Pall Corporation’s approach for testing multi-modular, sterilizing-grade filter assemblies is to provide assembly-specific Forward Flow limit values below the sum of the maximum allowable FF limit values for the individual filter modules (i.e. a multiplying or reducing factor).  This reduction in allowable Forward Flow is designed to reduce the risk that a marginal filter test failure (in the unlikely event that one is present) cannot be detected in a multi-element assembly.  It results in a tighter, more conservative test limit when compared to a linear multiplier.

While the use of multiplying factors is not a regulatory requirement, it is Pall Corporations’ philosophy on integrity testing to use practices that provide the maximum safety for large filter area installations.  This needs to be balanced with the risk that a set of integral modules may fail the test due to the application of a multiplying factor that is too stringent (“false fail”).  This approach is also described in the 2008 revision of PDA Technical Report No. 26 on Sterilizing Filtration of Liquids 1.

Pall Corporation’s basis for defining the appropriate multiplying factor for a specific multi-modular installation is based on several parameters including:

  • Type of membrane and number of elements
  • Statistical distribution of observed Forward Flow (FF) values for the specific filter
  • Mean deviation for Forward Flow distribution
  • Standard deviation of Forward Flow distribution
  • Point of first failure during microbial challenge (if observed)

Forward Flow limits obtained from Pall Corporation for multi-element filter assemblies will incorporate the appropriate multiplier.

PDA Journal of Pharmaceutical Science and Technology, Technical Report No. 26 Sterilizing Filtration of Liquids, Rev. 2008. Supplement Volume 62, No. S-5