Limit of Quantification

For the traditional plate count method the limit of quantification is 1 cfu/ml. However, the highest count accuracy is obtained when there are from 30 to 300 organisms per plate. If membrane filtration is the method used, then the limit is 1 cfu from the filtered volume. For example, for a large volume parenteral solution this may require filtering of between 1 to 5 litres. However, this limit can also be determined by any pretreatment required to solubilize or extract the microorganism from the sample.

So if 10-g sample has to be diluted in 90 ml of diluent and 1 ml enumerated using pour plates, then the limit could only be reported as less than 10 cfu/g of sample. At best the limit of quantification should be such that very low levels of microorganisms are detected and enumerated, with the same frequency for the rapid method as for the conventional. One way to do this is to carefully prepare a suspension, to contain as close as possible to 1 organism per ml. Multiple replicates (from 25 to 50) are enumerated by the conventional or rapid method. Even in the most carefully prepared suspension actual results achieved will probably be in the range of 0 to 5 cfus. Both methods are expected to detect and enumerate microorganism in this range equally successfully.

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