Detection

How can I detect Legionella?

There are three methods of detecting Legionella in your water:

  1. Have a microbiology laboratory culture a sample for viable bacteria, using media that is selective for Legionella. This process is slow (taking up to 2 weeks to yield valid results).
  2. Have a specialist laboratory analyse the sample for DNA fragments from Legionella. Usually samples can be turned around within about 24 hours, but this approach is expensive.
  3. Detect antigen from the cell surface in the sample. Using a simple test kit (such at the hydrosense test) Legionella pneumophilla serogroup 1 can be detected in as little as 25 minutes, with all testing undertaken at your own site.

When should I test for Legionella?

Each site is different, and the decision on how often to test should take into account:

  • the risk of Legionella growing in a particular water system (looking at the system design, the operating temperatures etc)
  • the means of generating aerosols, e.g. shower heads, cooling towers etc.
  • the exposed population (with a particular focus on the numbers of older and immunocompromised people who may be exposed).

A decision on Legionella testing might also be influenced by the effectiveness and frequency of monitoring of other control measures which are in place. We would always recommend testing in the following circumstances:

  • if an existing control measure (e.g temperature or biocide dosing) has failed or dropped ‘out of spec.’ even for a short period of time.
  • whilst the system has failed to meet the standards required in industry codes / guidance, and remedial work is being planned and implemented.
  • when personnel are about to enter an area of high aerosol exposure

In these circumstances waiting for traditional culture results is unlikely to provide adequate warning of a developing risk. Faster testing should be used to support informed decisions on continuing to operate equipment in these circumstances.

Testing has an important role to play during routine operations as well, and should be used along with other monitoring techniques (temperature, displides/ATP tests, chemical tests etc) to monitor the effectiveness of the Legionella control programme. High risk sites may consider testing weekly using a fast test, or even daily during periods of particular concern. Lower risk sites with effective systems in place may reduce the frequency of testing to monthly, or even quarterly once they have sufficient ongoing data on the effectiveness of their processes.