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AimsTo perform an international trial to derive alert and action levels for the use of quantitative PCR (qPCR) in the monitoring of Legionella to determine the effectiveness of control measures against legionellae.Methods and ResultsLaboratories (7) participated from six countries. Legionellae were determined by culture and qPCR methods with comparable detection limits. Systems were monitored over ≥10 weeks. For cooling towers (232 samples), there was a significant difference between the log mean difference between qPCR (GU l−1) and culture (CFU l−1) for Legionella pneumophila (0·71) and for Legionella spp. (2·03). In hot and cold water (506 samples), the differences were less, 0·62 for Leg. pneumophila and 1·05 for Legionella spp. Results for individual systems depended on the nature of the system and its treatment. In cooling towers, Legionella spp. GU l−1 always exceeded CFU l−1, and usually Legionella spp. were detected by qPCR when absent by culture. The pattern of results by qPCR for Leg. pneumophila followed the culture trend. In hot and cold water, culture and qPCR gave similar results, particularly for Leg. pneumophila. There were some marked exceptions with temperatures ≥50°C, or in the presence of supplementary biocides. Action and alert levels for qPCR were derived that gave results comparable to the application of the European Guidelines based on culture. Algorithms are proposed for the use of qPCR for routine monitoring.ConclusionsAction and alert levels for qPCR can be adjusted to ensure public health is protected with the benefit that remedial actions can be validated earlier with only a small increase in the frequency of action being required.Significance and Impact of the StudyThis study confirms it is possible to derive guidelines on the use of qPCR for monitoring the control of legionellae with consequent improvement to response and public health protection.

We thank Dr Gordon Murray for his supervision of the statistical analysis. AppendixThe APACHE II score is a simplified form of the original APACHE system, which attempts to measure reliably and objectively the severity ofdisease so that a patient's immediate prognosis may be estimated. The system is based on the principle that the outcome from an acute illness relates to the degree of disturbance in the physiology of the main organ systems, to the patient's age, and to the presence or absence of pre-existing severe chronic health problems.The main part of this scoring system is an acute physiology score comprising 12 readily available physiological measurements. The value of each measure is given a weight varying from 0 to 12 depending on its deviation from a predetermined normal range and on its relative importance to patient outcome as determined by an expert multidisciplinary panel and by a review of past experience. The resulting acute physiology score varies from 0 to 45; a higher score representing a greater degree of physiological derangement and a higher risk of death. To estimate prognosis this score is combined with additional points for advanced age and preexisting chronic health problems relating to the hepatic, cardiovascular, respiratory, renal, and immune systems to form the full APACHE II score. This score has a theoretical range of 0 to 71, but an observed range of 0 to 50 is more probable. The higher the score the greater the patient's acute risk of death.