UKAS accreditation tells you a laboratory has been independently checked to run a specific test, by a specific method, to a recognised standard. It does not, on its own, tell you the lab is accredited for the Legionella analysis you actually need, that your sample was taken properly, or that a clean number clears your system. The whole skill in buying lab work is knowing the difference.

If you are commissioning analysis — routine verification, a clearance sample after a disinfection, or an investigation behind a high count — the lab is both the easy part to get right and the easy part to get wrong. Here is how to choose one you can stand behind.

What you are actually paying for

You are not buying a number. You are buying a number you can defend — in front of an enforcing officer, an insurer, or your own board — when it matters. Accreditation is what makes that number worth defending.

UKAS is the UK’s national accreditation body. It assesses laboratories against ISO/IEC 17025, the international standard for testing competence. The detail that trips people up is that accreditation is granted test by test, not as a blanket stamp. Every accredited lab holds a published schedule of accreditation listing exactly which tests, sample types and methods are covered. A laboratory can be entirely genuine in saying “we are UKAS accredited” while being accredited for drinking-water chemistry, or for clinical samples, and not for the detection and enumeration of Legionella in water. The badge on the letterhead is not the same as the scope on the schedule.

HSE’s guidance on testing and monitoring points duty holders towards analysis by a UKAS-accredited laboratory, and is equally clear that how often you sample, and what for, follows the system and the risk assessment rather than a fixed calendar [1]. So the lab sits at the end of a chain you also have to get right: the right outlets, sampled the right way, at the right time, in the right bottle.

Five things to settle before you send a sample

Treat this as your selection framework. For each point there is a thing to confirm, a question to put to the lab, and the answer that should make you walk away.

Confirm the scope, not just the badge

Ask to see the schedule of accreditation and check that Legionella in water is on it by name. This is the single most useful five minutes you will spend.

Ask them: “Can you send the UKAS schedule showing detection and enumeration of Legionella in water samples?” Walk away if: you get “we’re UKAS accredited” with no document, or a schedule that covers everything except the test you came for.

Vet the whole sample journey, not just the bench

A result is only as good as the sample that produced it. BS 7592 is the UK code of practice for taking Legionella samples — it covers where you sample, the bottles, the neutraliser that stops residual disinfectant skewing the count, transport temperature and how quickly the sample must reach the lab [3]. A good lab supplies the correct bottles and tells you its limits up front.

Ask them: “What bottles and neutraliser do you supply, and what is your maximum time and temperature for samples in transit?” Walk away if: there is no neutraliser, no transit-time limit, and a cheerful willingness to test a bottle that has been sitting in a van for three days.

Pin down the method and what it measures

The two main routes are culture and PCR, and they answer different questions (more on that below). What matters here is that whichever method you use is actually within the lab’s accredited scope.

Ask them: “Which method will you use, and is that specific method on your UKAS schedule?” Walk away if: the headline test is accredited but the method they will actually run on your sample is not.

Check turnaround and how results are reported

Culture takes days of incubation, so a culture result is never same-day — build that lag into any clearance or restart plan. Results should arrive as colony-forming units per litre, mapped to the action bands in HSG274 with a clear note when a count needs a response [2]. The report is a working document, not a certificate to file.

Ask them: “What is the typical turnaround, and how will you flag a result that needs action?” Walk away if: anyone promises a same-day Legionella culture, which the biology does not allow.

Look at the people around the test

The sampler is the part buyers skip, and it is where most defensible-looking results quietly fall apart. If a service provider takes your samples and runs your programme, their competence is part of the purchase. The Legionella Control Association publishes a code of conduct for service providers that is a reasonable baseline to ask about [5].

Ask them: “Who takes the sample, are they working to BS 7592, and who is accountable if a result is queried?” Walk away if: no one can name who owns the sampling or which standard they follow.

Culture, PCR, and what the result actually means

Culture is the long-established route. It grows viable Legionella from your sample and counts the colonies, reported as cfu per litre. It needs days in the incubator, so it is slow, but it tells you about live organisms. PCR detects Legionella DNA and is much faster — useful for screening and for speed — but on its own it cannot tell live bacteria from dead. A positive PCR straight after a disinfection can reflect organisms you have already killed. The CDC notes that laboratory methods differ in what they actually detect, which is exactly why the method behind a number matters as much as the number itself [4].

For most routine UK verification, culture is the method people expect to see. PCR earns its place where speed counts, but confirm it is within the lab’s accredited scope and agree in advance how you will act on each kind of result.

When a new lab is not the answer

A laboratory cannot fix control. If your hot water is drifting cool, outlets are sitting stagnant, or a disinfection was not done properly, a drawer full of clean certificates buys you nothing — and a single bad count will still find you out. Sampling verifies control; it does not create it. If you are tempted to bolt a quarterly sample onto a system you cannot keep hot or keep moving, the money is better spent on the control first. A clearance sample after a repair or remedial works is worth doing well; let your site-specific monitoring plan decide the routine sampling, not reflex.

One honest limit

Accreditation proves the laboratory can perform the test reliably. It says nothing about whether your sample represented the system, or whether a clean number clears it. A negative from a poorly chosen outlet, or a bottle that sat warm overnight, is a confident-looking result that means very little. Lab selection is one link in a chain that runs through your risk assessment, a competent sampler and your written scheme — and those, not an article, decide what to test, how often, and how to read the figure that comes back.

FAQ

The duty is to control Legionella and to verify that control where your risk assessment calls for it. HSE guidance directs that analysis to UKAS-accredited laboratories so the result is reliable and defensible [1]. An unaccredited result is hard to stand behind if it is ever challenged, so accredited analysis is the sensible default even where you might argue the strict letter of the rules.

A lab can be UKAS accredited but still wrong for me — how?

Because accreditation is test- and method-specific. A lab might be accredited for water chemistry, or for clinical specimens, and not for the detection and enumeration of Legionella in water. Read the schedule of accreditation and check the exact test is listed, rather than trusting the marketing line.

Does it matter who takes the sample, or only who tests it?

Both, and the sampler is the part most people overlook. A perfectly accredited lab cannot rescue a sample drawn from the wrong point, in the wrong bottle, or left too long in transit. Sampling to BS 7592 by a competent person is what makes the lab’s number mean anything [3].

Sources

[1] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm [2] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [3] BSI, “BS 7592:2022 - Sampling for Legionella bacteria in water systems. Code of practice”. https://knowledge.bsigroup.com/products/bs-7592-sampling-for-i-legionella-i-bacteria-in-water-systems-code-of-practice-1 [4] CDC, “Laboratory Testing for Legionella”. https://www.cdc.gov/legionella/php/laboratories/index.html [5] Legionella Control Association, “Code of Conduct for Service Providers”. https://www.legionellacontrol.org.uk/