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Legionella testing – when do you need to take legionella samples?

Legionella Sample Testing
Legionella sampling
UKAS accredited Legionella sampling

Do I need to take Legionella samples? 

It is a common misconception that taking Legionella samples is a requirement for Legionella control in domestic hot and cold water systems.  In fact, microbiological monitoring of domestic hot and cold water is not normally a requirement for compliance with current guidance. However, Legionella sampling would be required in high risk situations or in systems where current controls are shown to be failing. For example, if temperature control is consistently unachievable or disinfection concentrations are not satisfactory then alternative precautions should be carried out such as weekly microbiological sampling.

In high risk properties such as hospitals and health care premises, taking samples is a requirement. For specific guidance on microbiological monitoring refer to the BS 7592:2008 – Sampling for Legionella Bacteria in Water Systems – code of practice. 

It is very important to know that taking Legionella tests from domestic water systems does not achieve compliance with the HSE guidance document The Control of Legionella Bacteria in Water Systems ACoP L8.  The first step to compliance is to carry out a Legionella risk assessment and, then, to put in place a control scheme based on the findings of the risk assessment.

Whilst the ACoPL8 guidance are not law, they are enforceable under the Health and Safety at Work Act 1974 and the Control of Substances Hazardous to Health (COSHH) regulations. A Legionella risk assessment will define actions required for compliance with the guidance, and provide a schedule for any required ongoing monitoring e.g. temperature and biological monitoring if required.   

How long does it take to get a legionella sample result?

The most common question about legionella samples is how long will the results take?  Legionella tests must be analysed at an independent UKAS accredited laboratory. The incubation time for a Legionella test, using the standard culture method, is 10 days, so this is not a quick test to carry out. In the event of a Legionella positive result, the customer will be informed immediately by phone or email with advice about what to do next.

Different methods are available for Legionella testing which reduce the time to get a result from the lab. Some methods can produce a result in as little as 24hours. Please contact the Dantek office for further information if a fast turnaround is required. 

Where do samples need to be taken?

For sites where Legionella monitoring is a requirement in hot and cold water systems, sampling should be carried out in accordance with: BS 7592 Sampling for Legionella Bacteria in Water Systems. Depending on the complexity of the system, the actual number of samples will be worked out. It is important to ensure the sample is representative of the water flowing around the system and not just of the area downstream of the fitting; samples should be taken from separate hot and cold outlets rather than through mixer taps or outlets downstream of TMVs or showers. This will help to identify where any potential colonisation is and will help to inform on appropriate actions going forward.

A sampling plan should be put in place detailing the sampling locations and the type of sampling to undertaken. Whenever possible, samples should be taken from locations considered most likely to contain the highest numbers of legionellae.

Locations for Legionella samples in cold water systems

  1. From the point of entry of the mains or nearest outlet to the point of entry into the building
  2. Cold water storage tanks
  3. From nearest & furthest outlets (Sentinel points) on each branch of the system

Locations for taking Legionella samples in hot water systems

  1. From the nearest outlet to the calorifier
  2. From the base of the calorifier
  3. From the nearest & furthest outlets (sentinel points) on each branch of the system for single pipe systems
  4. From nearest & furthest outlets (sentinel points) on each loop of a circulating system

How is a legionella sample taken?

The scope and type of sampling (pre or post flush) should be set out in the sampling plan. Methods for the different types of sampling are detailed below.  If not specified pre-flush samples will be taken as standard.

A sample will be collected in a new, unused, capped, sterile litre sample bottle using aseptic techniques.  This minimises the chance of cross contamination of the sample and reduces the risk of a resample being required.  The bottles need to be labelled with the exact location where the sample was taken and the exact time and date. 

Dantek Technicians are trained to take the sample using aseptic techniques to prevent cross-contamination.  It is important that the correct sampling technique is used to achieve the most accurate results so that correct preventative actions are completed.

What is an aseptic technique for Legionella sampling?

Aseptic sampling techniques reduce the likelihood of contamination and poor sample results.

When taking samples, the technician must observe the following best practice:

  • Never touch the neck of the bottle or the inside of the lid
  • Ensure the bottle does not come into contact anything which may introduce contamination
  • Do not rinse the bottle out prior to the sample being taken
  • Minimise the time that the bottle is open to atmosphere
  • Only use the correct sterile bottle containing sodium thiosulphate or appropriate neutraliser.
  • Wash hands when sampling
  • Use hand sanitiser when sampling
  • Samples must reach the lab within 48 hours
  • Correct transport procedures should be followed
  • Ensure an air gap is left in the sample bottle

When taking a sample from a cold-water storage tank, the samples are to be taken from the centre of the tank where possible, taking care not to collect any solids.  Move the bottle forward through the water during sampling, ensuring water is only collected from in front of the bottle.

