0.22 or 0.45 um collection filter for Bacterial retention testing

I have heard a number of FDA audits citing the following deficiency in study design:

Procedures designed to prevent microbiological contamination of drug product purporting to be sterile do not include adequate validation of the sterilization process.
Specifically, inadequate collection filters were used to confirm the retention of Brevundimonas diminuta during the performance of sterilizing filter validation. Pore size of 0.45um collection filters used during the filter validation is rated larger than the microorganism B. diminuta used during the filter validation. Due to the larger pore size of the collection filters, B. diminuta that may have penetrated the 0.22um sterilizing filters may not be detected by the collection filters and result in false negative results

Any thoughts on continued use of 0.45 filter

What they are saying is there is no proof that filtering a solution through your current filter is not sterlizing that solution. B. diminuta is ‘very small’ so I would even suggest 0.1 um filter.

See attached.

I’d tend to agree with the FDA, it is not logical to sample small Bd bacteria downstream of a 0.2um filter with a 0.45um filter disc, but it is stipulated in the standard methodology used in accordance with ASTM F838! In defence of the apparent lack of reason…I should add that the flat discs used for sampling will have a higher bubble point in comparison with equivalent pleated filters, and the challenge is much lower for these sampling filters. Unfortunately filter manufacturers tend to (have to) abide by the recognized ASTM procedure. I’d be inclined to modify it using 0.2 or as proposed 0.1um filters or you could add these downstream of the 0.45um sampling filter, so that it would cover ASTM and FDA requirements! My own experience is that the 0.45 um filters do suffice to capture bacteria that have penetrated through a marginally defective 0.2um filter cartridge. In recent years regulators have tended to pester injectable manufacturers who have not somehow proven that the filter is intact following exposure to the product…for both bugs and extractables…e.g. with viscous products the pressure drop might be higher than that used in ASTM F838. From the link : bacteria smaller than Brevundimonas…e.g. mycoplasma…this tends to effect processes esp. animal cell culture and water supply where some manufacturers use 0.1/0.03/0.04um filters to protect their (expensive) batches, but I’ve not seen regulatory requirements for pharma processes myself only for endoscopy washing equipment (mycoplasma tend to inhabit the ‘lower’ areas of the human body).
Filter sterilizing validation is fairly well characterised in PDA TR26 (the FDA are contributors).
Regards Moz