According to a current American Society for Testing and Materials (ASTM) standard (F838-05), a sterilizing-grade filter must completely retain Brevundimonas diminuta microorganisms when challenged with 107 bacteria per cm² of filtration area. Standard integrity tests applied by end-users, such as the bubble point or diffusion tests, have to be correlated with the bacterial challenge tests. In principle, porometric investigations can be used to confirm a filter’s pore size distribution around 0.2 µm. For mycoplasma retention, 0.1-µm-rated membrane filters are used.
The method used to determine whether or not a sterilizing-grade filter of a given removal rating will produce a sterile effluent in a manufacturing process is a process-specific bacterial challenge study. Ahead of such a study, it is recommended that the filter end-user begin with the selection of a filter that has been validated by its manufacturer as retentive for B. diminuta in accordance with test method ASTM F838-05 at a challenge level of 107 CFU per cm2 effective filter area. Typically, filters rated at 0.2 µm and 0.1 µm sterilizing-grade make this generic microbial retention claim.
The subsequent process-specific bacterial challenge study, performed with an appropriate challenge organism at the same challenge level, yet under ‘worst-case’ operating conditions, can confirm whether or not a selected filter will deliver a sterile final drug or vaccine product.
Sterile filter is a filter that reproducibly removes test microorganisms from the process stream, producing a sterile filtrate. In extremely simple terms, as described in PDA Technical Report 26, sterilizing filtration is the process of removing microorganisms (not viruses) from a fluid stream without adversely affecting product quality.
The specific technical requirements of sterilizing filtration have evolved over time, ultimately leading to the development of the ASTM F838 standard test method for 0.22µm rated sterilizing-grade membranes, which still applies today.
Sterilizing-grade designation is not pore size dependent. It is a functional definition. Functionality is firstly defined by qualification testing by filter manufacturer. It is then secondly defined by validation, normally by final user.
Filter vendors typically supply a large volume of supporting data to demonstrate filter performance under standardized test conditions, the responsibility for correctly implementing sterile filtration ultimately falls upon the user.
Simply selecting a suitable type and format of filter is not sufficient; in order to be truly considered “sterilizing-grade”, further process qualification must be performed. This requires an understanding of exactly which filter type to use, and how to successfully implement it.
Therefore it is important for filter users to understand the true requirement of the application in question. Installation of a sterilizing-grade filter module without the appropriate qualification cannot be considered to offer sterilizing-grade performance, and in many cases may be unnecessary.
Filter qualification typically involves demonstrating bacterial retention of the filter under process simulated conditions. Rather than using the standard Brevundimonas diminuta, challenge testing with a specific organism isolated from the manufacturing process may instead be required. In addition, it must be demonstrated that the filter is not adversely affected by the process solution itself, and a suitable non-destructive integrity test regime must also be developed to reliably demonstrate that the filter is fit for purpose prior to and after use.
It is also required that the bioburden upstream of the filter is understood and controlled in order to avoid the risk of non-sterile filtrate due to unusually high microbial loading (in addition to the other risks that would also create). To this end, organizations such as the PDA and EMA stated that (unless exceptional circumstances exist) no more than 10cfu per 100mL of solution should be present upstream of the final sterilizing-grade filter.
As a consequence, the support and guidance offered by filtration vendors can be extremely beneficial to user. Vendors can provide initial guidance around appropriate filter selection based on application needs, subsequent optimization of the filtration process though direct technical support, and finally input into the process qualification procedure necessary to ensure repeatable, high-quality manufacturing which satisfies all market and regulatory requirements.