Types of test
Laboratory test or rapid test?
A laboratory test is the recognised method and takes about two weeks. A rapid test gives an answer in around half an hour but covers fewer types of legionella and is a screening tool only.
Last reviewed October 2026 against HSE guidance.
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Side by side
| Laboratory culture kit | Rapid (lateral flow) test | |
|---|---|---|
| How it works | Sample posted to a laboratory and grown on a culture plate | Water filtered and applied to a test strip on site |
| Time to result | About 10 to 18 days | About 25 to 35 minutes |
| What it detects | Depends on the laboratory. Several sellers state all legionella species. | Legionella pneumophila only: serogroup 1, or serogroups 1 to 15, depending on the test |
| Result | A count in cfu per litre with a certificate of analysis | Positive or negative at a set threshold, typically 100 cfu/l |
| Accreditation | Look for a UKAS-accredited laboratory | No laboratory involved |
| Typical price | About £40 to £85 per sample | About £70 to £80 per test |
What HSE's guidance refers to
HSE's technical guidance, HSG274 Part 2, says analysis of water samples for legionella should be performed in UKAS-accredited laboratories with the current ISO standard methods in their scope of accreditation, and that sampling should follow BS 7592. That describes laboratory culture.
The action levels in the guidance are also written in cfu per litre, which is what a laboratory reports. See understanding your results.
When a rapid test is useful
- as a quick check while you wait for laboratory results
- to screen several outlets and decide where to send laboratory samples from
- to check again soon after cleaning or disinfection
A negative rapid result does not show the water is free of legionella. One seller of these tests states that its test is a screening test and cannot be used to certify water as safe or unsafe.
Questions people ask
Are rapid legionella tests reliable?
They are designed as screening tests for Legionella pneumophila at a set threshold. They do not detect other legionella species and do not give a count, so they do not replace laboratory culture.
Which test should a landlord use?
If you have decided to test, a laboratory kit from a seller using a UKAS-accredited laboratory gives a result you can keep on file. Remember that HSE says testing is not usually required for domestic systems.
Why does a laboratory test take so long?
Legionella grows slowly. The culture plates are incubated for around ten days before the laboratory can report a count.
Sources
- HSE: HSG274 Part 2, The control of legionella bacteria in 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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