Avoiding the Timebomb: Accuracy and Recovery in Legionella Testing

Tools to maximise the efficacy of testing to ensure the threat is detected before an outbreak of Legionnaires’ disease occurs

June 24, 2021

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As the world emerges from under the shadow of COVID-19, effective strategies to mitigate the risks posed by Legionella are going to be key. In Part 1 and Part 2 of this blog series, we explained why Legionella may become the unwanted encore to COVID-19. In Part 3 here, we discuss the good and bad of Legionella testing and how to maximize the chances of detecting Legionella present in water samples.


Testing water samples for Legionella is readily accomplished via culture in any accredited microbiology lab. US regulation requires filtration of the sample using a 0.45 micron (or smaller) filter. The filter is then placed onto a suitable growth media to foster growth which typically takes around three to seven days at which time the number of colonies is counted to determine the presence and density of Legionella species in the sample.  Turn-around time from sample to result is typically in the region of two weeks.


In a recent trial conducted by an independent accredited testing laboratory, the PALL pre-sterilized Supor® polyethersulfone (PES) 0.2-micron disc filter was compared to two other filters that were already in use by the lab from other suppliers in a head-to-head water sample elution test for Legionella. The PALL membranes are white with black gridding to facilitate easy colony counting and the low-binding Supor membrane is well known for its excellent flow rates, reproducibility, repeatability, and low extractables. In contrast to the filters currently in use by the lab, the Pall filters are pre-sterilized by gamma irradiation, which eliminates the need for sterilization and quality control, a time-consuming step that takes staff away from other productive activities.


In experimental and real-world testing, the Pall Supor filter achieved equivalent or higher bacterial recovery rates compared to the other products tested, while eliminating the need for a pre-sterilization step in the workflow [1]. As a result of these experiments, the laboratory adopted the Pall membrane for its Legionella testing.  Read the full Legionella case study including details of the experiments on our Legionella water testing page.


Figure 1: Individual value plot of Pall versus competitor membranes showing more consistent results from Pall membranes.


In a similar study comparing membrane performance at capturing and culturing Legionella pneumophila and Legionella anisa in water samples, the PALL Metricel® black membrane filter was quantitatively compared to two black nitrocellulose membrane filters from a competitor supplier [2]. The results of the experiments can easily be seen in figure 1. with the Pall filter demonstrating a consistent high recovery rate for all samples. In contrast, the competitor’s filters showed less consistent recovery with a much greater range of values for the samples tested. This type of inconsistency in filter performance is concerning as it can have a knock-on effect on the result of real-world Legionella testing, with the potential for labs using poor-performing filters to return a negative result from a sample that was actually positive. The connotations of such an error in the case of Legionella testing are stark, exposing building owners up to the possibility of reopening a building to human occupation when Legionella may still be present.  


Access the full Scientific Brief as well as more information about the variety of methods Pall provides for testing samples for Legionella, including high quality, consistent performance membranes for growth and counting, on our Legionella water testing page.  


This was the final of a three-part series on the looming threat of Legionnaires’ disease in a post COVID-19 world.  You can find the previous posts in the series here:

Part 1. The Ticking Timebomb. Legionnaires’ Disease in the Post COVID-19 World.

Part 2. Mitigating Risk: Steps to Avoid Legionnaires’ Disease Becoming Act 2 in the Global Pandemic. 


In the event of Immediate Critical Contamination and Outbreak, please call Pall’s Emergency 24hr Outbreak Hotline: 1-866-347-3428. Pall can rapidly deploy Point-of-Use Water Filters to deliver an immediate barrier against Legionella outbreak .

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