ASTM F838: Bacterial Retention Test for Sterilising-Grade Membrane Filters

A filter used to sterilise a liquid has to retain bacteria, not only particles of a given size. ASTM F838 is the test method that shows it: the membrane is challenged with a very large number of small bacteria, and the liquid that passes through is checked for any that got past.

What the test does

F838 determines the bacterial retention of membrane filters used for liquid filtration. The challenge organism is Brevundimonas diminuta (formerly called Pseudomonas diminuta), a small bacterium that is the industry standard for qualifying sterilising filters.

The challenge is 10 million bacteria (10⁷) for every square centimetre of the filter's effective area. That is far more than a filter would meet in a real process. The level is chosen so that the test is sensitive enough to find any oversized pore, and so that a filter that passes it has been shown to retain large numbers of organisms.

The liquid that passes through is filtered again through an analysis membrane, which is then cultured. If no bacteria grow, the filtrate was sterile under the test conditions. The result can also be given as a log reduction value: the logarithm of the number of bacteria challenged divided by the number that passed.

Sterilising grade

A membrane filter that gives a sterile filtrate under this challenge is described as sterilising grade. Such filters are usually rated 0.2 or 0.22 µm. The rating came from the finding that 0.45 µm membranes can let B. diminuta through when the challenge is high enough.

What the test does not cover

The standard is clear on its limits. It assesses a filter under standard challenge conditions; it is not the product- and process-specific validation that a pharmaceutical or biopharmaceutical sterilising filtration needs. That validation is done with the user's own product (or a close surrogate) and process conditions, and it remains the responsibility of the user.

How F838 links to the bubble point test

The bacterial challenge destroys the filter, so it cannot be run on every cartridge that is sold. Instead, the challenge results for a membrane grade are related to a test that does no damage, usually the bubble point or the diffusion (forward flow) test. A cartridge whose bubble point is at or above the figure related to full bacterial retention is then accepted as sterilising grade. See ASTM F316: bubble point and pore size.

Vens membrane cartridges and F838

Depending on the membrane and grade, Vens membrane cartridges for sterilising filtration of liquids can meet the ASTM F838 challenge:

• PES pleated cartridges (SulfoFil)

• Nylon 66 pleated cartridges (NylonFil)

• Hydrophilic PTFE pleated cartridges (FlouroFil X)

Tell us the liquid, the rating and the integrity test value your specification names, and we will confirm the cartridge. Validation with your own product and process is still needed, as the standard itself says.

More: ASTM standards for filters and the Vens filter test laboratory.

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ASTM is a trademark of ASTM International. Vens Hydroluft is not affiliated with ASTM International. The standards are described in our own words from the scopes ASTM publishes; work to the edition named in your specification. Whether a filter meets a requirement depends on the product, its rating and the conditions of use, so we confirm it for each enquiry. See also our product information disclaimer.