What the number means
Laboratories grow any legionella in the sample and count the colonies. The result is reported as colony forming units per litre of water (cfu/l). Results are often reported as 'not detected' below the laboratory's limit of detection.
HSG274 Part 2 action levels
| Result (cfu/l) | What HSG274 suggests |
|---|---|
| Not detected or below 100 | Under control. Continue the existing scheme. |
| Over 100 up to 1,000 | If only one or two samples are positive, resample and review control measures. If most samples are positive, the system may be colonised, so review controls and consider disinfection. |
| Over 1,000 | Resample promptly, review control measures and the risk assessment, and carry out remedial work, which may include disinfection. |
Always read the guidance for your own system type. Cooling towers (HSG274 Part 1) and healthcare settings (HTM 04-01) use their own approaches.
Pre-flush and post-flush samples
A pre-flush sample, taken as soon as the outlet is opened, shows what is happening at the tap or shower itself. A post-flush sample, taken after running the water, reflects the wider system. Comparing the two helps locate the problem.
After a positive result
- Check temperatures at the outlets and plant involved
- Look for dead legs, low use and scale
- Clean or replace the outlet fitting where the problem is local
- Disinfect where the system appears colonised
- Resample to confirm control, and record everything
We can sample and interpret results and put a response plan in place.
Frequently asked questions
Is any legionella in a sample a problem?
A low count does not always mean a system is out of control, but it is a warning sign. HSG274 sets out when to resample, review controls and consider disinfection.
How long do legionella results take?
Culture results usually take around 10 days from the laboratory. Rapid screening methods are available where speed matters.
Disclaimer: This guide is general information about UK requirements and is not legal advice. Always check the current legislation and guidance for your premises.