Steps to taking a Legionella sample:

  • Legionella samples must be taken in 1 litre sterile sample bottles containing sodium thiosulphate capable of neutralising up to 5ppm of free chlorine without effecting the bacteriological population.
  • Samples bottles, which are checked to ensure they are in date.
  • Samples bottles are checked before use to ensure that the lid seal is still intact.

Prior to removing the lid, the bottle should be marked with a permanent pen detailing the following information;

  • Customer
  • Site
  • Location of sample, building name, room location, cold water storage tank, calorifier, cooling tower sump, spa pool etc.
  • System identification, mains cold water, tank feed cold, domestic hot.
  • Pre or post-flush
  • Date & time of sample
  • Temperature of the water and/or chlorine dioxide reserve

How to take a pre-flush sample?

  • Do not disinfect the tap/outlet
  • Collect the sample immediately after the tap, or fitting, is opened, ensuring that the sample consists of only the first water which leaves the outlet.
  • Continue to fill the bottle leaving a 1 cm air gap at the top of the bottle.
  • Immediately replace the cap and invert the bottle several times.
  • Place the sample in a cool box and return to the office the same day.

How to take a post-flush sample?

  • Flush approximately 1 litre of water from the outlet.
  • Remove spray any inserts and flow directors from the outlet. It may also be necessary to remove and strainers and non return valves from TMV’s, strainers and solenoid valves.
  • Whilst wearing protective googles & gloves, make up a 200ppm solution of chlorine.
  • Clean the outside of the tap using the disinfectant solution.
  • Inject disinfectant solution into the outlet until it begins to run out of the tap using a wash bottle.
  • Descale and disinfect inserts and flow directors
  • Allow a two-minute contact time for the disinfection to take place.
  • Flush the outlet for 1 minute to remove the disinfectant.
  • Fill the sample bottle without adjusting the flow of water leaving a 1 cm air gap at the top of the bottle.
  • Immediately replace the cap and invert the bottle several times.
  • Place the sample in a cool box and return to the office the same day.
  • Rinse the outlet and surrounding area to remove any remaining disinfectant.

What should I do if the legionella sample comes back positive?

Firstly, don’t panic get in touch with Dantek and we will be able to guide you through the next steps to ensure you deal with the results in the correct way and to ensure your system is back under control as quickly as possible.  What action to take depends largely on the results and if this is the first positive result for the system.

If the first positive Legionella result is less than 1000cfu/ml:

If the minority of sample results are positive but less than 1000cfu/ml then arrange for a re-sample as soon as possible and review the control measures and risk assessment.

If the majority of the samples are positive but again with less than 1000cfu/ml then there is the possibility the system could be colonised at a low level.  Immediate review of control measures and risk assessment are needed and disinfection of the system must be considered.

If the 1st positive Legionella result is greater than 1000cfu/ml take the following action:

  • Arrange for a re-sample to be taken as soon as possible
  • Shut down any processes on the system where the positive sample was taken that can generate and disseminate airborne water droplets
  • Disinfect the system
  • Keep them shut down until sampling procedures and any remedial cleaning or other work has been done
  • Retest a few days after the disinfection and at frequent intervals afterwards until a satisfactory level of control is regained.
  • Review the risk assessment and control measures.

Following positive Legionella results a review of the Legionella risk assessment, control regime and record keeping/log books should be reviewed by checking the following:

  • They are current, up to date and accurate
  • That any remedial actions/recommendations have been completed
  • That the recommended control regime is in place. Identify the failing areas if there are any.
  • That good records are being kept of the activities undertaken
  • A record of the audit including any changes is placed in the log book

The question of whether to take samples or not can seem daunting however the key point to remember is taking samples will not affect levels of contamination on its own and does not necessarily offer compliance with the ACoPL8.  Ensure you keep your legionella risk assessment and control scheme up to date and seek advice from your water hygiene contractor.  If you have any concerns about Legionella samples, do get in touch with us as we are best placed to advice you about the Legionella control on your site.

Further reading:

BS 7592:2008 Sampling for Legionella bacteria in water systems – code of practice

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