A pre-flush sample and a post-flush sample can come from the same tap, minutes apart, and still answer completely different questions. One tells you what a person would breathe in if they turned that outlet on right now. The other tells you what the system feeding the outlet is delivering once the local fitting is taken out of the picture. Pick the wrong one and the lab gives you an honest, certificated answer to a question you never meant to ask.
So this is not really “which sample is better”. It is “which question am I trying to settle” — exposure at the point of use, or the quality of the water arriving behind it.
What each sample actually captures
A pre-flush sample is the first water out of the outlet, taken before you run it and without disinfecting the fitting. It includes whatever was standing in the tap, spray head, flexible hose or thermostatic mixing valve, plus any biofilm those parts are shedding. That is deliberately the unflattering water — the first slug a guest, patient or employee would actually meet on use [1].
A post-flush sample is taken after the outlet has been run, and (depending on the method) after the fitting itself has been disinfected, so that the local fitting’s contribution is cleared and you draw water coming in from the distribution system. It describes the supply feeding that point, not the point itself [1].
BS 7592 sets out both approaches and the detail of how each is performed; choosing between them is a sampling-strategy decision made before anyone arrives with a bottle, not something the laboratory decides [1]. Keep one sentence in your head and most of the confusion goes away: pre-flush looks at the outlet, post-flush looks at the system behind it.
The things that actually separate them
The two samples differ on more than timing. They include different water, they implicate different parts of the building, and a positive result from each sends you to a different place. A positive pre-flush points your attention at the fitting on the wall — descale the spray head, strip and disinfect the hose, check the TMV. A positive post-flush points further upstream, towards storage, the calorifier, long runs and dead legs. The effort differs too: a pre-flush sample is captured the moment you arrive, while a post-flush sample needs the outlet run, sometimes the fitting disinfected, and a wait before the bottle goes under.
Pre-flush vs post-flush at a glance
| Decision axis | Pre-flush (first draw) | Post-flush (run, often disinfected) |
|---|---|---|
| Question it answers | What is a user exposed to on first use? | What is the system delivering to this point? |
| Water it includes | Standing water plus the fitting: head, hose, TMV | System water, with the fitting’s influence removed |
| A positive points to | Local colonisation of the outlet or fitting | Contamination in storage or distribution |
| Best suited to | Representative exposure and sentinel monitoring | Tracing a source and clearing the outlet as a variable |
| Effort and timing | Minimal: sample on arrival, before anything runs | More steps: run, sometimes disinfect, wait, then sample |
Which to use when
For routine monitoring built around how much risk a user actually meets, the pre-flush, first-draw sample is usually the more honest picture, because it captures the outlet exactly as the next person would find it. Pair that thinking with your temperature checks, which are the real control evidence; the sample is a periodic cross-check, not the regime.
For investigating a positive or a suspected case, take both from the same outlet. A paired sample is what lets you tell a colonised shower head apart from a contaminated supply, and that distinction decides whether you replace a £15 fitting or open up the plant room.
For validating remedial work or a disinfection, the post-flush sample comes into its own: once you have cleaned the system you want to know what the system is now producing, with the old fitting either replaced or taken out of the equation. Sampling after a return to service or repair sits in the same family of checks.
For commissioning or a building brought back from low use, a post-flush sample tells you whether the supply you are reconnecting is sound, while a pre-flush sample tells you whether the outlets that sat idle have grown their own problem in the meantime.
Reading a paired result
The real payoff comes from taking both samples at the same outlet and comparing them. Four combinations, four very different conversations.
- Pre-flush positive, post-flush negative — the problem is local. The system water is clean; the fitting on the wall is the reservoir. Clean, descale or replace the head, hose or TMV, then re-sample to confirm.
- Pre-flush positive, post-flush positive — the problem is systemic. Contamination is arriving from upstream, so chasing individual fittings will not fix it. Look at temperature control, stagnation, storage and the wider distribution.
- Pre-flush negative, post-flush negative — both the outlet and the supply look clean at the moment of sampling. Reassuring, but it describes one outlet at one moment, not the whole building.
- Pre-flush negative, post-flush positive — uncommon and worth a second look before you act, as it can point to a sampling slip such as the outlet being run shortly before the “pre-flush” bottle was filled. Re-sample cleanly before drawing conclusions.
That matrix is why so many specialists default to paired sampling on a problem outlet: it converts a single number into a location. When the certificate lands, read it alongside what CFU counts actually mean, because the count without the sample type and method is only half the story.
A caveat worth reading before you book the lab
A flush sample is only as honest as the few minutes around it. If an outlet is quietly run, wiped or descaled before a “pre-flush” sample — by a cleaner, an over-helpful caretaker, or by you steadying the bottle under a briefly opened tap — you no longer have a pre-flush sample. You have a post-flush one wearing the wrong label, and the result will mislead. Brief whoever has access to the outlet, and record exactly what was and was not done.
Either sample type describes one outlet at one moment. Neither vouches for temperatures, stagnation, cleanliness or the rest of the system, and a clean result is never a reason to relax control. Sampling supports a written scheme decided by a competent person through a site-specific risk assessment; it does not replace that scheme, and nothing here is legal or clinical advice. Let your risk assessment and BS 7592 set which sample type you take, where, and how often [1][2].
FAQ
Do I disinfect the outlet before taking a post-flush sample?
It depends on the objective and the method. Disinfecting the fitting before a post-flush sample is one of the approaches BS 7592 describes, used when you specifically want the system water with the local fitting ruled out [1]. If you want to know what the supply delivers after remedial work, that step makes sense; if you are measuring user exposure, you would not do it. Decide the question first, then choose the method, and record which you used.
My pre-flush sample is positive but the post-flush is clean — what now?
That pattern usually means the fitting, not the supply, is the reservoir. The practical response is to strip, clean and descale or replace the spray head, flexible hose and any aerator, check and service the TMV, then re-sample to confirm the fix held. Treat it as a local job before assuming a system-wide problem.
Which sample type counts for routine monitoring?
There is no single mandated answer; the sample type, location and frequency follow your risk assessment and the BS 7592 strategy for the system [1][2]. Many programmes use first-draw, pre-flush samples at sentinel outlets because they best represent what a user is exposed to, and bring in post-flush sampling when investigating or verifying. Write down why you chose what you chose, so the record explains itself later.
Sources
[1] 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 [2] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm