require inspection consideration including but not limited to the following:
2.2.1.1 Pre‑use system leak‑tightness testing to rule out connection‑related leakage
During this test, pressurized non‑sterile gas is introduced upstream of the secondary (redundant) filter, while inter‑filter piping and the downstream side of the primary filter remain post‑sterilization and shall be maintained closed. Pre‑use leak‑tightness testing establishes a baseline reference for subsequent PUPSIT interpretation. Any piping‑connection leakage shall be rectified and the sterile system re‑established prior to filter integrity evaluation.
2.2.1.2 Filter wetting prior to integrity testing
Both filter stages shall be fully wetted. The primary filter downstream outlet shall be fitted with wetting‑liquid discharge assemblies such as drain valves for stainless‑steel installations or collection bags for SUT configurations.
Wetting modalities including water‑based, buffer‑based or product‑fluid wetting shall be selected according to formulation characteristics. For water‑sensitive products, direct product‑fluid wetting may be adopted to avoid adverse impacts from aqueous wetting agents.
For SUT systems, collection‑bag capacity shall match production scale. Undersized bags require frequent replacement, risking disruption to sterile conditions; oversized bags occupy excessive footprint and present waste‑handling hazards. Large‑volume filtrate‑collection bags shall be fitted with vent gas sterilizing filters to prevent environmental microbial ingress that would compromise downstream sterility.
2.2.1.3 Sequential PUPSIT execution for secondary and primary filters
Both filters have completed pre‑process sterilization; special attention shall be paid to preventing microbial contamination introduced during testing. Sterile Water for Injection shall be utilized where feasible. A post‑sterilization gas‑sterilizing filter shall be installed between secondary and primary filters to facilitate gas venting during secondary‑filter testing and supply of sterile compressed air for primary‑filter testing.
Upon completion of PUPSIT activities, additional associated inspection considerations apply:
① Compatibility between PUPSIT wetting fluid and subsequent process fluid shall be evaluated, addressing risks including dilution or chemical alteration of product components.
② PUPSIT for both single‑stage and redundant filtration systems shall be performed as an in‑situ integrity test. In‑situ testing avoids filter disassembly and re‑assembly that could introduce secondary contamination, preserving system sterility while improving operational efficiency and safety.
③ Piping assemblies shall possess adequate pressure rating to withstand integrity‑test pressures and routine process operating pressures. Insufficient pressure resistance may cause rupture or leakage, compromising test accuracy, triggering product contamination or unplanned production downtime. SUT tubing material and dimension selection shall account for maximum expected test and process pressures.
2.2.2 Operational risk‑control assessment
Given PUPSIT complexity and criticality, internal audits shall thoroughly evaluate documented workflows. Comprehensive standard operating procedures (SOPs) shall be established addressing operational details including but not limited to:
① Adequacy of filter wetting. Risk of compromising sterility through mid‑run collection‑vessel switching due to insufficient waste‑bag capacity from repeated wetting cycles shall be avoided.
② Alignment between wetting‑medium type and integrity‑test acceptance criteria, distinguishing acceptance limits for water‑wetted, buffer‑wetted or product‑fluid‑wetted filter conditions.
③ Residual wetting‑liquid concerns. Following successful PUPSIT, the system proceeds directly to product filtration. While Water for Injection / Sterile Water for Injection are common wetting media, organic solvents such as isopropanol or tert‑butanol are also applied for integrity testing. Demonstrating adequate removal of organic‑solvent residuals post‑PUPSIT poses challenges, requiring focused method development and validation.
④ Compressed‑air quality during PUPSIT execution; compressed air supplied upstream of the primary filter within redundant systems must be sterile.
⑤ Sequential individual testing of each filter stage in redundant configurations; simultaneous dual‑stage testing prevents accurate assignment of integrity‑test results to individual filters.
⑥ Maintain atmospheric pressure downstream of filters during testing. Restricted downstream headspace impedes gas permeation through filter membranes and may produce false‑negative results. This requirement justifies vent gas‑sterilizing filters fitted to SUT waste‑collection bags.
⑦ For stainless‑steel installations, SIP is performed prior to PUPSIT. Piping and valve arrangements for SIP venting and condensate removal shall be engineered in conjunction with PUPSIT hardware to prevent workflow impairment caused by suboptimal layout.
⑧ Evaluate impacts of PUPSIT operations on classified clean‑room environments, for example contamination risks from discharge of non‑sterile gas or liquid into Grade A zones.
Additionally, SOP review shall verify adequate operator training and proficiency in PUPSIT execution.
2.2.3 Data‑integrity review of test records
Integrity‑test records constitute key evidence for assessing process robustness, deviation investigation and traceability for sterilizing‑grade filtration workflows. Data‑integrity evaluation forms a core component of internal process audits, covering:
① PUPSIT operational logs documenting test timestamp, operator identity, test methodology and results for each execution.
② Associated supporting monitoring records including environmental parameters such as temperature and clean‑room classification.
③ Deviation‑handling reports. Abnormal test outcomes or operational errors shall be documented, together with corrective actions and resulting outcomes to support timely issue identification and resolution.
Part 03 Regulatory Evolution and Mitigation Strategies
1 Updates to international regulatory frameworks
As Chinese pharmaceutical manufacturers expand into global markets, close attention to international regulatory revisions has become essential. The 2022 revision of EU GMP Annex 1 introduced transition periods for industry adaptation. To sustain alignment with global standards and support international market access, manufacturers shall track regulatory updates and correspondingly revise validation strategies and production processes. Sustained engagement with domestic and international regulators supports awareness of evolving expectations and industry best practices.
2 Discussion of PUPSIT alternative approaches
Regulators including EMA maintain a stringent stance regarding PUPSIT necessity to mitigate filter‑defect and flaw‑masking hazards. Nevertheless, certain process constraints render PUPSIT technically impracticable in specific scenarios. For instance, minimal‑volume formulations may incur unacceptable product dilution or material loss from PUPSIT operations, a circumstance explicitly acknowledged within the 2022 revision of EU GMP Annex 1. Where PUPSIT cannot be implemented, alternative risk‑control measures founded on comprehensive risk assessment are permissible.
Per EMA GMP Annex 1, risk assessments shall address the following non‑exhaustive set of factors:
① Filter manufacturing and supplier quality controls;
② Filter transportation, storage, unpacking and installation activities;
③ End‑user filter sterilization operations;
④ Filtration‑system architecture including single‑stage, redundant or dual‑stage configurations and pre‑filtration steps;
⑤ Physicochemical characteristics of the product feed stream and filter compatibility.
Risk assessments adopted as PUPSIT alternatives require comprehensive analysis and supporting validation demonstrating equivalent risk‑control rigor. Manufacturers shall engage proactively with regulators, submitting detailed validation datasets and risk‑assessment reports for regulatory acceptance.
Part 04 Conclusion
Robust sterile‑process performance is paramount for sterile‑product manufacturing, particularly within aseptic filling operations. Implementing PUPSIT principles alongside redundant filtration architectures substantially reduces contamination risk. Appropriate system design, rigorous validation workflows and thorough audit practices enable pharmaceutical enterprises to maximize sterilizing‑filtration process robustness and overall aseptic‑process stability. As domestic and international regulatory frameworks converge, manufacturers shall continuously optimize filtration‑process design, strengthen supplier collaboration and execute robust risk assessments to deepen process understanding and control maturity. These measures support dual objectives of regulatory compliance and pharmaceutical quality, underpinning the production of high‑quality sterile medicinal products.
