Process validation of sterilising grade filters is a fundamental task to ensure and verify the performance of the filter used within the process conditions and fluid stream. Process validation means that the specified membrane filter is subjected to a bacteria challenge test in accordance with ASTM 838-05, utilising the process parameters of the filtration process and, if possible, the actual fluid product. If the fluid has properties that are detrimental to the challenge organisms, a placebo as close as possible to the actual drug product, is required. The effect of the fluid stream on the organisms needs to be tested using viability tests.
Over-sterilisation is sometimes cited as a concern. Beside other key attributes, sterility is key to an injectable drug product; therefore I would hesitate to utilise the word "over-sterilisation". Safety and sterilisation robustness are essential, and one has to ensure that sterility is achieved by appropriate measurements, qualification and validation. If one wants to classify something as "over sterilised", it could be through the use of either redundant filtration or filter ratings, which are inappropriate for the application.
More important than adding filters or tighter pore sizes are process validation procedures, which verify that the filter does what it is supposed to do: retain the organism level and species found in the upstream fluid and create a sterile effluent. When a sterilising grade filter is properly validated and evidence has been obtained that it performs to required specifications, why would one need a second one or move to a tighter pore size?
The only reason would be insurance, which creates an immediate insecurity notion. Another "over-sterilisation" aspect could be the European restriction of a maximum 10 cfu/100 mL in front of a sterilising grade filter. Why are end-users of sterilising grade filter bound to such a restriction, when the filters are commonly challenged with 107 organisms per cm2? If one utilises the example of a 1000 L volume with a bioburden of 10 cfu/100 mL being filtered through a 10 in. filter element of 0.6 m2, the challenge would be 16 organisms per cm2, far below the validation challenge test level of 107 per cm2. That sounds very much like overkill!