There are several predictable debates and considerations that all aseptic filling operations face whether they be a newly defined project or part of an upgrade plan. These typically include such items as determining the appropriate barrier technology (RABS vs Isolation), appropriate air classification and airlocks strategy, designing for compliant operational flows for personnel, materials and tooling, identification of applicable regulations and guidelines, and on and on. Recently, one topic that is gaining momentum and becoming a growing subject of active debate in industry circles, is Pre-Use Post Sterilization Integrity Testing, PUPSIT, for point of use sterile liquid product filters. To offer clarity, demystification, and guidance on this topic, it is helpful to answer/address the most obvious and usual questions that always come up among the full spectrum of personnel (including the author!) involved in filling operations.

First introduced in the EMA Guide to Good Manufacturing Practice for Medicinal Product in 1997, the intention of PUPSIT is to verify the integrity of a sterilized filter assembly before use, to check for damage and loss of integrity caused by the filter preparation prior to use. Sounds reasonable, but wait, if I perform post fill integrity testing, as is common throughout industry, why should I test pre-use? Seems that will only add complexity to the filling circuit and risk sterility.

Well, the concern is a phenomenon known as flaw masking of minor filter flaws in the filter membrane, which may result from the sterilization or the installation process. The flaws can become basically “plugged” during the filtration process while processing a batch. The result of this clogging effect is that a non-integral filter may pass a post use integrity test because of the foulant embedded in the filter membrane. This flaw may have allowed a portion of liquid to pass through the flaw, compromising the sterility of a batch.

To focus on this concern, a PDA/BioPhorum was formed to address PUPSIT and concluded its work activities in 2020. The study activities found that although filter masking is a real phenomenon, it is extremely unlikely to occur during sterile product manufacturing and that filters containing minor flaws from the sterilization processes are extremely rare based on industry data. However, a solid rationale mitigating the use of PUPSIT would be reliant on scientific data and knowledge, at least for small batch processing.

At the time of this writing, the requirement to perform PUPSIT boils down to what market will the products serve, as regulatory requirements relative to PUPSIT are not presently homogeneous throughout the world. If a product is exported for use in the EU, Australia or a company that is following WHO guidance, then PUPSIT is necessary; India FDA guidelines offer a differentiation for the requirement between stainless steel housings and single-use filtration circuits; the U.S. FDA, CFDA and Japan FDAs are less prescriptive on the PUPSIT requirement. However, in drafting the revision to Annex 1, the EC worked closely with the World Health Organization and PIC/S to maintain existing global standards; each organization reviewed the revised annex in a parallel public consultation. Notably, working team members included representatives from the FDA. Therefore, it is expected that all PIC/S participating authorities will adopt this revised stance on PUPSIT in their guidance documents, including FDA and PIC/S itself.

The circuit design for PUPSIT is like that used for post filter integrity testing, requiring full wetting of the filters and the associated sequencing for venting and testing. Utility services, again similar, are electrical power and compressed gas (air, nitrogen) for the testing circuit. PUPSIT is performed at the point of use filtration assembly once it is fully in-place and ready for sterile filtration of the incoming bulk product. Gas venting and wetting fluid discharge and collection need to be designed to maintain the integrity of the filtration and liquid path using sterile barrier filters. Post integrity testing can subsequently also be done at the point of use location, or performed in an alternate location for product, gas venting and barrier sterile filters. Safety considerations when using IPA as a wetting fluid need to be provided.

Planning to implement PUPSIT in the immediate or future state for exiting or newly planned filling operations, seems to be the most prudent path to follow given the current and potential state of regulatory guidelines. PUPSIT circuit designs and the planned sequence of operation needs to risk assessed against the challenge of maintaining product path sterility. The PDA/BioPhorum studies can be consulted for guidance on assessment protocols.

Concurrently to planning for PUPSIT capability implementation, and as may be needed, a program to secure and establish scientific data based on the results of post-use filter integrity and product sterility without the use of PUPSIT could be completed.

There is much information for PUPSIT available for study and research. The intent of this writing was to offer some clear guidance for understanding the intent and current state of the PUPSIT subject.