For some types of products, such as vaccines and insulin, sterilization by heat is not possible. In this case, sterilization by filtration into a previously sterilized container can be used. The filter should have a nominal pore size of no more than 0.22µm. However, it should be remembered that although these filters can remove bacteria and moulds, viruses and mycoplasmas might not be removed by this method.

In order to reduce the risks associated with the filtration method, double filtration may be advisable. There is usually a pre-filter before the main one anyway, but in addition, a final filter, just prior to filling, should also be used where possible.

Fibre shedding filters and asbestos filters may not be used.

There should be documented evidence of filters integrity tests having been performed after use and in some cases also before use. This requires the use of equipment such as a bubble-point tester. In addition, validation of the method will have produced standard times and pressure differentials for a given volume of liquid. Any variations from this should be noted and investigated.