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General information for laboratory, pharmacy and clinic staff. This is not medical advice and not a validation procedure.
Short answer
Filtration sterilization passes a liquid or gas through a sterilizing-grade filter that has been validated to remove microorganisms, so the filtrate is sterile. It removes organisms rather than killing them. FDA’s 2004 aseptic-processing guidance calls it a common method of sterilizing drug product solutions and says such filters usually have a rated pore size of 0.2 µm or smaller; EU GMP Annex 1 sets a maximum nominal pore size of 0.22 µm.
Definition and scope
FDA’s guidance says a sterilizing-grade filter “should be validated to reproducibly remove viable microorganisms from the process stream, producing a sterile effluent.” The Annex 1 glossary defines it the same way: a filter that, when appropriately validated, removes a defined microbial challenge from a fluid or gas and produces a sterile effluent.
The important word is validated. A pore-size number on a package is a rating; sterility of the filtrate is a result that has to be demonstrated for a given fluid, filter and set of conditions. For the device itself, see our guide to the sterilizing filter; this page looks at filtration as a method and at what to verify.
How filtration sterilization works in practice
Removal, not killing
Heat, radiation and chemical methods act on organisms in or on the item. Filtration instead holds organisms back on the way through, so what matters is retention. That is why both documents focus on challenge testing and integrity testing rather than on an exposure time. Our guide to aseptic technique covers the handling that has to follow, because a sterile filtrate is only useful if it stays sterile.
Why 0.22 micron, and why 0.2 appears too
FDA describes filters that “usually have a rated pore size of 0.2 µm or smaller”, and names Brevundimonas diminuta (ATCC 19146) as a common challenge organism for 0.2 µm rated filters because of its small size (0.3 µm mean diameter). Annex 1 (paragraph 8.79) refers to a nominal pore size of a maximum of 0.22 µm that has been appropriately validated to obtain a sterile filtrate. A 0.22 micron filter is therefore a rating that fits the regulators’ wording, but both documents tie the claim to validation, not to the number alone.
Why a rating alone is not enough
FDA notes that a filter can contain a number of pores larger than the nominal rating, and that the probability of organisms passing is considered to increase as the number of organisms (bioburden) in the fluid increases. Its guidance therefore asks for microbiological challenges that simulate worst-case production conditions, and for manufacturing controls that keep the bioburden of the unfiltered product low.
PES and other membrane materials
A PES (polyethersulfone) membrane, like any other membrane polymer, is a material choice, not a sterility claim. FDA says the specific type of filter membrane used in commercial production should be evaluated in filter validation studies, and that production should use identical filters (for example, identical polymer construction and pore size rating) to those validated. Annex 1 (8.79) says the filter selected should be compatible with the product, and (8.81) that adsorption of product components and extraction or leaching of filter components should be evaluated.
Integrity testing before and after
FDA says integrity testing can be done before processing and should routinely be done after use, to detect leaks or perforations; forward flow and bubble point are two tests it names. Annex 1 (8.87) expects an integrity test of the sterilised filter assembly before use (PUPSIT) and a non-destructive integrity test after use, with examples of bubble point, diffusive flow, water intrusion or pressure hold. PUPSIT may be handled by an alternative approach only where a thorough risk assessment supports it. In both documents the test specification must correlate to the retention capability shown in validation.
Where it matters for labs, clinics and supply decisions
Annex 1 (8.79) places filtration where a product cannot be sterilised in its final container: the liquid is filtered through a validated sterilizing-grade filter and then aseptically filled into a previously sterilised container. Annex 1 (8.80) also notes that a sterile filtration process carries additional risks compared with other sterilisation processes, and says an additional filtration close to the point of fill should be considered as part of the contamination control strategy. See EU GMP Annex 1 and media fill simulation for the surrounding requirements.
Document checklist drawn from the two sources
| Ask for or check | Why (source) |
|---|---|
| Retention validation data for your fluid and conditions | FDA: the filter user is responsible for reviewing validation data, which should be applicable to the user’s products and conditions of use. |
| Challenge organism and how the product was handled in the test | FDA: B. diminuta is common for 0.2 µm filters; Annex 1 (8.84): use the actual product where possible, with the organism justified. |
| Membrane type and polymer matching what was validated | FDA: the specific membrane type should be evaluated; identical filters should be used in production. |
| Compatibility, adsorption and extractables evaluation | Annex 1 (8.79, 8.81): compatibility with the product; adsorption and leaching evaluated. |
| Integrity test method and specification | FDA and Annex 1 (8.87): specification consistent with retention validation; tests before and after use. |
| Use limit per filter | FDA: routinely discarded after a single lot unless repeated use is justified and validated. |
| Process parameters recorded | Annex 1 (8.86): for example minimum time to filter a known volume and the pressure difference across the filter. |
For format questions in a lab, see the clinical and laboratory filter selection guide and what a syringe filter is.
Common misunderstandings
- “0.22 micron means sterile.” The number is a rating. Both sources require validated retention and integrity testing before the filtrate is treated as sterile.
- “Filtration kills bacteria.” It removes them; the guidance speaks of removing viable microorganisms.
- “The membrane material decides sterility.” Material is evaluated for compatibility and extractables, and the specific membrane must be covered by validation.
- “One validation covers any fluid.” FDA says filter performance may differ significantly for various conditions and products.
FAQ
Is a 0.22 micron filter the same as a 0.2 micron filter?
Not automatically. FDA describes sterilizing-grade filters as usually rated 0.2 µm or smaller, while Annex 1 gives a maximum nominal pore size of 0.22 µm. In each case the claim depends on validation for the fluid in use.
Does a PES membrane make a filter sterilizing grade?
No. PES is a membrane material. Whether a filter is sterilizing grade depends on validated removal of a defined microbial challenge, and the material must also be compatible with the fluid.
When is filtration used instead of other sterilization?
Annex 1 (8.79) describes it for solutions or liquids that cannot be sterilised in their final container, followed by aseptic filling into a sterilised container.
Can a sterilizing filter be reused?
FDA says sterilizing filters should be routinely discarded after a single lot; repeated use is for cases that can be justified, with validation covering the maximum number of lots.
Sources
- FDA, Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing – Current Good Manufacturing Practice (September 2004), section IV.B Filtration Efficacy: fda.gov/media/71026/download. Date checked: 1 November 2026.
- European Commission, EudraLex Volume 4, Annex 1: Manufacture of Sterile Medicinal Products, C(2022) 5938 final (22 August 2022), paragraphs 8.79-8.87 and glossary: health.ec.europa.eu. Date checked: 1 November 2026.
