Legionella test kits: what can they actually tell you?
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Key points
- Only one kind of kit gives a real legionella count: the postal kit backed by a UKAS-accredited laboratory culturing to BS EN ISO 11731.
- Dip slide kits cannot detect legionella. They measure general bacteria. Rapid on-site tests are screens with detection limits, not measurements.
- A kit result is one outlet on one day. No kit certifies a system as safe or compliant.
- Judge a postal kit by the laboratory behind it: UKAS scope for legionella, the culture method, the bottles, the return packaging, the report.
- Technique beats kit. A perfect kit sampled badly tells you less than an ordinary one sampled to a plan.
Search for a legionella test kit and you will find three quite different products wearing the same label, priced from a few pounds to tens of pounds, all implying they answer the same question. They do not. This review does not rank brands; it does something more useful, which is to separate the three kinds of kit, state plainly what each can and cannot tell you, and give you the checklist that separates a worthwhile kit from a waste of money.
The three kinds of kit
1. The laboratory-backed postal kit. A box of sterile bottles containing sodium thiosulphate, with instructions, labels and prepaid return packaging. You take the samples, post them back, and a UKAS-accredited laboratory cultures them to BS EN ISO 11731, reporting species and a count in cfu/L after around ten days. This is not a DIY approximation of laboratory testing; it is laboratory testing, with you doing the sampling. It is the only kit category that produces a legionella count you can read against the action levels.
2. The dip slide kit. Paddles coated with nutrient jelly that you dip, incubate and read against a chart. These measure the total viable count of general aerobic bacteria. Legionella does not grow on them, so a dip slide cannot detect legionella, full stop. They have a legitimate role as a trend gauge on treated systems such as cooling towers, but sold as a legionella test they are the wrong tool. The full comparison is in dip slides versus laboratory testing.
3. The rapid on-site test. A strip or cartridge, similar in spirit to a lateral-flow test, that detects legionella antigen or DNA within minutes to hours. Rapid tests are genuine screens: a positive result is worth acting on. But they have detection limits, they typically read one sample at one moment, and a negative result means only that legionella was not detected above the threshold. They are triage tools for situations that need a fast signal, not a substitute for culture when you need a count.
What no kit can tell you
Every kit shares the same structural limits. A result describes one outlet, on one day, from one sample. It says nothing about the outlet you did not test, nothing about next month, and nothing about whether your controls are being carried out. And no kit, however good, touches the legal duty: that is met by assessment, control, monitoring and records, not by any purchase. A kit result records; it never certifies. This is why our guide to whether testing is a legal requirement matters more than any product page.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
The checklist: how to judge a kit before you buy
- Which laboratory analyses it? A postal kit should name its laboratory. No named laboratory, no sale.
- Is legionella on its UKAS schedule? Check the schedule on the UKAS register, or use our testing laboratory directory, where we have done the check.
- Which method? Culture to BS EN ISO 11731 for a count; qPCR is a faster screen with different meaning.
- What's in the box? Sterile bottles with sodium thiosulphate, labels, instructions that cover pre-flush and post-flush choices, and return packaging. Our guide on taking a sample shows why each matters.
- What does the report look like? You want a count in cfu/L with species identification, not a bare pass or fail.
- What is the real price? Watch for kits sold cheap with the laboratory fee added at return. Published per-sample prices start from around £45 in the directory.
Where kits genuinely fit
Used honestly, kits earn their place. A landlord with a stored-water system who wants verification beyond temperature checks; a duty holder confirming that disinfection worked after a positive result; a small premises whose written scheme includes periodic sampling: all are served well by a laboratory-backed postal kit. The same kit fails when it is bought as a substitute for the monitoring routine, the monthly temperatures and weekly flushes that actually control the risk day to day. Check those first with the legionella temperature checker, and see our legionella testing kit page and legionella testing for the wider picture.
Buy the laboratory, not the box. The only kit that tells you about legionella is the one whose sample ends up on a culture plate in an accredited laboratory; everything else is a gauge or a screen wearing the same label.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Frequently asked questions
Are legionella test kits accurate?
A postal kit backed by a UKAS-accredited laboratory is as accurate as laboratory culture itself, because that is what it is: you take the sample, the laboratory cultures it to BS EN ISO 11731. The weak point is technique at the tap, not the kit. Dip slide kits are accurate at measuring general bacteria, which is not legionella. Rapid on-site tests are screening tools with detection limits, so a negative rapid result does not establish that legionella is absent.
Do DIY legionella test kits work?
It depends entirely on which kind you mean. A kit that sends your sample to an accredited laboratory works, in the sense that you get a genuine legionella count, provided you sample correctly. A dip slide “works” only as a general cleanliness gauge and cannot detect legionella at all. A rapid antigen-style test can flag legionella above its detection threshold within minutes or hours, but it is a screen, not a measurement, and a clear screen is not evidence of a clear system.
How much do legionella test kits cost?
Dip slides cost a few pounds each and rapid on-site tests sit in the tens of pounds. Laboratory-backed postal kits are priced per sample analysed: published prices in our testing laboratory directory start from around £45 per sample, usually including the bottles and return packaging. The price differences reflect what you are buying: a general bacteria gauge, a quick screen, or an accredited legionella count.
Does using a test kit meet my legal obligations?
No kit, of any kind, meets a legal obligation by itself. The duty under HSWA and ACOP L8 is to assess the risk, control it, monitor the controls and keep records. A kit result is at most one record inside that system. A duty holder with a cupboard of negative test results and no risk assessment or monitoring log has not met the duty, and a duty holder who has never bought a kit but has sound assessment, controls and records usually has.
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