Sterile filtration is a validated process — not simply a pore-size specification
Sterile filtration is used in pharmaceutical and biopharmaceutical manufacturing when a liquid or gas stream requires microbiological control and the process cannot rely solely on terminal sterilisation.
Selecting a filter therefore involves considerably more than specifying 0.2 µm. Membrane chemistry, microbial retention, formulation compatibility, throughput, adsorption, process conditions, sterilisation method and integrity testing all influence whether a filter is suitable for a defined process.
What is sterile filtration?
Sterile filtration is a membrane-filtration process used to remove viable microorganisms from a process stream without relying on heat to sterilise the fluid itself. It is particularly relevant to heat-sensitive pharmaceutical and biological products for which terminal sterilisation may not be suitable.
In a typical aseptic process, filtration is only one part of a larger contamination control strategy. The upstream bioburden, filtration train, equipment interfaces, downstream sterile boundary and aseptic filling operation all remain important.
This sequence is illustrative rather than universal. The actual filtration train should be developed around the product, process bioburden, particle load, formulation characteristics and validated manufacturing strategy.
Bioburden reduction is not the same as sterilizing-grade filtration
Reducing the microbial load
A bioburden-reduction step is used to reduce microorganisms upstream of a subsequent process operation. It can protect downstream equipment and reduce the microbial challenge presented to a later critical filtration stage.
A filter used for bioburden reduction should not automatically be described as sterilizing grade.
Validated microbial retention
A sterilizing-grade filtration step is based on demonstrated microbial retention under defined conditions and requires validation appropriate to the filter, product and process.
The nominal micron rating is therefore only one part of the complete filter specification.
0.2 µm versus 0.22 µm: what does the rating actually mean?
The terms 0.2 µm and 0.22 µm are both widely encountered in pharmaceutical sterile filtration. FDA guidance treats them as interchangeable nominal pore-size ratings in the context of sterilizing-grade filters.
This does not mean that every membrane labelled 0.2 or 0.22 µm has the same physical pore structure or the same performance.
A 2025 perspective by Kapila, Messerian and Zydney in the Journal of Membrane Science highlights that validated commercial 0.2/0.22 µm sterile-filter membranes can differ substantially in polymer chemistry, membrane morphology and pore-size distribution. These differences can affect permeability, filter capacity and product yield.
Bacterial retention and Brevundimonas diminuta
Brevundimonas diminuta is widely used as a model challenge organism when evaluating membrane filters intended for sterilizing liquid filtration.
ASTM F838-20 defines a standard bacterial-retention test using B. diminuta. The standard challenge is 107 organisms per cm² of effective filtration area.
However, ASTM F838-20 also makes an important distinction: the standard method is designed to assess membrane retention under defined challenge conditions. It does not replace product- and process-specific validation for a pharmaceutical or biopharmaceutical sterilizing-filtration process.
Why process-specific validation still matters
The actual filtration process can expose the membrane to conditions that differ significantly from a standard bacterial challenge test.
Relevant parameters may include:
- the actual product or an appropriately justified surrogate;
- product bioburden and relevant process microorganisms;
- filtration time and total contact time;
- operating pressure and differential pressure;
- flow rate and total filtration volume;
- temperature;
- filter conditioning and wetting conditions;
- sterilisation method;
- product compatibility and potential membrane adsorption.
Membrane selection: PES, PVDF and PTFE are not interchangeable
Sterile-filtration performance depends not only on nominal pore size, but also on the physical and chemical characteristics of the membrane. Different membrane polymers can have different wettability, adsorption behaviour, permeability, chemical compatibility and interaction with the process formulation.
This is particularly important in biopharmaceutical processing, where product recovery can be affected by adsorption of proteins, surfactants or other formulation components onto product-contact surfaces.
PES membranes are widely used for aqueous pharmaceutical and biotechnology liquids. Hydrophilic PES can provide high permeability and is frequently considered for buffers, media and other aqueous process streams.
Hydrophilic PVDF is commonly considered where low protein adsorption and recovery of valuable biological product are important. Actual adsorption remains formulation- and filter-specific.
Hydrophobic PTFE is particularly relevant to air, process gas and vessel-vent filtration because it permits gas transmission while resisting penetration by liquid water under appropriate operating conditions.
PES versus PVDF: there is no universal winner
It is tempting to reduce membrane selection to a simple statement such as “PES has higher flow” or “PVDF has lower protein binding”. In a regulated bioprocess, that is usually too simplistic.
Na, Suh, Cho and Baek compared commercial hydrophilic PVDF and PES sterile filters in a 2022 study published in Membranes. Both filter types demonstrated bacterial retention, but they differed in morphology, hydrophilicity, permeability and filtration behaviour depending on the process fluid.
Research on concentrated protein formulations has similarly shown that filtration behaviour can change with protein concentration, formulation composition, viscosity, surfactant content and filter material.
