According to the EU GMP guideline (annex 1), the bioburden should be monitored before sterilisation and testing should be performed on each batch.
For routine commercial manufacturing, bioburden testing should be performed on the bulk solution, immediately before its sterile filtration. If a presterilising filter is additionally installed, then sampling for bioburden testing may be performed prior to the prefiltration, provided that no holding time is scheduled for the solution between the two filtration steps.
