In most pharmaceutical manufacturing plants, microbial testing of
purified water is conducted by using membrane filtration in which a 100-ml
aliquot is used. There is no need to worry about standard deviation
between plates since a single membrane filter is used.
In 9215B. Pour Plate Method of Standard Methods for the Examination of
Water and Wastewater, I do not see anything about plating duplicate 1.0 ml
and 0.1 ml aliquots of a water sample from an undiluted sample and a 1.0-ml
aliquot from a 1:100 dilution.
If you want to perform duplicate plate counts, I have no objections.
However, see Table 9020 VI of Section 9020 for the acceptable range of an
allowable logarithm difference for duplicate counts.
If you have a properly designed purified water system, the microbial counts
in a distribution loop are normally low such as between <1 and 10 CFU/ml.
I would think hat it would be somewhat difficult to establish an allowable
standard of deviation for 2 plates at a sampling site with such low counts.
Donald J. English Microbiological Quality Consulting LLC
Florham Park, New Jersey 07932
On Thu, May 28, 2020 at 7:18 PM Kyriaki Antoniou <[log in to unmask]>
<[log in to unmask]> wrote:
> Dear all,
> Could you give me your knowledge/opinion on the below matter?
> Microbiological water analysis, in pharmaceutical company, is performed in
> duplicate for each sample. What is the best (allowable) Standard
> deviation/RSD between the two results from the duplicate testing? Is there
> any guideline to follow? If not, can you set your own RSD as per USP <1225>?
> Best Regards
> Kyriaki Antoniou
> Microbiology Department Manager
> Remedica Ltd
> Tel.: +357 25-553000 | Direct Tel.: +357 25-553508 (Internal: 508) | Fax:
> +357 25-390192
> Site address: Remedica Ltd, Aharnon Street, Limassol Industrial Estate,
> 3056 Limassol, Cyprus-EU
> Mailing address: Remedica Ltd, P.O. Box 51706, CY-3508, Limassol Cyprus-EU
> Web: www.remedica.eu<http://www.remedica.eu/>
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