Earlier work on therapeutic-protein filtration also demonstrated that adsorption of proteins and surfactants can differ between membrane materials and even between different filters of nominally similar material.
| Selection factor | PES | Hydrophilic PVDF | Hydrophobic PTFE |
|---|---|---|---|
| Typical starting point | Aqueous liquids, buffers, media and compatible process solutions | Biological and protein-sensitive liquid applications | Air, gas, vessel vents and compatible solvents |
| Wettability | Hydrophilic | Hydrophilic | Hydrophobic |
| Protein adsorption | Can be low, but must be evaluated with the actual formulation | Often selected where low protein adsorption is important | Not normally the first choice for aqueous protein-liquid filtration |
| Integrity testing | Suitable validated wet-membrane methods depend on filter grade | Suitable validated wet-membrane methods depend on filter grade | Suitable hydrophobic-filter methods may include water intrusion |
| Sterilizing-grade status | Must be established for the exact filter grade and configuration; membrane material or nominal pore rating alone is insufficient. | ||
Filtra currently offers PES membrane capsule filters and PVDF membrane capsule filters in multiple pore ratings for precision liquid-filtration applications. Selection for a critical sterile-filtration process should always include confirmation of the exact filter grade and validation documentation required for that process.
For gas and vent applications, Filtra also offers hydrophobic PTFE capsule filtration .
Filter capacity, flux and differential pressure
A sterilizing filter must provide the required microbial retention, but it must also process the batch within the validated operating window.
Practical filter performance is influenced by the interaction between the membrane and the process stream. A solution containing particles, aggregates, proteins or precipitated material may progressively foul the membrane, reducing flux or increasing differential pressure.
For protein formulations, higher concentration can also increase viscosity. This can increase the pressure required to achieve a given flow rate and may change the economics of filter sizing.
Important parameters during filterability studies
- initial and time-dependent flux;
- throughput per unit membrane area;
- differential pressure development;
- batch volume and required processing time;
- product viscosity;
- protein or active-component recovery;
- membrane adsorption;
- filter fouling or plugging behaviour;
- required safety margin for production scale.
Small-scale filterability testing is therefore valuable before selecting the final membrane area for pilot or production processing.
Why prefiltration can determine final-filter performance
A final critical membrane should not automatically be expected to absorb the complete particulate burden of an upstream process.
Clarification and prefiltration can remove particles, aggregates or other fouling material before the final membrane stage. In suitable applications, this can preserve membrane capacity, reduce premature pressure rise and make final filtration more predictable.
Sterile filtration inside a single-use fluid path
Capsule filters are particularly useful in single-use processing because the membrane and housing are supplied as one self-contained device. The filter can be incorporated directly into tubing assemblies between bags, bottles, process equipment and receiving containers.
However, once the capsule becomes part of a complete single-use assembly, validation must consider more than the filter alone.
Product-contact materials, tubing dimensions, connectors, filter membrane, sterilisation method, operating pressure, hold-up volume and downstream sterile-boundary design should therefore be evaluated as a complete system.
Filtra's Single-Use Bioprocess Solutions combine process bags, bottles, tubing, connectors and capsule filtration into configurable fluid paths for pharmaceutical and biotechnology applications.
Filter integrity testing: verifying the installed membrane
Microbial-retention validation demonstrates what a particular filter grade can achieve under defined conditions. Integrity testing has a different function: it provides a non-destructive method of verifying that the installed membrane filter remains within defined physical integrity limits.
The test method and acceptance limit must correspond to the exact membrane, filter configuration, wetting fluid and validated filtration process. Common integrity-test methods include Bubble Point, Diffusion / Forward Flow and Water Intrusion, depending on the membrane type and application.
Determines the pressure at which gas displaces wetting liquid from the largest wetted membrane pores and produces continuous gas flow.
Measures gas transport through a fully wetted membrane at a defined pressure below the bubble point. The measured gas flow is compared with the validated acceptance limit for the filter configuration.
Used with suitable hydrophobic membrane systems to evaluate water intrusion under controlled pressure without conventionally wetting the membrane with an alcohol-based wetting liquid.
Integrity testing does not replace microbial-retention validation
It is important to distinguish between a filter integrity test and a microbial retention study.
A microbial-retention validation establishes whether a filter can achieve the required microorganism retention under defined challenge and process conditions. An integrity test, by contrast, measures a physical property of the membrane that has been correlated with the validated retention performance of that particular filter grade.
Consequently, passing a Bubble Point, Diffusion / Forward Flow or Water Intrusion test does not by itself demonstrate that a process is sterile. The integrity result must be interpreted against validated limits for the exact membrane and filter configuration.
What is PUPSIT?
PUPSIT means Pre-Use Post-Sterilisation Integrity Test.
