Sample size for Purified water

May I ask that 1ml and 100ml sample size should be used for analyzing the Microbial limit test in gmp purifed water

Generally, a 1 mL sample of purified water should not be used for the Microbial limit test (MLT), this is because it will not be a representative sample of water collected from the purified water system/storage tanks. Hence 100 mL of purified water should be used for MLT, and the test should be performed by filtration method (100 mL of purified water should be filtered through 0.45-micron nylon filter paper, and the filter paper should be put on the media plate (SC Agar) for incubation to observe total aerobic microbial count (TAMC). The same procedure should be repeated and the second filter paper should be put on SD Agar plate and incubated for a specified period at a specified temp for total fungus and yeast counts. The media plate along with other plates (positive and negative controls) should be incubated at the specified temperature and number of days to observe microbial growth, if any.

The USP <1231> recommends a test volume of 1 mL aliquot for the pour plate method and 100 mL for a membrane filtration method, in order to get a statistically valid microbial count. For pharmaceutical purified water with100ml sample for membrance filtration, TNTC result may be obtained and don’t get a statistically valid Microbial count. Can I add another testing for using 10ml sample size ?

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