Prefiltration. An Often Overlooked, but Critical Driver of Success

Adding a prefiltration step to your sterile filtration protocol can speed up and improve the quality of your results

July 1, 2021

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In a four-part “Filtration Nation” blog series, we take a deep dive into some of the important but often overlooked fundamentals of sterile filtration and explore some of the critical success factors that can make or break your filtration protocol.


In Part 1 below, prefiltration is examined. Part 2 will further discuss sterile filtration and integrity. Part 3 will focus on differential pressure monitoring, and finally, Part 4 will delve into improving sterile filtration throughput. Want to know more now? Check out our Sterile Filtration Resources.


Filtration Nation Part 1. The importance of prefiltration


Sterile filtration is an often-overlooked part of many laboratory protocols, taken for granted in favor of the more exciting parts of the workflow. However, sterile filtration is often the foundation on which the rest of the experiment is built and quite capable of influencing the final results. Many people do not realize that done poorly sterile filtration may not prevent, and can even introduce, contamination that can compromise further downstream processes and analyses.


Defining “Sterile”


To be considered a sterile filter under the FDA definition, the filter must be able to remove particles down to 0.2 µm in diameter. Quality Control (QC) testing is typically undertaken using the microorganism Brevundimonas diminuta, which, as its name might suggest, is very small indeed, just 0.3 µm diameter. QC testing of filters involves challenging the filter with a predefined concentration of B. diminuta and ensuring the effluent retains a minimum of 107 CFU (Colony Forming Units) per cm2 per modified ASTM F838, current revision.


Prefiltration: removing the rocks and pebbles before filtering out the sand


The problem for filtration in the laboratory is that the extremely small size of the pores in the sterile filter makes it unsuitable for the filtration of large or even medium-sized particles, clogging quickly in their presence and even becoming damaged to the point that unwanted material of all sizes can pass through. In most cases, a 1% solid load is too much for efficient final membrane filtration.


Many researchers, in their eagerness to get on with the experiment and not wishing to extend the duration of their protocol, attempt to use only a final filter for their sterilization step with the potential for delays, errors, and general headaches downstream. It is far better and ultimately quicker to take the time to prefilter your solution, taking care to size your prefilter appropriately to deal with the larger debris present before moving on to final, sterile filtration. Ultimately, using the analogy of a river, we want to remove the rocks and pebbles before trying to filter out the sand.


In a recent scientific brief, Director of Scientific Laboratory Services here at Pall, Joseph Baaklini, outlines the proper selection of prefilters and discusses how they can improve your filtration, accelerate your protocol, and give you confidence in the sterility of your final medium. You can find this brief on the Scientific Resource Library along with a host of other concise, highly focused briefs dealing with all the major issues, concerns, and challenges of filtration.


This is part one of a four-part series looking at key issues in filtration. Stay tuned for Part 2 where we will further discuss sterile filtration and understanding integrity.

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