Validity Assessment of Michigan’s Proposed qPCR Threshold Value for Rapid Water-Quality Monitoring of E. coli Contamination

Michigan’s water-quality standards specify that E. coli concentrations at bathing beaches must not exceed 300 E. coli per 100 mL, as determined by the geometric mean of culture-based concentrations in three or more representative samples from a given beach on a given day. Culture-based analyses require 18–24 h to complete, so results are not available for issuing beach notifications (advisories or closings) until the day following collection. This one-day delay is problematic because E. coli concentrations at beaches can change markedly from one day to the next. qPCR-based E. coli concentrations, by contrast, can be obtained in only 3–4 h, making same-day beach notifications possible. Michigan has proposed a qPCR threshold value (qTV) for E. coli of 1.863 log10 gene copies per reaction as a potential equivalent value to the state standard, based on statistical analyses of a set of training data from 2016–2018. The main purpose of the present study is to assess the validity of the proposed qTV by determining whether the implied qPCR-based beach notification decisions agree well with culture-based decisions on two sets of test data (from 2016–2018 and 2019–2020), and whether performance of the proposed threshold is similar on the test and training data. The results show that performance of the proposed qTV on both sets of test data was consistently good (e.g., 95% agreement with culture-based beach notification decisions during 2019–2020) and was at least as good as its performance on the training data set, supporting its use as an equivalent value to the state standard. ### Competing Interest Statement The authors have declared no competing interest.