If a water sample comes back positive, or your risk assessment recommends it, you may need Legionella testing — but many building managers are unsure what it actually involves, when it's required, or how to read the results. This guide explains Legionella testing in plain terms, from the sampling process to the action levels that decide what happens next.

What is Legionella testing?

Legionella testing is the laboratory analysis of water samples to find out whether Legionella bacteria are present in a water system, and in what concentration. Samples are taken from representative points around the system — such as tanks, outlets and returns — and sent to an accredited laboratory, where they are cultured and the results reported as colony-forming units per litre (CFU/L).

It's a direct check on the biological safety of your water. Where a risk assessment tells you where risk could occur, testing tells you whether bacteria are actually there.

Legionella testing vs Legionella risk assessment

These two are often confused. A Legionella risk assessment is a survey of your system that identifies where conditions could allow Legionella to grow and sets out how to control that risk. Legionella testing (water sampling) is the physical measurement that confirms whether bacteria are present. A risk assessment does not always require routine sampling — but it will tell you if and when testing is needed for your particular system.

When do you need Legionella testing?

Testing isn't automatically required for every building, but it is recommended or expected in situations such as:

Where your risk assessment specifically calls for it.
Where water temperatures cannot reliably be kept outside the range Legionella thrives in (20–45°C).
In higher-risk systems — for example those with stored water, showers, spa pools, or cooling towers.
In healthcare and care settings, where more frequent monitoring applies.
After remedial work, or following a positive result, to confirm the system is back under control.
Where control measures cannot be assured, or a system has been out of use.

The HSE's guidance document HSG274, which supports the Approved Code of Practice L8, sets out when sampling for Legionella is appropriate.

How does Legionella testing work?

A competent technician takes water samples following the recognised standard for Legionella sampling (BS 7592). The process typically runs as follows:

Samples are collected from pre-agreed points identified in the risk assessment.
They are kept in suitable conditions and delivered to a UKAS-accredited laboratory.
The lab cultures the samples — results usually take around 7 to 10 days, as the bacteria need time to grow.
Results are reported in CFU/L, with an interpretation of whether action is required.

Using a UKAS-accredited laboratory matters: it means the results are produced to a recognised quality standard and will stand up to scrutiny in an audit or investigation.

Understanding your results

HSG274 sets out commonly used action levels for hot and cold water systems, which broadly work like this:

Not detected or up to 100 CFU/L — system considered under control; maintain your regime.
Over 100 and up to 1,000 CFU/L — resample and review your control measures; if levels persist, take action.
Over 1,000 CFU/L — immediate action required, including a review of controls and remedial measures such as disinfection.

Results should always be interpreted alongside your risk assessment and, where needed, with a water treatment specialist — the right response depends on the type of system and the wider picture, not the number alone.

How often should you test for Legionella?

There is no fixed interval that applies to every building. Your risk assessment sets the frequency, based on the type of system and how well control measures (such as temperature) can be maintained. Some systems need no routine sampling; others — particularly complex or higher-risk ones — are tested regularly. Reviewing this is part of ongoing water hygiene management.

Legionella testing in London

London Water Treatment provides Legionella testing and control services across London, including UKAS-accredited water sampling, risk assessments and remedial work. If your risk assessment has flagged the need for testing, or you've had a positive result and need to act, get in touch for advice and a quote.

Frequently asked questions

Is Legionella testing a legal requirement?

Testing itself is not automatically mandatory, but controlling Legionella risk is a legal duty under health-and-safety law and ACOP L8. Your risk assessment determines whether sampling is needed to demonstrate that your system is under control.

How long do Legionella test results take?

Because the laboratory has to culture the sample, results typically take around 7 to 10 days. Faster methods exist for indicative screening, but culture remains the standard for confirmed results.

What's the difference between water sampling and Legionella testing?

They describe the same activity from different angles: water sampling is the act of collecting the water, and Legionella testing is the laboratory analysis of that sample for Legionella bacteria.

Legionella Testing: What It Involves and When You Need It