What is BS 7592 and why does sampling method matter?
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Key points
- BS 7592 is the British Standard for legionella sampling: how to plan sampling, choose points, take, preserve, transport, and document samples.
- It exists because the laboratory can only analyse what reaches the bottle. Poor technique turns a sampling exercise into expensive noise.
- It is voluntary, not a legal requirement, but sampling to the standard is the recognised way to produce defensible results.
- Sample points should be chosen from the risk assessment, targeting the outlets and components most likely to harbour legionella, not the most convenient tap.
- A clear result is a snapshot, not a certificate. Sampling supports the control regime; it does not replace it.
BS 7592 is the British Standard that governs how water samples are taken for legionella analysis: how to plan the sampling, which points to sample, how to draw the water, how to preserve and transport the sample, and how to document everything from tap to laboratory. Most recently revised in 2022, it exists for one reason: a laboratory can only analyse the water that arrives in the bottle, and the way that bottle is filled decides whether the result means anything at all.
This matters more than it first appears. Legionella culture is a slow, sensitive laboratory method, and the numbers it produces, the CFU per litre figures behind the action levels, are only as good as the sampling behind them. A carefully analysed sample of the wrong water, taken the wrong way, gives you a precise answer to the wrong question.
What the standard actually covers
BS 7592 is a method standard, not a duty. In broad terms it deals with four things:
- Planning. Deciding why you are sampling and what question each sample is meant to answer, whether that is routine surveillance, checking a disinfection, or investigating a problem. The plan should trace back to the risk assessment, because the assessment is what identifies the vulnerable parts of the system.
- Sample points. Choosing outlets and components that represent the risk: distant outlets at the ends of long pipe runs, little-used outlets on a flushing regime, calorifier bases, storage tanks, and points feeding vulnerable occupants. Convenience sampling of the kitchen tap tells you about the kitchen tap.
- Technique. How the sample is drawn so it reflects the water in the system, including whether you want the water as it first emerges or after running, and how to avoid contaminating the bottle. Bottles contain a neutraliser so that any residual disinfectant does not keep killing bacteria between the tap and the laboratory, which would falsely flatter the result.
- Handling and records. Labelling, keeping samples cool, getting them to the laboratory promptly, and keeping the paperwork that links each bottle to a named point and time. Without that chain, a result cannot be traced back to the part of the system it describes.
BS 7592 versus the documents around it
| Document | What it governs | Status |
|---|---|---|
| ACOP L8 | The duty: assess, control, record | Approved Code of Practice, special legal status |
| HSG274 | The technical control regime, including when sampling is appropriate | HSE guidance |
| BS 8580-1 | How to carry out the legionella risk assessment | British Standard, voluntary |
| BS 7592 | How to plan and take the water samples | British Standard, voluntary |
| Laboratory culture method | How the laboratory grows and counts the organism | International standard method, used by accredited labs |
The division of labour is clean. L8 and HSG274 tell you whether and when to sample, the risk assessment tells you where, BS 7592 tells you how to take the sample, and the laboratory method tells the lab how to analyse it. A result you can act on needs all four links in the chain to hold.
Why sampling quality decides whether the result means anything
Consider two samples from the same outlet. One is taken after flushing the tap for several minutes: it mostly reflects the fresh incoming water, and it will often come back clear even if the outlet's local pipework is colonised. The other is taken as the water first emerges, capturing what has been sitting in that branch. They can produce very different numbers, and only one of them answers the question you actually care about, which is whether the water people draw from that outlet carries a risk. Neither is wrong in every context, but you have to know which question you asked, and the sampling record has to say which was done. That is the level of discipline BS 7592 is for.
A legionella result is only as good as the sampling behind it. The laboratory can count what is in the bottle; it cannot know whether the bottle was filled from the right place, in the right way.
What to ask a sampling provider
If you commission sampling, a few questions separate a competent job from a box-ticking one:
- Will the sampling plan be based on my risk assessment, and will each sample point be named and justified?
- Is the sampling carried out to BS 7592, and is the laboratory analysis done by a UKAS-accredited laboratory using the standard culture method?
- How are samples preserved, kept cool, and transported, and how quickly do they reach the laboratory?
- Will the report link every result to a specific named outlet, with the date, time, and technique used?
- If a result comes back above an action level, what support is offered to interpret it against the action levels and my written scheme?
A provider who answers these clearly is doing the job properly. One who cannot tell you which points they sampled or why is selling you bottles, not assurance.
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.
Where sampling fits in the bigger picture
For most hot and cold water systems, temperature control is the primary defence and sampling is the exception, used where the risk assessment justifies it: healthcare, complex systems, cooling towers, spa pools, and after a problem. Our guide to legionella testing covers when testing is and is not needed, and the testing labs directory lists UKAS-accredited laboratories. Results belong in your records alongside temperatures and flushing logs in a log book, kept for at least five years. And one discipline above all: a nil result is a snapshot of one point on one day. It records, it never certifies.
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
What is BS 7592?
BS 7592 is the British Standard for sampling for legionella in water and related materials, most recently revised in 2022. It sets out how a sampling exercise should be planned and carried out: how to choose sample points, how to take the sample so it represents the water in the system, how to preserve and transport it, and how to document the chain from tap to laboratory. It exists because a laboratory can only analyse the water that arrives in the bottle, so a badly taken sample produces a confidently misleading result.
Is BS 7592 a legal requirement?
No. It is a voluntary British Standard with no legal force and no special status like ACOP L8. Its value is credibility: a sampling exercise carried out to BS 7592 follows the recognised method, which makes the results far harder to challenge and far easier to rely on when deciding what action to take. When commissioning sampling, specifying BS 7592 is the simplest way to require a competent job.
Where should legionella samples be taken from?
Sample points should represent where the risk actually is, not where it is easiest to reach. That typically means outlets furthest from the calorifier or storage tank, little-used outlets on a flushing regime, the base of a calorifier, and any point the risk assessment has flagged. Both hot and cold systems may need points. The exact locations should come from the legionella risk assessment, because the purpose of the sample is to test the parts of the system most likely to let legionella grow.
Does a clear legionella sample mean the system is safe?
No. A sample is a snapshot of the water at one point, on one day, under one set of conditions. A nil result tells you legionella was not detected in that bottle; it does not certify the system. Legionella numbers fluctuate with temperature, stagnation, and season, and biofilm can harbour the organism even when the running water is clear. Sampling supports temperature control and the written scheme of control; it never replaces them.
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