New addition to Legionnaires’ FAQ page

Who can be appointed to test or monitor legionella?

Testing of water quality may be carried out by a service provider, such as a water treatment company or consultant, or by the operator, provided they are trained to do so and are properly supervised.   The type of test required will depend  on the nature of the water of the system. HSG274 Legionnaires’ disease Technical guidance  provides further details for both evaporative cooling systems and hot and cold water systems.  

How do I test or monitor legionella from my water system?

Where monitoring for legionella is considered appropriate, the sampling method should be carried out in accordance with BS7592 and the biocide, if used, neutralised where possible. Water samples should be tested by a UKAS-accredited laboratory that takes part in a water microbiology proficiency testing scheme such as that run by Public Health England.  The laboratory should also apply a minimum theoretical mathematical detection limit of = 100 legionella bacteria per litre of sample for culture-based methods.

How often should I test water for Legionella?

It depends on the system that you have and the outcome of your risk assessment. For open systems, such as cooling towers, evaporative condensers and spa pools etc, routine testing should be carried out at least quarterly. However, there may be circumstances were more frequent sampling may be required.

For hot and cold water systems, which are generally enclosed, i.e. not open to the elements and significant contamination in the same way as cooling towers, microbiological monitoring is not usually required.  But there may be circumstances where testing for legionella is necessary eg where there is doubt about the efficacy of the control regime or where recommended temperatures or disinfection concentrations are not being consistently achieved . Further guidance is available in HSG274 Part 2

How do I interpret legionella test results?

Consider what the results mean in the context of your water system. Your subsequent specific actions will depend on your risk assessment. Further information about what action to take when certain levels of legionella are identified can be found in HSG274 Part 1 (paras 1.114 – 1.129 and table 1.10) for evaporative cooling systems; and HSG274 Part 2 (paras 2.119 – 2.125 and table 2.2) for hot and cold water systems.


Current HSE position on quantitative polymerase chain reaction (qPCR) in the management of legionella risks

HSG274 and HSG282 recommend routine legionella sampling be undertaken as part of the checks on the effectiveness of control regimes in cooling towers, evaporative condensers and spa pools, as well as in some hot and cold water systems and other risk systems. 

BS 7592 and BS 6068 sampling and analytical culture by UKAS-accredited laboratories remain the gold standard methods for the detection of legionella in environmental samples. 

However, legionella culture methods do have certain acknowledged disadvantages including:

  • long incubation period;
  • poor reproducibility;
  • poor sensitivity;
  • inability to detect viable but non-culturable cells; and
  • inhibition due to competing microbial flora. 

Sampling can also form an important part of public health investigations into cases and outbreaks of Legionnaires’ disease.  The disadvantages associated with the culture method, can impede or slow down these investigations. 

Over the previous few years, there has been an increase in the use of the quantitative qPCR assay in public health investigations.  With this assay, results can be obtained in 2 days rather than the typical 10 days for the culture method and consequently – this can deliver substantial time savings.

There are currently three key areas where HSE recognises the benefits and one key limitation of the use of the qPCR assay conducted to ISO 12869:2012, as an alternative to the traditional legionella culture-based methods:

Rapid detection of legionella bacteria – the high negative predictive value (NPV) of qPCR means that it is suitable for use as a negative screening tool to rapidly rule out potential sources, for example, in an outbreak situation, and to support public health investigators in prioritising resources. 

Indication of the effectiveness of cleaning and disinfection – the high NPV of qPCR, means that negative qPCR results may be a useful indicator for the restarting of system implicated in the source of an outbreak following remedial actions, such as cleaning and disinfection.

Complementary tool for the rapid routine monitoring of legionella trends at dutyholder sites – it is important that data from such tests can be properly interpreted, to enable informed decisions on the effectiveness of control measures to be made by a competent person. 

Interpretation of results – the results of positive qPCR samples are difficult to interpret, as the assay detects legionella DNA from both live and dead bacterial cells and the units of measurement for qPCR (genomic units/litre) are not directly comparable with the action and alert levels for culture (expressed in colony forming units) published in HSG274.

Article source: http://hse.gov.uk/legionnaires/faqs.htm#Testing-monitoring