How to Test for Legionella in Water (4 Effective Methods)

If you have any reason to believe that your water supply might contain legionella, you should test for this dangerous bacteria as soon as possible.

In this guide, we’ve shared the most common, effective testing methods for legionella in water.

๐Ÿ“Œ Key Takeaways:

  • Legionella is an invisible contaminant, so the only way to detect it is by testing your water.
  • You can test for the presence of legionella bacteria with a lateral flow at-home test kit or a laboratory test.
  • Laboratories may use several different legionella testing methods, including culture tests, PCR tests, lateral flow tests, and DFA tests.

๐Ÿค” Why Test For Legionella In Water?

Legionella is a group of hardy bacteria that grow in warm or stagnant water. You should test for legionella in your water if you have any reason to believe that this bacteria is contaminating your water supply.

Why is testing for legionella so important? Because legionella growth in domestic water systems increases the likelihood of the bacteria being released into the air with water vapor. Breathing in legionella bacteria increases your risk of contracting legionnaires’ disease, a type of pneumonia that can be life-threatening.

The best way to protect your family from legionnaires’ disease is to conduct a legionella water test if you have any concerns about this bacteria in your drinking water supply.

Bacteria on a petri dish

๐Ÿ“† When Should You Test For Legionella?

There are three occasions on which we recommend testing for legionella in your water system:

If Someone Gets Sick

If someone in your home contracts legionnaires’ disease and you can’t trace the cause of the disease to any outside source, you should conduct a test of the water in your home.

Testing in this manner should help you to determine for certain whether or not your home’s water is responsible for the infection.

If Your Home Has Recently Been Unoccupied

If you’re returning home after several months, and your home has been unoccupied during this time, you should test for legionella in your home’s water system.

Over long periods of non-use, legionella may grow in the stagnant water in your plumbing and water pipes. A legionella water test can rule out this dangerous bacteria before you start using your water supply again.

If You Notice Biofilm Buildup

You should also test for legionella in your water system if you notice biofilm buildup or other microbial growths on your shower head, fixtures, and faucets. Biofilms are microscopic structures that may accumulate on surfaces, forming black, red, or pink staining, or black slime.

If biofilm is present on your fixtures, it’s a good indication of legionella growth, and conducting a legionella test can confirm this.

Biofilm buildup in sink

๐Ÿ“‹ 4 Effective Legionella Testing Methods

Now we know why you should test for legionella bacteria, what are the most effective methods of legionella testing?

We’ve listed the 4 testing methods below.

1) PCR Method

The polymerase chain reaction (or PCR) method is a quick method of legionella testing used by laboratories. Test results from this method are usually obtained within 72 hours.

PCR tests are faster and more convenient than other methods, but the presence of other contaminants (like heavy metals, chemicals, and debris from the environment) could mask legionella and produce false negative results.

2) Culture Method

The culture method uses a BCYE or BCYE-modified agar to detect legionella colonies. This method is slower than the PCR method, taking between 4 and 10 ten days to produce results.

It’s also possible for the culture method to produce false negative results because if legionella colonies are inhibited by other types of microbial flora, they may not be visible. This is especially likely if the sample is taken from saunas and hot tubs, cooling towers, and fountains.

Culture method for detection of legionella in tap water

3) Lateral Flow Method

One emerging method of at-home DIY testing for legionella is the lateral flow method.

Lateral Flow Immunochromatographic Assay (LFICA) technology provides almost immediate results (usually within 30-60 minutes) on-site, but it’s typically only capable of detecting one type of legionella: the Legionella pneumophila sg1 antigen, which causes around 90% of legionnaires’ disease cases.

4) DFA Method

The DFA method, or direct epifluorescent monoclonal antibody method, is a CDC-developed method that provides the most sensitive and specific results.

A DFA test doesn’t typically provide false negative results. However, not all laboratories have the expertise and equipment to use this method of testing.

