Bacteria Retention Claims for Sterilizing Filters and Why They Differ Among Manufacturers

Bacteria Retention Claims for Sterilizing Filters

Why Are They Different Among Manufacturers?

Bacteria retention tests are normally conducted following ASTM F838 protocols (Standard Test Method for Determining Bacterial Retention of Membrane Filters Utilized for Liquid Filtration).

This test method determines the bacterial retention characteristics of membrane filters for liquid filtration using Brevundimonas diminuta as the challenge organism. B. diminuta, is widely considered to be representative of the smallest known bacterium and is recognized as an industry standard for qualifying sterilizing filters.

For the purposes of this test, a B. diminuta culture is allowed to grow to a known concentration of colony-forming units per ml (CFU/ml). The concentration needs to be sufficient so that the required filter challenge can be carried out with a reasonable volume of challenge fluid.

The test requires a challenge of 10⁷ bacteria per cm² of effective filtration area. This provides a normalization of the test across devices with different filtration areas. The required volume of the challenge solution is calculated from the measured concentration and the effective surface area of the device to be tested so that the device sees the required challenge level. The filtrate is run aseptically through a known retentive filter with appropriate growth media. This filter is then incubated to determine the number of active bacteria (ideally zero) that penetrated through the membrane of the test device. If no growth is seen on the collection membrane, then the test filter fully retained all of the challenge bacteria. Retention of 10⁷ bacteria per cm² is characterized as a 7-log reduction per cm².

How then do some manufacturers claim an 11-log reduction? The answer is that they are claiming the reduction for the entire device, not on a per cm² basis. The math works out as follows:

• In order to assure a minimum challenge of 10⁷ bacteria per cm² the total challenge volume is pushed so the calculated challenge is slightly higher - let’s call it 2x10⁷ bacteria per cm².
• A typical 10” pleated sterilizing membrane filter has approximately 0.7 m² of effective area. This converts to 7000 (or 7x10³) cm².
• To calculate the bacteria challenge for the entire filter:

2x10⁷ bacteria per cm² X 7x10³ cm² per device = 14 x 10¹⁰ = 1.4 x 10¹¹ bacteria per 10” device.

Based on this calculation the DEVICE retained > 10¹¹ bacteria, so the device retention can be claimed as an 11-log reduction - even though it is still a 7-log reduction per cm².

Because of the many filter sizes we offer, CPF has chosen the 7-log reduction per cm² characterization so that the same retention claim applies to all sterilizing products.

When comparing sterilization claims between manufacturers, make sure to check whether the claim is based on area, and not the entire device.