How to take water samples for Legionella testing

Legionella risk and pat blog's

If you are a landlord or a business owner, it is your responsibility to keep your tenants and employees safe. Although installing fire alarms and exit signs is a good start, you also need to consider water safety. Providing people with access to safe water within a building you are responsible for is essential. 

Luckily, implementing water management programs has never been simpler. All you need is to schedule regular water tests with a professional water management company. In this article, we explain everything you need to know about water testing in order to prevent an outbreak of Legionella and maintain water hygiene. 

Why take water samples and test for Legionella?

Regularly testing your water source is important in order to ensure the water that you are using is safe and hygienic. There are numerous waterborne diseases that could be manifesting in your water systems, the most common of which is Legionella bacteria. 

Legionella, often referred to as L. pneumophila, is a waterborne pathogen that causes Legionnaires’ disease, which affects the immune system and can be fatal. The likelihood of Legionella developing in your water system depends on a series of different factors, including water temperature, water source, and the presence of stagnant water. However, it is an extremely opportunistic pathogen so it can multiply quickly. 

Most of these factors fluctuate, meaning that even if your water was taken to testing laboratories 6 months ago, there is a chance that Legionella could have developed since. It is therefore essential to implement a water management plan to regularly test your water for Legionella. This can prevent people from becoming ill and ensure that you are running a safe establishment.

What is Legionella

When should you sample for Legionella?

When and how often you should sample your water to test for Legionella depends on different factors such as the type of water system you use and the outcome of your last assessment. 

For example, if you are using an open-water system, such as cooling towers or spa pools, routine testing should occur every few months. However, if you are using a cold water system or a hot water system that is usually enclosed, you don’t have to test as frequently. In fact, by law, you don’t have to test at all, but we recommend carrying out tests at least once a year.

Sampling techniques for Legionella

There are three key ways to test for Legionella: a culture test, a PCR test, and a DFA test. The method you use will depend on how quickly you need the results. When testing your water samples, you will either do so in bulk or with a swab. We always recommend bulk testing potable water systems as it provides you with a more accurate account of water safety. Here is a breakdown of each detection method. 

Culture test

A culture test is performed by detecting Legionella colonies on BCYE or BCYE-modified agar. The culture method is not particularly popular due to the fact that you need to send off the sample, which you don’t get back for an additional 4-10 days. 

Therefore, if you have a serious reason to be concerned about the safety of your water, you could be putting more people at risk during the time you are waiting for results. Moreover, this method is likely to produce false negatives as the Legionella culture might not appear on the test if they are surrounded by other microbial flora. 

PCR test 

The PCR test is a step up from the culture method thanks to rapid detection. If you are in a rush for answers, you should consider performing a real-time PCR test on your water system. However, it is important to recognize that PCR tests also have a high likelihood of false negative results. This is especially true for environmental water samples where debris and heavy metals can mask the presence of Legionella. 


The DFA test is considered the gold standard method of water testing. It is an extremely efficient and sensitive test, making the results close to perfect. The only issue with this test is that it needs to be carried out by a licensed professional.

Safety precautions when sampling water systems

It is important to remember that all man-made water systems are different, which means they all need to be approached and handled differently. The number one way to stay safe whilst sampling water is to take the time to read the current Legionella risk assessment for the water system you will be testing. 

By thoroughly reading previous risk assessments, you will be able to better gauge the environmental samples and water systems you are dealing with. Within the previous assessment, there should be instructions on how the water is currently being treated and what the last tester did when sampling the water. 

Taking all of this information into account will help you safely approach the sampling of water within building water systems. However, if you don’t feel confident with environmental sampling, you should seek the services of a professional company such as Legionella Risk and PAT

What equipment is needed when sampling for Legionella?

The equipment list for testing for Legionella is fairly extensive, so it is important you prepare everything beforehand: 

  1. One liter sample bottles
  2. Something to disinfect the area 
  3. Labels to keep detailed records
  4. Thermometer with surface probes
  5. Disposable gloves
  6. Torch
  7. Packing materials for the sample bottles
  8. Clean plastic bags (e.g., ziplock bags)
  9. Scissors
  10. Elastic bands
  11. Personal protective equipment
  12. Sterile swabs (if taking swab samples)

Book a Legionella risk assessment

Should you take pre- or post-flush water samples?

You can take both pre-flush or post-flush water samples, depending on how you want to test.  Pre-flush samples utilize water directly from the outlet, whereas post-flush samples will let the water initially run through the water pipes. Although you can use both kinds of samples, you should look up the specific sample requirements of the test you are performing. If your sampling procedures are incorrect, your sample could get contaminated and you could end up with false positives or false negatives. 

Dip sampling for Legionella

Dip sampling involves taking a substantial water sample and collecting it in a sterile container. A dip sample is recommended where possible as you obtain more of the sample, hence increasing the chances of being able to accurately detect elevated levels of Legionella. 

Swab sampling for Legionella

Swab sampling is a more localized form of sampling. It involves taking a swab on the inside of a water source. If you choose to take a swab sample, you need to ensure that the entire swab is submerged into the water and fully coated. 

What do the results mean?

When you get your results back, you will see that either L. Pneumophila has been detected or that it has not been detected. If L. pneumophila hasn’t been detected, it will show up on the results as <100 cfu/l, which means that the possible levels of Legionella in the environmental samples you submitted are too low to even show up. This could mean that there is nothing there, that there is the presence of dead bacteria, or that the bacteria present is so low it’s nothing to be worried about.

If L. pneumophila has been detected, your results will show one of two things. The first is >100 cfu/l – <1,000, which means that you have a more than 100 but less than 1000 limit of detection, indicating low levels of Legionella in your water system. When this happens, you should begin a further investigation to double-check how harmful your water system is. 

The final result that might show up on your results is >1,000 cfu/l. This is a dangerous result that requires immediate corrective action. The action levels for this intense result should be fast as the bacteria is extremely dangerous and numerous. 

Legionella risk assessment services

All water systems have the potential to develop Legionella, which is why it is essential to keep on top of your water management. Whether you are worried about the water in your family home or are a business owner or landlord responsible for your building’s water safety, Legionella Risk and PAT is here to ensure your water is safe to use.

Our Legionella risk assessment involves:

  • Checking all water sources, such as water outlets, water heaters and pipework in your property
  • Assessing who is at risk from these dangers
  • Performing water temperature checks
  • Identifying if there are any Legionella bacteria in the water system
  • Putting in place the necessary safety measures to control and minimize the threat of Legionella bacteria spreading 
  • Documenting the evidence gathered, while doing the assessment and presenting you with your Legionella risk assessment report on the same day

Book a Legionella risk assessment now

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