How to Test for Legionella in Water

Legionella bacilli are a group of gram-negative bacteria that can cause several diseases, the most severe and well-known being Legionnarires’ Disease.

According to an EPA fact sheet, Legionella are relatively resistant to standard water disinfection, and potable water is the “most important source” of these bacteria.

If you have any reason to suspect your water might contain Legionella, you’ll learn the best Legionella water test methods in this guide.

🧪 Use An At-Home Lateral Flow Test

If you prefer to use an at-home DIY testing method for Legionella, our recommendation is the lateral flow method.

This type of testing for Legionella typically uses antibodies that bind to Legionella antigens, producing visible results within a short time frame.

Good to Know: 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.

A DIY-friendly lateral flow test should include a sample collection vial/syringe and a test strip. Here are the instructions for using a lateral flow Legionella test at home:

1. Collect a water sample in the included vial or syringe.
2. TEST SCENARIO 1: Mix the sample with the buffer solution provided, then apply a few drops of the mixture to the sample well on the test strip.
3. TEST SCENARIO 2: Send the water through the included hollow fiber filter, then use a buffer solution to extract bacteria from the filter.
4. Wait for the solution to move along the strip – it’ll encounter antibodies that bind to Legionella antigens if any are present in the water sample.
5. Analyze your results. A visible line will appear on the test strip if Legionella antigens are present, which indicates a positive result for Legionella contamination.

Lateral flow tests are designed to be quick and simple to use, providing quick on-site results. However, they’re not as sensitive as the more complex laboratory methods.

For that reason, we recommend lateral flow Legionella testing for preliminary screening, but don’t suggest it as a substitute for professional testing.

If Legionella bacteria are detected in a lateral flow test, our advice is to then follow up with a laboratory test for more precise data.

🚰 Collect a Water Sample for Lab Testing

The more popular Legionella testing option is to use a professional laboratory test.

Dozens of certified laboratories in the US offer Legionella testing services. There are a few different types of tests available, which we’ve discussed in more detail later.

The advantage of laboratory testing for Legionella in water is that it can be much more precise and comprehensive, which means the results themselves are more detailed and accurate.

The lab test method also takes the pressure off you, because all you need to do is collect your water sample.

Here’s the general process for collecting water samples for Legionella testing:

1. Choose and buy your preferred laboratory water test, then wait for the sample-taking kit to be mailed to your home.
2. Select the tap, showerhead, or other fixture, to collect the water sample. Wash your hands or put on a pair of gloves.
3. If the testing laboratory has provided a sterile container, keep it sealed until you come to use it.
4. Run the faucet for a few minutes.
5. Open the container and hold it below the tap until it fills. Make sure you don’t contaminate the container with any surfaces or your hands.
6. Cap the container immediately to prevent contamination.
7. Return the container to the shipping box and send it back to the testing laboratory as soon as possible after you’ve taken the collection. Follow any specific transportation and storage instructions provided by the laboratory.

We choose Tap Score, provided by an EPA-certified testing lab, whenever we need to test our water. Tap Score offers three tiers of Legionella testing, with the highest tier testing for 28 analytes.

Looking for more guidance on how to accurately take samples of water systems for laboratory testing? Legionella Control has some helpful advice in this article.

Methods of Laboratory Testing for Legionella

Different labs use different methods of testing for Legionella, depending on the required test results and the lab’s equipment and preferred processes.

When you ship your water sample off to a lab, it should get tested with one of the following methods:

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.

To use the PCR method to detect Legionella, scientists extract DNA from the water sample, then subject the extracted DNA to PCR, where specific genetic markers of Legionella are replicated millions of times. Scientists can then analyze the resulting DNA using techniques like gel electrophoresis or fluorescence.

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. First, scientists filter the water sample, then spread the filter onto a medium that supports Legionella colony growth. After an incubation period, scientists use various techniques to identify and catalog the colonies.

This method is slower than the PCR method, taking 4-10 days to produce results. It’s generally considered the gold standard for Legionella detection in hot and cold water systems because of its high sensitivity and specificity.

However, the culture method could 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.

3) DFA Method

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

This method of testing involves using specific antibodies tagged with fluorescent dyes to detect the presence of Legionella. Scientists filter the water sample to concentrate any bacteria present, then treat the filter with a solution containing antibodies that bind to Legionella antigens. If Legionella is present, the antibodies attach to the bacteria and form a complex that fluoresces under a microscope.

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.

📝 Interpreting Your Results

Once you have your results, you can determine whether or not you need to take any action.

The results you receive depend on the laboratory you choose. For instance, some tests are only designed to detect one type of Legionella, while others may test for several 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.

If Legionella is detected in your water, you will need to respond with a four-step process:

1. Identify the root cause of contamination – This could be stagnant water in an infrequently used fixture, or dead legs or dead ends in your plumbing system, or storage issues in your building water systems. The CDC’s guide, Monitoring Your Building Water, is a helpful read for building owners who want to establish a water management program for Legionella.
2. Shock your water system – Use disinfectant chemicals to flush out your entire building water system to kill the bacteria.
3. Install a water treatment system – Reverse osmosis, ultrafiltration, and UV purification are all good ways to remove or kill Legionella in your water. A point-of-entry UV system will kill legionella in water that enters your home, although it won’t address Legionella that accumulate in fixtures or appliances at the point of use.
4. Take other precautions – This could include keeping up with regular maintenance for air conditioning units and humidifiers, flushing your hot water pipes to prevent water from stagnating, draining and cleaning drinking water fountains, and more.

Our guide on how to treat legionella in water discusses the steps you should take in more detail.

📑 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 routine environmental sampling and legionella testing to ensure that you’re never unknowingly exposed to this bacteria.

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.