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How to interpret Legionella Water Samples

Detected Legionella results are more common than assumed, and can be worrying to receive. Legionella management programmes often include water sampling for Legionella bacteria, and sometimes other bacteria such as Total coliforms & E.coli and Pseudomonas aeruginosa. We are going to look at Legionella detected results, how to interpret and actions to take. Depending on your water system and usage, you may have been recommended sampling, or you are looking to test the effectiveness of your control programme.

When should I sample for Legionella?

How often you sample the hot and cold water system will depend on the recommendations from the Legionella risk assessment. The risk of the system, non-compliant temperatures, and lack of control regime are some factors taken into account when recommending water samples and the frequency of testing. If there has been confirmed Legionnaires Disease, it is essential to test your water system.

Where should I sample for Legionella?

The locations of where to test for Legionlla and the amount to be taken will vary from system to system. The Legionella Risk Assessment report will highlight the overall risk of the site, areas, and those that use the water system. Risk areas within a hot and cold water system can include low-use outlets and areas that can create aerosols/water droplets. Sentinel outlets (nearest and furthest) with a few outlets in between are often recommended to give an overview of the system. For example:

  • If your site has a small water system with high usage and low risks - samples at the sentinel outlets on an annual basis to test the effectiveness of the control programme is likely to be the best course of action.

  • If your site has a large water system with non-compliant temperatures and deadlegs - you are likely to be recommended for regular testing in multiple areas across the system.

Although water hygiene companies will recommend the areas and frequency of where to test, it is the responsibility of the site Duty Holder to decide.

Other risk systems include:

  • Cooling Towers

  • Jacuzzi and S

pa Baths

  • Water Fountains

  • Air Conditioning Units

  • Showers and Spray Taps

Legionella Testing and Analysis Process

Samples should be taken by operators that have been properly trained to do so. Once samples have been collected, they are taken to a laboratory. The HSE state ‘UKAS-accredited laboratories remain the gold standard’. Results of a laboratory test are usually available between 10 to 12 days. This is due to the incubation period and nature of testing.

Law and Guidance

Currently, testing water for Legionella bacteria is not a legal requirement in the UK, although necessary in the safe management of certain systems which include cooling towers, evaporative condensers and spa pools. It is also the only method to determine and monitor whether Legionella bacteria is in your water system. Testing for Legionella will determine if your current control programme is effective in the management of control.

For a full and comprehensive Legionella control programme, water sampling should be inclusive.

Types of Legionella Bacteria

The laboratory certificate of analysis will highlight the type of Legionella found within the system. Legionella pneumophila Serogroup 1 is the most serious form, with the likelihood to lead to Legionnaires disease being exposed. Serogroups 2-14 can still cause diseases, although less dangerous forms. The final group contains the various other forms of Legionella that does not cause disease but shows the presence of Legionella and therefore the suitable environment for serogroup 1 and serogroup 2-14.

Interpreting Results

The laboratory will provide the results as colony-forming units per litre of water tested, which will be presented as ‘cfl/l’. Water companies are required to keep results within the limit of 100 cfu/l. However, for healthcare settings and hospitals, all detected results should be investigated further due to the potential immune-suppressed service users and patients.

Other settings should monitor this level on a case by case basis. For results over 100 cfu/l, the first point of call is usually to disinfect the outlet and re-sample to determine whether this is an isolated issue. If there are multiple areas showing results of detected Legionella, it may be likely there is a systemic issue that will need further investigation and action. Actions include system disinfections, review of the Legionella risk assessment and review of control programme. Re-samples should be carried out to confirm the reduction of Legionella bacteria has been achieved.

For results that are over 1000 cfu/l, the settings should immediately carry out a review of the Legionella risk assessment and control programme, action any remedial and corrective actions highlighted, and disinfect the system. Again, re-samples should be carried out to confirm the reduction of Legionella bacteria to reasonable levels.

The overall importance is to identify and remedy any corrective actions your systems require to stay safe and compliant.

For further advice on water sampling, contact us on / 01296 914916 for a no obligation consultation.

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