Legionella Test Kits
Posted 11 February 2011 - 03:39 PM
Posted 24 February 2011 - 11:26 PM
There is no quick test for Legionella that will give you accurate results. The gold standard for testing for Legionella is culture method. This requires you to take a sample and send to a lab. Results generally take 7-10 days. If you would like more information on lab testing for Legionella please email me at firstname.lastname@example.org.
I'm going to dispute what "Jackie" says. The generally recognised standard test for Legionella is indeed the "culture method", however there are a few quick tests around which actually give accurate results. The results may not always agree with the lab result - but that doesn't necessarily make them innacurate - I'll explain more of that at the end of the post - first let me describe the faster test methods that are available.
The CULTURE test method (ISO 11731) takes a water sample (and usually preprocesses this sample to concentrate the bacteria, and suppress non-legionella bacteria in your sample). This is then spread onto special agar plates that encourage legionella bacteria to grow whilst suppressing some, but not all, other bacteria. The plates are incubated for several days (a confirmed result will normally take at least 12 days from collecting the sample). Any bacteria growing on the plates then need to be checked to see if they are legionella. Although widely accepted as the "gold standard" method - the method can give both false positive and false negative results. Of particular concern are "viable but non culturable bacteria" - these are bacteria (possibly at the most virulent stage of their life cycle) which can cause infections but which will not grow on the artificial culture media. The culture media is optimised for growing Legionella pneumophila bacteria and so whilst it can detect most Legionella species, non-pneumophila species may go underreported. This combined with lab-to-lab variability means that probably at best 80% of samples are detected via this route.
There are variations on the method including most probable number liquid based suspensions and slightly different media but all must be performed in a specialist laboratory. One variation of the culture method which does allow for potentially faster results is marketed by Phigenics in the USA. This uses a dipslide which is innoculated on site before returning to the lab for analysis - by looking for "microcolonies" they are able to report results quicker than normal - but the same draw backs apply.
DNA fingerprinting (or PCR detection as it is generally known) offers a much faster alternative. For example, Genesystems (part of the Pall filtration group) offer a system which can yield results in less than a day. It is a relatively expensive process, and needs some specialist equipment and a degree of expertise. Unlike culture PCR will detect DNA fragments from both whole organism and fragments of bacteria. Although often criticised for detecting "dead bacteria" this may not be a bad thing provided the detection level is not too sensitive since you don't get dead bacteria without first having live bacteria. "Accuracy" figures depend on the study and statistical analysis, but are typically around 80%. Depending on the method it can detect only L. pneumophila or all L. species.
There are some ANTIGEN based detection methods (and I'll specifically mention The hydrosense Legionella Field Test as I have a vested interest) which provide the ability to detect antigens from the cell surface (the same technique used to identify Legionnaires' disease in urine samples). This has many of the benefits of PCR but is simple enough to perform on site and get results in less than half an hour. Multiple trials report a diagnostic sensitivity of ~ 80%, and very high specificity for L. pneumophila sg1 (which as you probably know is the causative agent for the vast majority (some reports say >90%) of infections). These tests aren't "cheap" but usually compare favourably to lab tests.
Now when people compare these three methods they often run into a problem: all three are measuring different things, and therefore may not get good correlation. They are all a measure of Legionella risk, but the culture method is assessing risk by counting how many cells have survived the transportation to the lab, the preprocessing stages and been disposed to grow on artificial media. PCR measures how much DNA is in the sample whilst Antigen techniques such as hydrosense detect molecules from the surface of the bacteria. To further complicate matters people often assume in their data analysis that because the culture method is widely recognised it is 100% reliable and therefore any deviation from this is a failing on the alternative method - this is simply not true. Moreover culture methods may be least effective when the bacteria are at their most infectious (when seeking a new host to infect). To get the most information you would want to run all three types of test on a frequent basis, but practicality and cost will mean this is unlikely, however many Hydrosense users use the fast on-site test in between less frequent slower lab tests,
Reply to this topic
0 user(s) are reading this topic
0 members, 0 guests, 0 anonymous users