Evaluation of an MPN test for the rapid enumeration of Pseudomonas aeruginosa in hospital waters.

In this study, the performance of a new most probable number (MPN) test (Pseudalert(®)/Quanti-Tray(®)) for the enumeration of Pseudomonas aeruginosa from hospital waters was compared with both international and national membrane filtration-based culture methods for P. aeruginosa: ISO 16266:2006 and UK The Microbiology of Drinking Water - Part 8 (MoDW Part 8), which both use Pseudomonas CN agar. The comparison based on the calculation of mean relative differences between the two methods was conducted according to ISO 17994:2014. Using both routine hospital water samples (80 from six laboratories) and artificially contaminated samples (192 from five laboratories), paired counts from each sample and the enumeration method were analysed. For routine samples, there were insufficient data for a conclusive assessment, but the data do indicate at least equivalent performance of Pseudalert(®)/Quanti-Tray(®). For the artificially contaminated samples, the data revealed higher counts of P. aeruginosa being recorded by Pseudalert(®)/Quanti-Tray(®). The Pseudalert(®)/Quanti-Tray(®) method does not require confirmation testing for atypical strains of P. aeruginosa, saving up to 6 days of additional analysis, and has the added advantage of providing confirmed counts within 24-28 hours incubation compared to 40-48 hours or longer for the ISO 16266 and MoDW Part 8 methods.