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Fresenius Environmental Bulletin, Vol: 21, Issue: 5A, Page: 1202

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Quality Control (QC); heterotrophic plate count (HPC); warning limit; (WL); control limit (CL); BACTERIA; Environmental Sciences
article description
A method was developed to monitor precision for heterotrophic plate count (HPC) using both blind duplicates and lab replicates as part of a project looking at pilot drinking water distribution systems. Precision control charts were used to monitor for changes in assay variability with time just as they are used for chemical assays. In adapting these control charts for the HPC assay, it was determined that only plate counts >= 30 cfu per plate could be used for Quality Assurance (QA) purposes. In addition, four dilutions were used for all known Quality Control (QC) samples to insure counts usable for QC purposes would be obtained. As a result there was a 50% increase in the required labor for a given number of samples when blind duplicates and lab replicates were run in parallel with the samples. The distributions of the duplicate and replicate data were found to be significantly different and separate control charts to monitor precision were used. The data did not have a normal distribution so a probability based analysis for setting up the warning limit (WL) and control limit (CL) was developed and compared with the standard method of the National Institute of Standards and Technology (NIST) guidelines. The procedure developed gives a method for monitoring precision for HPCs so that decreases in precision can be identified and corrective action taken.