I have analyzed PT a sample from an external PT provider, the matrix was skimmed milk powder (lyophilized), and the required tests were: aerobic plate count, Enterobacteriaceae count, Coliforms count, and E. coli count. the results that I got were questionable; the counts were much lower than the assigned values (lower by an order of magnitude). I tried to investigate the possible reasons for what happened; I check media preparation, incubators temperature, sterility checks of 3M Petrifilms and all were good. the problem that the number of CFUs capable of growing on plates was much lower than the real number so I suspect that the problem happened during the delivery of samples, although the sample was delivered with two other samples and their results were good. Anyone has been through this? Any other possible explanations?
Haya Rashed Abdullah
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Hi there, to my opinion, although the result si "only" questionable ( as opposed to unsatisfactory), you should run a 5 M investigation, although you might ot get a definite root cause. Also was the expected value low? have you ever calculated you uncertainties of measurement? Once every thing on your side is OK, only then could you place the cause onto transportation and storage.
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