Upon completion of a bacterial retention test, the entire filtrate must be analyzed to confirm the retention capability of the sterilizing-grade filter. There are generally two methods for recovery and analysis:
1. Direct passage through a suitable analytical (recovery) filter or membrane installed downstream of the test filter.
2. Collection of the filtrate in a sterile container, followed by filtration through an analytical membrane.
Typically, a 0.45 μm or sterilizing-grade membrane is used to capture Brevundimonas diminuta or other microbial challenge organisms. The installation of the analytical filter must not affect the intended pressure differential across the test filter. Partial sampling of the filtrate is insufficient to validate sterilizing filtration, as a small fraction of bacteria may have penetrated the filter and remain present in the unsampled and unanalyzed portion of the filtrate.
Acceptance Criteria
Provided the positive control is valid, the sterilizing-grade filter meets the acceptance criteria if no challenge bacteria are detected in the filtrate from all three test filters.
Handling Test Failures
If bacterial penetration is detected in one filter and the root cause has not been confirmed, retesting may be conducted following an investigation and risk assessment (e.g., testing three filters from the same batch as the failed membrane). If a definitive root cause for the test failure can be identified, the filters from the questionable batch must be retested.