I have been working in filtration for many years here in the Bay Area and many times the simple question comes up - what is a sterilizing grade filter? Why do I need 0.2 um vs 0.45 um? Why is one filter called a "Biorburden" reduction filter and the other certified as "Sterilizing Grade". As I am actively working on projects where these same discussions are raised I wanted to share an article I recently discovered that gives the history of sterilizing grade membranes, the regulatory standards that developed, and offers great insight on the topic. A sterilizing grade filter needs to meet ASTM 838-05 - boiled down to the simplest of definitions this means that the filter needs to meet a very specific bacterial challenge test where the filter removes 1 x 10^7 CFU/cm2 of membrane using a very specific organism B. Diminuta. I also previously worked with one of the authors Maik Jornitz who is an expert on process filtration and has published many books and articles on the topic. Here is a link to the article and I hope you find a wealth of information there:

http://www.pharmtech.com/assuring-sterility-astm-f-838-83

As always I am available to help with filtration training or discuss regulatory guidance with respect to filtration as it is an industry topic I really enjoy working with. Feel free to contact me anytime if you need help with Filtration!