How do you test for legionella?
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Key points
- Decide whether you need to test at all. Sampling is targeted, not routine: for most well-controlled systems, temperature monitoring is the primary control and laboratory sampling is used only when the risk assessment calls for it.
- Use a laboratory whose UKAS accreditation scope covers legionella in water, cultured to BS EN ISO 11731. Accreditation for other tests does not count.
- Plan the sample points before the kit arrives: representative outlets from the risk assessment, each one named, each bottle labelled.
- Get samples to the laboratory within 24 to 48 hours, kept cool, and expect a culture result in around ten days.
- A result only earns its keep when it is read against the action levels, acted on, and recorded in the log book.
You test for legionella by sending water samples from representative outlets to a UKAS-accredited laboratory, which cultures them for around ten days and reports a count in colony-forming units per litre. But the laboratory run is only the middle step. A test that answers a real question starts before the kit arrives, with the decision about whether you need to sample and which outlets to sample, and ends after the report comes back, with the action you take and the record you keep. This page walks through the whole process in order.
Step 1: Check whether you need to test
Sampling is not a routine requirement for most hot and cold water systems. HSG274 is clear that where temperature control is working, temperature monitoring is the primary control and sampling is used to answer specific questions: verifying a treatment regime where temperature control is not practical, checking a system after a positive result or a case, investigating an out-of-range reading, or meeting a specific regime such as a cooling tower or healthcare premises. Our guide on whether legionella testing is a legal requirement sets out the legal position, and legionella testing explains when sampling is and is not needed.
If your only reason for testing is a vague sense that you probably should, put the effort into your monitoring records first. An inspector asks for the risk assessment, the written scheme and the monitoring log long before they ask for a laboratory report.
Step 2: Choose an accredited laboratory
The analysis must be done by a laboratory with legionella in water on its UKAS schedule of accreditation, using the culture method BS EN ISO 11731. “UKAS-accredited” on its own proves nothing: accreditation is specific to named tests, and a laboratory can hold it for food or water chemistry without it covering legionella. The check takes five minutes on the UKAS register, and we have done it for the laboratories in our legionella testing laboratory directory, which lists confirmed scope, method, turnaround and published prices side by side.
Step 3: Plan the sampling points
A sample can only speak for the outlet it came from, so the sampling plan decides whether the exercise tells you anything useful. Take the outlets from your risk assessment and written scheme: typically the sentinel outlets (the nearest and furthest hot outlets from the calorifier), the calorifier itself via its drain valve, the cold water storage tank if you have one, and any outlet the assessment flagged as a concern, such as a little-used shower.
A worked example for a small office with one calorifier and a cold water tank might be four samples: the calorifier drain, the hot tap nearest the calorifier, the hot tap furthest from it, and the cold tap nearest the tank. That spread covers the hot plant, the hot distribution at both extremes, and the stored cold, for the price of four analyses rather than a scattergun dozen. Write the plan down before the kit arrives, with each point named exactly as it appears in your records.
Step 4: Take the samples
Each sample goes into the laboratory's sterile bottle, which contains sodium thiosulphate to neutralise any residual disinfectant, and is filled to around one litre. Cold outlets are normally sampled first-draw, before any flushing, and hot outlets after the tap has run up to temperature, because the two answer different questions. The full procedure, including the pre-flush and post-flush choice, labelling and the mistakes that invalidate a sample, is in our guide on how to take a water sample for legionella.
Step 5: Pack and send promptly
Legionella continues to live and die in the bottle after sampling, so the count the laboratory reports should reflect the water at the outlet, not what happened in transit. Samples should reach the laboratory within 24 to 48 hours, kept cool in a cool bag or the insulated packaging the laboratory supplies, never frozen and never left in a hot van or on a sunny windowsill. Most laboratories supply prepaid courier or postal return packaging with the kit, which removes the friction that causes delay.
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.
Step 6: Read the result against the action levels
The report gives a count in colony-forming units per litre for each sample, or a not-detected result. HSG274 Part 2 sets the action ladder: below 100 cfu/L the system is considered under control; between 100 and 1,000 cfu/L you review the control scheme, resample and investigate; above 1,000 cfu/L you take immediate remedial action. What each rung means in practice, and what to do at each one, is set out in legionella CFU levels explained and legionella sampling and action levels.
The laboratory run is the middle of the job, not the job. A useful legionella test starts with a sampling plan from the risk assessment and ends with an action and a record in the log book.
Step 7: Record the result and act on it
File the laboratory report in your log book alongside the monitoring records, note the outcome and any action taken, and keep records for at least five years. If the result triggers action under the ladder, record that too: what was found, what was done, and the resample that confirmed the fix. And before you book the next round of sampling, make sure the routine controls are running: hot water at 50°C or more at outlets, cold below 20°C, and little-used outlets flushed. The legionella temperature checker tests your readings against the HSG274 thresholds in a minute, and it is the check you will do far more often than any laboratory test.
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
How much does legionella testing cost?
Published prices in our testing laboratory directory start from around £45 per sample for a standard culture analysis, and most laboratories supply the sample bottles and prepaid return packaging either included or for a small additional charge. The per-sample price usually falls when you send several samples together. What you should not pay for is a test you do not need: whether sampling is required at all is a question your risk assessment answers first.
How long does a legionella test take?
The standard culture method, BS EN ISO 11731, grows and counts the bacteria, and that growth takes time: laboratories typically report in around ten days, plus the transport time to get the sample to the bench. Some laboratories offer a faster qPCR screen, which detects legionella DNA and can report within a day, but it cannot distinguish live from dead cells, so it is a triage tool rather than a like-for-like replacement for culture.
Can I test for legionella myself?
You can take the sample yourself using a kit supplied by a UKAS-accredited laboratory, and for a simple domestic system that is often entirely reasonable. The laboratory analysis itself is never a DIY step: the bottle goes back to the lab for culture. What matters is that the sampling follows a plan, each outlet is sampled and labelled correctly, and the result is recorded and acted on. Larger or higher-risk premises usually use a competent contractor to sample as part of the written scheme of control.
How often should you test for legionella?
There is no single legal testing frequency for most hot and cold water systems: sampling is done when the risk assessment calls for it, not on a universal calendar. Specific regimes do set frequencies. Cooling towers are sampled at least quarterly under HSG274 Part 1, healthcare premises sample under HTM 04-01, and spa pools are sampled at least monthly under HSG274 Part 3. Outside those regimes, most well-controlled systems rely on temperature monitoring and sample only when something indicates it.
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