Integrity Testing for Food & Beverage Filters

Integrity tests are non-destructive tests and as such, can be performed before filtration and after filtration without affecting the filter itself. At the same time, they provide confidence that the filter is intact and providing the expected performance and retention characteristics.

Integrity testing during validation, relates to the microbial removal of final filters based on membranes used during product development. For this purpose:

• Filters with a range of integrity test values under set conditions are challenged with microbes
• The results of the microbial challenge tests are plotted against the integrity test values 
• Based on the desired microbial removal level - sterilizing grade or a specific log reduction value (LRV) - an acceptable test limit for the integrity test is established (e.g. mL/min or mbar pressure drop)

Integrity Test Values and Microbial Challenge

A correlated integrity test can confirm filters are integral and suitable for continued service, for example:

• It confirms the filter is the right grade, has been installed correctly and is not damaged
  • One can integrity test an empty filter housing, with closed upstream and downstream valves closed to confirm that all housing o-ring seals are intact

• It confirms the filter meets the manufacturer’s specifications
• It is important for process validation, monitoring and batch record files, and could be helpful during process documentation review
• Finally, it allows for faster identification of contamination root cause
  • If the integrity test passes and there’s contamination downstream of the filter the following root causes are most likely:
    • Contaminated sampling port
    • Contaminated filling heads
    • Contaminated closures
    • Contaminated bottles

At Pall, 100% of the filters are integrity tested in manufacturing before release. There will be a production integrity test limit specific to the manufacturing site’s integrity test method and the results of the validation. Additionally, there will be a customer integrity test limit suitable to use with Palltronic® integrity test devices supplied by Pall for customer use (e.g. Palltronic Compact Touch Integrity Test Device). 

There are two integrity tests recommended to confirm the integrity of microbial reduction and sterilizing grade membrane filters: Forward Flow and Pressure Decay. The forward flow value is correlated to the bacterial challenge test results. The Pressure Decay value is calculated from the Forward Flow value and the upstream volume of the housing.

These tests are correlated to a set level of microbial removal or reduction during the initial filter validation. As non-destructive tests, they may be used after sanitization and at the end of a production batch to show the filter assembly is integral. These tests are based on the same physics that apply to the flow of gas through a wetted pore.

Integrity Test and the physics of flow of gas through a pore

Forward Flow Test 

• The pressure within the upstream volume of the filter housing is maintained at the, filter specific, Forward Flow test pressure.
• The total gas flow, made up of diffusive flow through wet pores and bulk flow through open pores or defects, through the filter is measured by the test device upstream or downstream Forward Flow measures the flow required to maintain the test pressure. 

Pressure Decay Test (also known as Pressure Hold)

• The pressure within the upstream volume of the filter housing is stabilized at the, filter and housing specific, Pressure Decay test pressure.
• The total gas flow through the filter is measured by the test device upstream.
• Pressure Decay measures the drop in the pressure 

Water for Intrusion Test (WIT)

• Water flow is measured through a hydrophobic filter and correlated to microbial retention.

Bacterial retention testing is the most sensitive integrity test for sterilization ability and microbial reduction but this testing is destructive and therefore cannot be used if the filter needs to be put into service after testing as the filter would be contaminated with the test bacteria. Non-destructive integrity tests, if correlated, are an indicator of bacterial challenge retention.