Its purpose is to verify the integrity of the sterilised filter assembly after preparation and sterilisation but before product filtration begins.
Current EU GMP Annex 1 addresses PUPSIT explicitly for sterilising filtration. It also requires a non-destructive post-use integrity test for a sterilizing-grade filter used to sterilise a fluid, prior to removal of the filter from its housing.
Annex 1 recognises that PUPSIT may not always be possible because of specific process constraints. In such cases, the alternative approach requires a documented, science- and risk-based justification with appropriate controls.
Why PUPSIT becomes a system-design question
PUPSIT is not simply a test programme on an integrity tester. The filtration system itself must support membrane wetting, venting, test-gas connection, drainage and subsequent product processing without compromising the downstream sterile boundary.
This becomes particularly important in single-use systems, where the filter, tubing, connectors, vents and receiving container may be delivered as one integrated fluid path.
A 2026 review in Applied Microbiology and Biotechnology discusses the engineering challenges associated with single-use PUPSIT systems, including wetting, venting, hold-up volume, dead legs, assembly complexity and the need to minimise opportunities for operator error.
For this reason, PUPSIT should be considered during fluid-path design rather than added as an afterthought once the single-use assembly has already been specified.
What should be defined before selecting a sterile-filtration system?
| Parameter | Why it matters |
|---|---|
| Filtration objective | Distinguish clarification, bioburden reduction and validated sterilizing-grade filtration. |
| Process fluid | Composition, pH, salts, proteins, surfactants and solvents can influence compatibility, adsorption and filtration behaviour. |
| Required microbial retention | Determines the required filter grade and validation documentation. |
| Batch volume & flow | Influence required membrane area, processing time and scale-up. |
| Particle / aggregate load | Determines whether clarification or prefiltration is needed to protect the final membrane. |
| Protein recovery | Membrane adsorption and formulation interactions can be significant when filtering high-value biological products. |
| Pressure & temperature | Must remain within both filter operating limits and validated process conditions. |
| Sterilisation method | Gamma irradiation, autoclaving or SIP suitability is configuration-dependent and must be confirmed for the exact assembly. |
| Integrity-testing strategy | Define the test method, wetting fluid, acceptance limits, PUPSIT requirements and post-use integrity testing before finalising the filtration system. |
| Fluid-path architecture | Connections, tubing, vents, receiving container and downstream sterile boundary must be evaluated as one system. |
| Documentation | Validation data, material declarations, sterilisation information, traceability and filter-specific integrity limits must match the selected configuration. |
Why a pore-size specification is only the beginning
A robust sterile-filtration specification connects four different questions:
- Can the filter provide the required microbial retention?
- Can it process the actual formulation within the required operating window?
- Can filter integrity be demonstrated before and after the critical process step?
- Can the filter be integrated into the complete aseptic fluid path without creating new process risks?
Answering only the first question by specifying “0.2 µm” is therefore insufficient for a critical pharmaceutical filtration process.
Continue with the individual integrity-test methods
Each integrity-test method is based on a different physical principle. The following Knowledge Center articles explain the methods, operating principles and interpretation in more detail.
Scientific and technical references
- Kapila, S., Messerian, K. O. & Zydney, A. L. (2025). Perspectives on sterile filter performance through insights obtained by membrane characterization methods. Journal of Membrane Science, 728, 124130.
- Na, J., Suh, D., Cho, Y. H. & Baek, Y. (2022). Comparative Evaluation of the Performance of Sterile Filters for Bioburden Protection and Final Fill in Biopharmaceutical Processes. Membranes, 12(5), 524.
- Sterile Filtration of Highly Concentrated Protein Formulations: Impact of Protein Concentration, Formulation Composition, and Filter Material. Journal of Pharmaceutical Sciences, 2015, 104(10), 3319–3329.
- Mahler, H.-C. et al. (2010). Adsorption behavior of a surfactant and a monoclonal antibody to sterilizing-grade filters. Journal of Pharmaceutical Sciences, 99(6), 2620–2627.
- Yun, K. H., Sharma, K., Kim, H. U. & Bae, T.-H. (2023). Modification of a PES microfiltration membrane to enhance sterile filtration by inhibiting protein adsorption. Journal of Industrial and Engineering Chemistry, 123, 311–319.
- Glenz, M., Eiermann, P., Manser, B. et al. (2026). Single-use solutions for PUPSIT: requirements, challenges, and solutions. Applied Microbiology and Biotechnology, 110, Article 138.
- ASTM International. ASTM F838-20 — Standard Test Method for Determining Bacterial Retention of Membrane Filters Utilized for Liquid Filtration.
- European Commission. EudraLex Volume 4, EU Guidelines for Good Manufacturing Practice, Annex 1 — Manufacture of Sterile Medicinal Products.
- U.S. Food and Drug Administration. Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing — Current Good Manufacturing Practice.