Results
Understanding legionella test results
Laboratories report legionella as colony forming units per litre (cfu/l). HSE's guidance for hot and cold water systems sets two action levels: above 100 cfu/l and above 1,000 cfu/l.
Last reviewed October 2026 against HSE guidance.
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HSE's action levels for hot and cold water
| Result | What HSE's guidance says to do |
|---|---|
| Not detected, or up to 100 cfu/l | No action level is set for general systems. In healthcare premises with susceptible patients, the guidance says any detection should be investigated. |
| More than 100 and up to 1,000 cfu/l | If only a minority of samples are positive, resample. If results are similar again, review the control measures and risk assessment. If the majority of samples are positive, the system may be colonised at a low level: review immediately and consider disinfection. |
| More than 1,000 cfu/l | Resample and carry out an immediate review of the control measures and risk assessment, including possible disinfection. Retest a few days after disinfection and at frequent intervals until control is achieved. |
Summarised from HSG274 Part 2, Table 2.2. Read the guidance itself before acting on a result.
What to do with a positive result
- Reduce exposure. Avoid using showers and other outlets that create spray on the affected part of the system until you have advice.
- Check temperatures. Stored hot water should be at 60°C or above, hot outlets at least 50°C within a minute, and cold below 20°C.
- Review the risk assessment. Look for stagnant water, dead legs, dirty tanks and scaled showerheads.
- Take competent advice. A water hygiene specialist can resample properly and carry out cleaning and disinfection if needed.
- Retest. Confirm the system is back under control.
If anyone using the building has symptoms of pneumonia, they should seek medical advice and mention the result.
What a negative result does and does not tell you
“Not detected” means legionella was not found in that bottle, from that outlet, on that day. It does not certify the whole system, and it does not last. Legionella can grow again within weeks if water sits warm and still.
That is why HSE treats temperature control and regular use as the main safeguards and describes temperature monitoring as a reliable method for confirming the system is under control.
Questions people ask
What is a safe level of legionella in water?
HSE's guidance does not define a safe level. For hot and cold water systems it sets action levels at more than 100 cfu/l and more than 1,000 cfu/l, with stricter expectations in healthcare premises.
What does cfu/l mean?
Colony forming units per litre. It is the number of legionella colonies grown from the sample, scaled to one litre of water.
What should I do if my legionella test is positive?
Reduce exposure to spray, check the water temperatures, review your risk assessment and take advice from a competent water hygiene specialist. Above 1,000 cfu/l the guidance calls for an immediate review and possible disinfection.
Sources
- HSE: HSG274 Part 2, The control of legionella bacteria in hot and cold water systems
- HSE: Hot and cold water systems
- HSE: Legionella and landlords' responsibilities
Checked against these sources in October 2026. This page is general information, not legal advice.
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