How to Test for Legionella Bacteria in Water

Keeping a building’s water systems safe is one of those responsibilities that sits quietly in the background until something goes wrong. Most facilities managers know temperature checks, flushing routines and general upkeep are vital, but the area that often raises the most questions is testing for Legionella. The process is not complicated, although it does need a structured approach and a clear understanding of why each step matters.

Legionella bacteria can appear in water systems that are not properly controlled, especially where warm temperatures and low flow create the perfect setting for growth. Knowing how and when to test helps you stay compliant, protect building users and avoid the disruption that follows when results come back higher than expected.

Why Testing Matters and When You Should Do It

Testing plays a specific role in water safety. It does not replace routine monitoring, nor is it designed to be carried out continuously. Instead, it acts as a verification tool that supports your control measures and highlights issues that may not be visible from temperatures or usage patterns alone.

Understanding the risk profile

Every building has its own set of conditions. Some have long pipe runs that are difficult to heat evenly, others have water storage tanks where sediment and biofilms can form, and some simply have outlets that are rarely used. Any of these can increase risk. A well structured Legionella risk assessment should spell out when testing is appropriate. If you need help understanding the typical elements of a risk assessment, our page on Legionella Risk Assessment Services provides useful background:https://www.titanwater.co.uk/water-hygiene-services/legionella-risk-assessment-services

Trigger points for testing

Testing becomes especially important at moments when your normal controls might not tell the full story. Common examples include system commissioning, major refurbishment works, unexpected temperature failures, or when a cooling tower or hot water system has been offline for longer than planned. In high risk environments such as care homes, hospitals or leisure facilities, testing is often built into a scheduled programme for added assurance.

If you are developing or reviewing your monitoring plan, and need a bit of expert support, you may find it helpful to refer to our Legionella Testing Services page.

How Testing Works and What Methods Are Available

Legionella testing must follow recognised procedures so results are reliable and meaningful. In the UK, laboratories typically follow the guidance in BS 7592, which covers the practical steps for sampling. While you do not need to memorise every detail, a basic understanding makes it easier to select the right method and interpret the results properly.

The culture method

Culture testing is the traditional laboratory process most building operators rely on. Water samples are taken from selected outlets, preserved using the correct bottles and transported to a UKAS accredited lab. Once there, any Legionella bacteria present are grown on specialised media. The advantage is simple. It tells you clearly whether live organisms are present and in what concentration. The drawback is the wait. Results often take up to two weeks.

For compliance work, culture testing remains the main reference point and is widely accepted by regulators and auditors.

PCR testing

PCR analysis detects Legionella DNA. It is extremely sensitive and can provide results within hours, which is helpful when you need a rapid assessment. Because PCR can pick up the genetic material of bacteria that are no longer alive, it is often used alongside culture testing rather than as a replacement. For investigation work or early indication of problems, it is a valuable tool.

On site testing devices

There are handheld swab tests and antigen based kits that provide quick feedback. These are useful screening tools, although they cannot replace laboratory analysis. They help locate areas that need deeper investigation and support early decision making while you wait for full results.

Choosing the Right Sampling Points

A good sampling plan is essential. It is not enough to pick a random tap or tank. Legionella thrives in areas where water is warm, slow moving or stagnant, so your sampling points need to reflect this.

Typical locations

Most plans include outlet samples from the farthest points on hot and cold distribution systems. Storage tanks, calorifiers, showers, thermostatic mixing valves and any little used outlets are also common choices. When you select samples, look at the system as a whole and consider where risk is likely to arise. The goal is to build a representative picture, not to test every outlet.

Technique and documentation

Sampling must be carried out with sterile equipment and clear handling procedures. Small details matter, such as whether the water is taken before flushing or after, since each option answers a different question. Accurate records of temperatures, times, locations and sample types form part of the evidence trail that supports your water safety plan.

What Results Mean and How to Respond

When the report arrives, the numbers need to be seen in context. Most facilities managers look first for whether Legionella has been detected, but interpreting the level and planning the next steps are just as important.

Low or non detected results

If Legionella is not detected or appears at very low levels, your control measures are likely working. This is the moment to maintain confidence in your routines rather than relax them. Temperature checks, flushing, tank maintenance and record keeping all play a part in sustaining those conditions.

Rising levels or persistent positives

A low level detection does not always signal a major failure, but it should prompt a review. Look at temperature records, outlet usage and recent maintenance.

Sometimes a simple adjustment or targeted clean resolves the issue. Persistent results may suggest biofilm build up, design limitations or hidden stagnation points. Addressing these early prevents escalation.

If you want to understand more about the cleaning and disinfection process, our page on Tank Cleaning and Disinfection offers a good foundation for system behaviour.

High results that require immediate action

If levels are significantly elevated, urgent steps are needed. This may include temporary restrictions, deep cleaning, disinfecting parts of the system or commissioning a full investigation. Keeping your duty holder informed and ensuring records reflect the decisions you take are crucial.

Our Legionella Investigation Services page gives a deeper overview of what steps should be taken when Legionella is persistently being detected in your water systems: https://www.titanwater.co.uk/water-hygiene-services/legionella-investigation-services

Supporting Long Term Control

Testing for Legionella is most effective when it sits inside a well organised water safety plan. When sampling, analysis, monitoring and maintenance all support each other, your systems become far more predictable and easier to manage. A consistent routine protects both compliance and operational continuity.

If you want to explore related areas such as tank hygiene, further system control measures or broader water safety responsibilities, take a look at our wider Water Hygiene Services section for deeper guidance.