Manufacturers of cartridge filters validate their filters in house, in accordance with a standard methodology described in ASTM F8381 using Brevundimonas diminuta as the challenge organism; in brief, at a pressure of 2 bar, a sterilising grade membrane filter is challenged with bacteria grown in an aqueous buffer. A positive control filter of 0.45µm pore size is used to ensure that the bacteria grown are small enough to penetrate any defect in a 0.2µm filter.
It is important to note that both liquid and gas sterile filters in pharmaceutical processes are required to prevent the breakthrough of bacteria in a liquid challenge. Gas sterile filters are often wetted during the process, either inadvertently with the product, or during steam sterilisation. In both scenarios, a filter that is validated with a liquid challenge will ensure sterility.
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