The sterile manufacturing process using sterilizing grade membrane filtration has been practiced reliably and safely over the years. However, to prove its reliability and safety, the integrity of the filter must be tested. Typically, filter integrity testing is performed prior to assembling and using the filter unit. Sterilizing grade filtration is an acceptable process only when the integrity of the filter has been tested. This fact is stated in various guidelines that recommend the use of integrity tests before and after filtration. The integrity of the filter assembly should be verified by an appropriate method, such as such as a bubble-point pressure test or a forward-flow pressure test, before and after use. Irregular leakage flow rates should be noted and examined. The results of these filter integrity checks must be entered in a master batch record. Of all the structural features that characterize a filter, none appears to be more relevant to particle retention, whether viable or impervious particles, than its pore size.