๐Ÿงช How To Test Water For Legionella

Let’s look at the steps you should take if you want to test a domestic water system with a laboratory legionella test.

  1. Purchase the test. Choose a test from a trusted laboratory (we like this Tap Score legionella test). The test should include everything you need to collect water samples at home.
  2. Take a water sample. Take a sample of water from your faucet, collecting the first draw of water without waiting for the faucet to run for a time.
  3. Take other water samples. Your test instructions might ask you to take multiple water samples in different areas of your home, such as a sample from your shower or samples from various faucets. Make sure each sample is in its own labeled vial.
  4. Ship the samples. Package and return the samples to the laboratory. Make sure the samples are returned within two days of taking them.
  5. Wait for your results. This could take a few days to over a week, depending on the testing method used.

๐Ÿ†š At-Home Vs Laboratory Testing For Legionella

At the moment, most legionella testing methods are conducted in a laboratory. The reason for this is that these tests are complex and often expensive to carry out, and there are a number of processes that must be followed strictly in order for the results to be accurate. Conducting these tests at home may lead to inaccurate results.

If you do prefer to test for legionella bacteria yourself, there’s one rapid test that you can buy: a lateral flow test (the same sort that’s used for COVID testing and in pregnancy tests).

This test should give you accurate results, is much faster than laboratory testing, and may also be cheaper. However, there aren’t many lateral flow legionella tests available for residential water systems as of yet.

So, what’s the best option – the at-home test method or laboratory testing?

It depends on your personal preferences.

For instance, you might prefer the reassurance of a professional laboratory test, where your only responsibility is collecting samples and you know for certain that the test will be carried out by a responsible person and the results will be accurate. You might be happy to spend a bit more money and wait slightly longer for your test results to be delivered.

On the other hand, you might want to get your results back as quickly as possible, and you don’t feel that you can wait for regular testing in a laboratory. In this case, you might prefer an at-home method of testing, which gives results almost instantly.

We personally recommend laboratory testing for legionella. Since legionella bacteria is such a dangerous contaminant, it’s essential that your test results are as accurate as possible, so you can take appropriate action as soon as your results are returned to you.

Water test

๐Ÿ”Ž What’s The Best Method Of Legionella Testing?

There is actually no best method of legionella testing because all methods have their setbacks. Whether you choose the culture method, the PCR method, lateral flow testing, or the DFA method, false-negative results are a possible outcome.

For this reason, we believe that the best method of testing for legionella in water systems is to combine multiple methods of testing for the same water sample.

That way, the test results can be compared against one another and retests can be conducted if inaccuracies are suspected.

Plus, the quality of a specific test method depends on the competency of the person conducting the test. Some tests, like the DFA method, require expertise that some laboratories may not have.

๐Ÿ“– How To Read Your Legionella Test Results

If you buy a laboratory water test designed for residential use, the test results will be easy for the average (non-professional) person to understand.

The exact results you receive depend on the laboratory you choose. For instance, some tests are only designed to detect one type of legionella bacteria, while others may test for a number of common legionella bacteria.

Your results data should indicate whether or not legionella was detected (POSITIVE = legionella detected and NEGATIVE = legionella not detected), and if so, at what concentration.

The laboratory may share information specific to your results, including the risk factors associated with the concentration of legionella bacteria found in your water system, and recommended methods of controlling legionella levels.

Water testing data sheet report

๐Ÿ“‘ Final Word

The presence of legionella is always a cause for concern, so you should react quickly if a test for this contaminant comes back positive.

If legionella is an ongoing problem in your home, you might need to develop a water management program and pay for regular legionella testing to ensure that you’re never unknowingly exposed to this bacteria.

However, in this case, we strongly recommend shocking your water or using a water treatment method to kill or remove legionella from your water. You should also employ other methods of reducing the likelihood of legionella growth, to prevent this bacteria from contaminating your home’s water system in the first place.

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