If a sterilizing-grade filter fails the integrity test, it may be damaged, but other potential causes of failure include instrument issues, system leakage, personnel operation errors (e.g., improper assembly, incorrect parameter settings), and incomplete wetting. The investigation and retest procedures for filter failure should be documented.
Analysis of Failure Due to Incomplete Wetting
Incomplete wetting may result from insufficient flushing of the filter, leaving some pores un-wetted. In post-use testing, it may also occur if the filter has adsorbed hydrophobic contaminants or if other formulation components have altered the surface wetting characteristics of the membrane. Changes in wetting properties can affect integrity test performance. For example, pipeline materials may enter the product stream and be adsorbed by the membrane, or product residues may not be adequately cleaned, thereby altering the membrane’s wetting properties and leading to integrity test failure.
To obtain valid integrity test results, the porous structure of the filter must be completely wetted, as the physical principle of integrity testing relies on gas flow through a liquid-filled pore. Membrane wetting can be influenced by the following factors:
• Membrane polymer: Some polymers are easier to wet than others, depending on the critical surface tension of the membrane material.
• Pore size: Smaller pores are more difficult to wet.
• Wetting fluid: Certain wetting fluids may exhibit repulsive interactions with the polymer matrix.
• Product residues or contaminants: Residues or contaminants may alter the hydrophilicity of the membrane polymer, repel the wetting fluid, or reduce surface tension.
• Pressure conditions: Maanufacturer-recommended pressure values should be followed to ensure complete membrane wetting.
• Temperature conditions: Temperature affects the surface tension of the wetting fluid.
In addition to the above, process- or application-specific factors not covered in the manufacturer’s instructions may also affect integrity test results.
To distinguish between actual filter damage and other causes that may lead to “false failures,” the following verification steps should be taken:
• Was the appropriate integrity test method selected, and were the correct test parameters used?
• Was the correct wetting fluid and wetting procedure applied?
• Is the test system leak-free?
• Was the temperature of the filter assembly stable and within specification during testing?
When retesting is required, follow these steps: Rewet the filter cartridge according to specifications and repeat the test.
If the filter fails the integrity test again, apply the following steps: Use more aggressive wetting conditions, increase flush volume/time, increase differential pressure, and/or apply back pressure.
If the filter fails the integrity test once more, proceed with the following:
• Perform an integrity test using a reference fluid with low surface tension to determine whether changes in wettability are unrelated to filter integrity.
• If the filter fails the test with the reference fluid, the filter is deemed to have failed the integrity test.
If the filter passes the integrity test at any point during the above failure analysis, it may be considered intact.
A summary diagram: Decision tree for integrity test failure analysis.