5.1 Since all sterilizing filtration processes are performed under positive pressure, this test method is designed to assess the retentivity of a sterilizing filter under process conditions.

5.1.1 A challenge of 10⁷ bacteria per cm² of effective filtration area is orders of magnitude higher than one would expect to encounter in a sterilizing filtration process. This level was selected in order to provide a high degree of assurance that the filter would quantitatively retain large numbers of organisms. This concept is important, in view of the requirement to provide a quantitative assessment in validating a sterilization process.

5.1.2 The analytical procedure utilized in this test method provides a method to assign a numerical value to the filtration efficiency of the filter being evaluated. This value, coupled with a knowledge of the number and types of organisms (bioburden) indigenous to the process, may then be utilized to determine the probability of obtaining a sterile filtrate. Conversely, the numerical value of the filtration efficiency may be used when one must meet a specified probability of sterility assurance to calculate the volume of fluid that may be filtered in order to maintain that level of assurance.