For hot and cold water systems, HSG274 Part 2 sets three response bands: under 100 CFU/litre is consistent with a system under control, 100 to 1,000 means review the control scheme and resample, and over 1,000 means immediate remedial action. Cooling towers, healthcare premises and spa pools work to their own regimes, which this tool applies for you. The bands are explained in full in sampling and action levels and what CFU levels mean.
Your result, in context
What to do next, step by step
Record this result. A sample result only counts if it is written down: date, outlet or asset sampled, the count, the lab report reference, and every action you took. Log it now in your legionella log book and keep the lab report with it — records need to be kept for at least five years.
Email me this interpretation + the free template
The risk assessment template your monitoring records and sample results slot into, following the five-step approach in ACOP L8 — one email you can file as part of your written record. Print or save this page to keep the interpretation itself.
This is general guidance based on the action bands in HSG274, HTM 04-01 and HSG282. It is not a laboratory interpretation service and it does not confirm that your system is safe or compliant. A single result is a snapshot of one outlet on one day — the detail of the report, including the species and serogroup found, belongs in a conversation with your laboratory or competent person.
The action levels this tool uses
The bands below are the ones set out across our guidance on sampling and action levels. “CFU/litre” (colony-forming units per litre, also written cfu/L) is the number of live legionella cells in the sample that grew into countable colonies.
| System | Under control | Review and resample | Immediate action |
|---|---|---|---|
| Hot and cold water (outlets, calorifier, tank) — HSG274 Part 2 | Under 100 CFU/litre | 100 to 1,000 CFU/litre | Over 1,000 CFU/litre |
| Cooling tower or evaporative condenser — HSG274 Part 1 | Up to 100 CFU/litre | Over 100 and up to 1,000 CFU/litre | Over 1,000 CFU/litre |
| Spa pool — HSG282 | Not detected | — | Any detection |
| Healthcare premises — HTM 04-01 | Works to lower thresholds than HSG274 Part 2; take any detection to your water safety group, whatever the count. | ||
Three things the bands do not tell you. First, a result describes one sample from one point on one day — legionella lives in biofilm and its distribution is patchy, so a low count at one outlet is not an all-clear for the system. Second, “not detected” and a count under 100 are different results: one found nothing at the method's detection limit, the other found a little. Third, the response to a count is always to fix the system and prove the fix with another sample, not to watch the number.
Important
This tool is general guidance, not advice on your specific premises and not a laboratory interpretation service. It never states that your system is safe or compliant. For any result in the review or immediate-action bands, and always in healthcare premises, care homes, cooling towers and spa pools, involve a competent person, usually one working for a Legionella Control Association registered company. This site does not provide medical advice; if you are worried about someone's health, contact a doctor.