Written and maintained by CASRAI Editorial Board
Last updated
This page describes products and general use; it is not medical advice.
In short: a sterilizing filter (also called a sterilizing-grade filter) is a filter that has been validated to remove a defined microbial challenge from a liquid or gas and produce a sterile effluent. A pore rating of about 0.2 µm is typical, but the rating alone does not make a filter sterilizing grade; the validation does. Who uses it: sterile drug manufacturers working under aseptic processing rules, and laboratory staff who need to understand what a filter label does and does not claim.
What a sterilizing filter is
The FDA’s aseptic processing guidance says a sterilizing grade filter “should be validated to reproducibly remove viable microorganisms from the process stream, producing a sterile effluent,” and that such filters “usually have a rated pore size of 0.2 µm or smaller.”
The EU GMP Annex 1 glossary defines it as “a filter that, when appropriately validated, will remove a defined microbial challenge from a fluid or gas producing a sterile effluent,” and adds that such filters usually have a pore size equal to or less than 0.22 µm.
USP chapter 1229.4 (USP 38 text) puts a number on the validation: a sterilizing-grade filter is one “capable of retaining a minimum 1 x 107 cfu of B. diminuta (ATCC 19146) per square centimeter of effective filter area,” when tested under ASTM F838. These are typically microporous membranes with nominal pore-size ratings of about 0.2 µm.
How it works: rating plus validated retention
A sterilizing filter is a membrane that retains microorganisms by size. The FDA guidance explains why the rating is not enough on its own: a filter “can contain a number of pores larger than the nominal rating,” and the probability of passage rises with the number of organisms in the challenge.
That is why the definition rests on a microbial challenge test. Brevundimonas diminuta (ATCC 19146) is used because, in the FDA’s words, it “is a common challenge microorganism for 0.2 µm rated filters because of its small size (0.3 µm mean diameter).” The FDA also says validation “should include microbiological challenges to simulate worst-case production conditions,” together with integrity test results for the filters used, and that “filter performance may differ significantly for various conditions and products.”
Practical meaning: a filter labelled 0.2 µm is not automatically a validated sterilizing-grade filter for your fluid. See what a syringe filter is and the clinical and laboratory filter selection guide for the general lab picture; this page covers only the sterilizing-grade definition.
Types and variants
EU GMP Annex 1 (2022) refers to sterilising grade filters in several roles.
| Role | What Annex 1 says | Annex section |
|---|---|---|
| Liquid sterilization | Liquids may be sterilised by a sterile sterilising grade filter, nominal pore size of a maximum of 0.22 µm, appropriately validated to obtain a sterile filtrate. | 8.79 |
| Gas filtration at point of use | Gases are filtered through a sterilising grade filter with a nominal pore size of a maximum of 0.22 µm. | 6.19 |
| Redundant (second) filter | Where a redundant filtration system is used, the integrity of the primary filter is tested. | 8.92 |
FDA 2004 guidance and EU Annex 1 (2022) side by side
The two documents agree on the principle and differ in wording and detail. Check which one applies to your product and jurisdiction.
| Topic | FDA aseptic processing guidance (2004) | EU GMP Annex 1 (2022) |
|---|---|---|
| Pore size | Usually a rated pore size of 0.2 µm or smaller | Usually equal to or less than 0.22 µm; nominal maximum 0.22 µm for liquids (8.79) and gases (6.19) |
| Challenge organism | B. diminuta (ATCC 19146) named as a common challenge for 0.2 µm filters | “A defined microbial challenge” (glossary) |
| Integrity test timing | Can be performed before processing; “should be routinely performed post-use” | Test before use (8.87) to check for damage from filter preparation; non-destructive post-use test before removal from the housing |
| Redundant filtration | “Should be considered in many cases” | Primary filter integrity is tested where a redundant system is used (8.92) |
| Reuse | “Routinely discarded after processing of a single lot” | Discarded after a single batch; not used continuously for more than one working day unless validated (8.94) |
Integrity testing
Integrity tests confirm that an individual filter is intact and performs as it did in validation. The FDA names forward flow and bubble point tests as two integrity tests that can be used, and says the integrity specification “should be consistent with data generated during bacterial retention validation studies.”
Annex 1 (8.87) lists bubble point, diffusive flow, water intrusion and pressure hold tests as examples, and notes that a pre-use test may be impossible in some cases, such as very small volumes, in which case an alternative needs a documented risk assessment.
USP 1229.4 describes two of the principles in plain terms. For bubble point, gas displaces a wetting liquid from the largest membrane pores, resulting in bulk gas flow through those pores. For diffusion, a wetted filter provides a liquid layer across which gas can flow by diffusion.
How to choose
- Review the validation data. The FDA says the filter user must review the filter manufacturer’s validation data, and that performance may differ for different conditions and products.
- Use the same filter that was validated. The FDA guidance says filters used in production should be identical (same polymer, same pore rating) to those validated.
- Confirm the integrity test specification is consistent with the retention validation.
- Consider whether a redundant filter is appropriate (FDA: “should be considered in many cases”).
Safe use and handling basics
- Discard after a single lot or batch. The FDA says sterilizing filters “should be routinely discarded after processing of a single lot”; Annex 1 (8.94) adds a one-working-day limit unless longer use has been validated.
- Integrity test after use. The FDA says post-use testing should be routine; Annex 1 (8.87) asks for a non-destructive post-use test before the filter is removed from its housing.
- Follow the manufacturer’s instructions for use and your own validated procedure.
Common mistakes
- Treating a 0.2 µm label as proof of sterilizing grade. The definition requires validated retention of a defined challenge.
- Skipping the post-use integrity test.
- Reusing a filter across lots or batches (see the discard rules above).
- Assuming validation done for one fluid applies to another; the FDA notes performance may differ for various conditions and products.
Frequently asked questions
What makes a filter “sterilizing grade”?
Validated retention, not just pore size. USP 1229.4 describes it as retaining at least 1 x 107 cfu of B. diminuta per square centimeter of effective filter area under ASTM F838 conditions; the FDA and EU definitions both require appropriate validation to produce a sterile effluent.
Is 0.2 µm the same as 0.22 µm?
Both are used. The FDA guidance refers to a rated pore size of 0.2 µm or smaller; Annex 1 refers to a nominal pore size of a maximum of 0.22 µm. Neither wording makes the rating alone sufficient.
Can a sterilizing filter be reused?
The rule in both texts is discard after a single lot or batch. The FDA allows repeated use only where it can be justified, with the validation covering the maximum number of lots; Annex 1 allows no more than one working day of continuous use unless validated.
Related CASRAI pages
See also what an autoclave is for steam sterilization, HEPA filter certification for air filtration in controlled environments, and aseptic process simulation.
Sources
- U.S. FDA, Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing – Current Good Manufacturing Practice (2004), section on filtration efficacy: https://www.fda.gov/media/71026/download (checked 2026-10-30)
- European Commission, EU GMP Annex 1: Manufacture of Sterile Medicinal Products, C(2022) 5938 final, 22 August 2022; glossary and sections 6.19, 8.79, 8.87, 8.92, 8.94: https://health.ec.europa.eu/system/files/2022-08/20220825_gmp-an1_en_0.pdf (checked 2026-10-30)
- USP General Chapter 1229.4 Sterilizing Filtration of Liquids, USP 38 text (mirror copy): https://www.drugfuture.com/Pharmacopoeia/usp38/data/v38332/usp38nf33s2_c1229_4.html (checked 2026-10-30). ASTM F838 is cited here as described in the USP chapter.
