Starting this week, we enter one of the most concerned topics in the validation module: bacterial retention validation.
Bacterial retention validation is used to confirm the filter membrane rating and to demonstrate, using representative challenge microorganisms, the complete removal of microorganisms from a specific product or product family. Validation of sterilizing-grade filters requires consideration of two main factors:
1. Membrane Rating: Conducted via bacterial challenge using applicable standardized tests or equivalent methods.
2. Process Validation: Conducted by the filter user or a laboratory commissioned by them (e.g., the filter manufacturer or a contract laboratory), using representative microorganisms to demonstrate that the filter can completely remove microorganisms from each product or product family under production conditions. A scientific rationale must be established for each product family.
These two filter testing concepts are not interchangeable and must be validated independently. The purpose of these tests is to demonstrate that the sterilizing filtration production process yields a sterile filtrate.
The objective of bacterial retention validation studies is to obtain documented evidence demonstrating that, under simulated process conditions, the filtration process can consistently remove high levels of standard bacterial or relevant microbial contamination isolates suspended in the product or a substitute fluid.
The study objective determines whether to use membrane discs or full-scale process filters. If the objective is to validate the bacterial retention efficacy of a specific membrane material, using small test discs is generally considered acceptable. The detection method used to determine the physical integrity of the process filter should be correlated with the bacterial retention test results.
Process time and differential pressure can affect the results of bacterial retention tests. Performing bacterial challenge tests over the complete process time allows for the evaluation of time-dependent factors, including filter compatibility, maintenance of integrity, and time-dependent breakthrough.
When conducting product bacterial retention validation for membrane filters, the following items must be considered:
• A thorough evaluation of the filtration process should be conducted, including solvent properties (e.g., aqueous, acidic, alkaline, organic), filtration time, process differential pressure, process flow rate, process temperature, and filter design parameter specifications.
• Product bacterial retention validation studies should include multiple filter membrane batches (typically three batches). In some cases, where product properties are considered damaging to the filter membrane, the exact number of filters and experimental design shall be determined based on the specific process.
• Among the three batches used in the bacterial retention validation study, at least one batch should have physical integrity test values (during pre-study or pre-use testing) that pass but are close to the pass/fail criteria provided by the filter manufacturer. If low bubble point filters (low production specification filters) are not used in the validation, the bubble point value of the standard solution filter/cartridge used in actual production must be higher than the minimum bubble point value of the filters actually used in the validation test.
• Physical integrity test values for the filter membranes used in the bacterial retention validation study should be included in the study report. Physical integrity testing should be performed using the corresponding wetting fluid with established test standards (e.g., water, product, or other wetting fluids) and completed prior to microbial challenge.
• If challenge microorganisms are detected downstream of any filter after the bacterial challenge test, an investigation must be conducted. If the investigation confirms that the test microorganisms penetrated a filter that met integrity test criteria, the suitability of that filter model under these operating conditions must be reconsidered.
• For a product family with identical components but varying concentrations, validation can be performed by challenging at the limit concentration, with intermediate concentrations accepted by extrapolation. If a specific product is identified as the worst-case representative, a rationale and supporting data must be provided.
• Reuse of liquid filters is generally not recommended for pharmaceutical processes. However, if a sterilizing-grade filter is reused, the rationale must be documented, and adequate validation or qualification must be performed in accordance with applicable regulations and guidelines.