Evaluation of Legionella real-time PCR against traditional culture for routine and public health testing of water samples

Aims: To evaluate the usefulness of Legionella qPCR alongside traditional culture for enumeration of Legionella from water samples as part of both routine and public health investigation testing. Methods and Results: Routine water samples (n = 2002) and samples from public health investigations (n = 215) were analysed by culture and qPCR for Legionella spp., Legionella pneumophila and L. pneumophila sg-1. A negative qPCR result was highly predictive of a negative culture result for all water systems (negative predictive values, NPV from 97.4 to 100%). Positive predictive values (PPV) were lower (0-50%). Results for qPCR were generally larger than culture with average log(10) differences of 1.1 for Legionella spp. and 1.2 for L. pneumophila. Alert and action levels of 1000 and 10 000 GU per litre, respectively, are proposed for Legionella qPCR for hot and cold water systems (HCWS). The use of qPCR significantly reduced the time to results for public health investigations by rapidly identifying potential sources and ruling out others, thus enabling a more rapid and efficient response. Conclusions: The high NPV of qPCR supports its use to rapidly screen out negative samples without culture. Alert and action levels for Legionella qPCR for HCWS are proposed. Quantitative PCR will be a valuable tool for both routine and public health testing. Significance and Impact of the Study: This study generated comparative data of >2000 water samples by qPCR and culture. Action and alert levels have been recommended that could enable duty holders to interpret qPCR results to facilitate timely Legionella control and public health protection.