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Asked by : Pradeep Mishra, Quality Assurance Executive, Reliance Life Sciences
Industry : Biotechnology
Functional Area : Quality Management
Keywords : filtration sterilty
Activity: Question posted: 11 09 2010 16:09:33 +0000, 2 answers, 8447 views, last activity 11 10 2010 23:00:18 +0000
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During Sterility testing .45 micron filter is being used not  .22 micron why? smallest bacteria size is .3 micron.

  Answered by     Gautam Pangu, Senior Scientist, Vindico Pharmaceuticals  | 11 10 2010 23:00:18 +0000
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A 0.45 micron filter actually means that  0.45 micron is the size of the biggest pore. There will be a lot of pores that are smaller then the same and they would retain the microorganisms, even smaller than the mentioned pore size. Also, a 0.22 micron filter generally requires a lot of pressure to push the liquid through (this is true for any syringe filtration) and chances of membrane break-up are more (at least on a lab scale, may not be true for manufacture scale). A 0.45 micron filter would reduce that required pressure and allow more recovery, so it is a cost-effective option.

  Answered by     Pravinchandra Vankawala, Principal Scientist, Wanbury Limited  | 11 10 2010 05:33:45 +0000
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The classification of membranes is done on the basis of the pore size, the sterilization grade filters are classified on the basis of retention of Brevundimonas diminuta. 0.45u should retain 10 e5 cells per cm2 and 0.22u should retain 10 e7. The selection of the membranes need to be done on the basis of the bio-burden of the sample.

However while doing the sterility testing we are sure that the load of the microorganisms is not much hence can use the 0.45u membrane, the other point is the flow rate - but is not much of importance.

The most important point is that 0.22u membrane the recovery of microorganisms is not as good as 0.45 because of pore inhibition. This is the phenomenon exhibited when a bacteria gets trapped in the pore and hence ceases to grow.

Hence the 0.45u membrane has been validated for sterility testing !

As far as usage of 0.45 microns filter at lab scale is concerned, there are various benefits, which are as follows.
1. Fast flow rates, therefore decreases test time.
2. Better media nutrition across the filter through bigger pores of 0.45 microns filters in comparison to 0.2 microns filters
3. Mentioned in USP

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