Does a gas filter require bacterial retention validation?

When it is necessary to perform a liquid microbial challenge test on a gas filter using challenge microorganisms, the requirements for the challenge microorganism and the challenge level are identical to those for liquid sterilizing-grade filters. Depending on your specific needs, you may refer to our previous step-by-step guide series on bacterial retention validation:

• Is Brevundimonas diminuta Absolutely Required for Bacterial Retention Validation?

• What Preparations Are Needed for Bacterial Retention Validation?

• Developing a Bacterial Retention Validation Test Protocol

• How to Adjust the Bacterial Retention Test Protocol?

• How to Analyze Bacterial Retention Test Results?

Meanwhile, due to the hydrophobic characteristics of gas filters, conducting a liquid bacterial challenge test requires special attention to the following points:

• Pre-Challenge Integrity Testing: Available methods include the Bubble Point, Diffusive Flow / Forward Flow (Pressure Hold / Pressure Decay), and the Water Intrusion Test (WIT).

• Pre-Wetting: Because of the hydrophobic nature of gas filters, prior to an aqueous-based microbial challenge, the filter must be pre-wetted using a low surface tension liquid (e.g., a 25% v/v aqueous tert-butanol solution or a 60% v/v aqueous isopropanol solution).

• Solvent Removal: Before the challenge, the alcohol solution must be thoroughly removed to prevent any interference with the challenge test.

• Challenge Protocol: The protocol design and operational procedures for the challenge test are identical to those used for liquid filters.

• Post-Challenge Integrity Testing: This step confirms that the filter was not damaged during the challenge process. At this stage, unlike pre-challenge testing, the Water Intrusion Test or other direct water-based tests can typically no longer be selected. This is because surviving or non-surviving contaminating microorganisms can both affect the filter's hydrophobicity, and any filter material that remains wetted may skew the test results. Instead, the same solvent solution used for the pre-challenge integrity test should be used to perform a Bubble Point or Diffusive/Forward Flow test.

• Filtrate Analysis: The analysis of the filtrate is identical to that for liquid filters.

• Result Interpretation: There is currently no unified standard specifically for sterilizing-grade gas filters. However, conventional sterilizing-grade gas filters on the market today typically already meet the standards for liquid sterilizing-grade filters, which require the retention of a challenge microorganism (usually Brevundimonas diminuta) at a level greater than 107 CFU/cm2 of Effective Filtration Area (EFA).