A Legionella result is only as good as the moment the bottle was filled. Touch the cap thread, top up from the wrong outlet, let the bottle warm on a dashboard for a day, and the laboratory will faithfully analyse a sample that no longer represents the system. This is the field technique for getting that moment right.
Where to sample and whether to take a pre-flush or post-flush sample are decisions made before you arrive on site, in the sampling plan and the risk assessment [1][2]. If those decisions are not settled, start with BS 7592 sampling: how to plan representative Legionella water samples and Pre-flush vs post-flush sampling: which to use when. This procedure assumes the plan is already settled and walks through the physical task at the outlet: bottles, asepsis, fill, label, temperature and the paperwork that travels with the sample.
Before you go: competence, kit and lab liaison
Sampling looks simple and is easy to do badly. The operative should be trained in aseptic technique and in the specific protocol the laboratory expects, because a result that ends up in an enforcement file or an outbreak investigation has to survive scrutiny of how it was taken [4].
Talk to your UKAS-accredited laboratory first. They supply the sample containers, and those bottles are not generic. For first-draw (pre-flush) samples in particular, the bottle contains a neutralising agent, commonly sodium thiosulphate, to quench any residual disinfectant the instant the water enters, so the count reflects the pipework rather than continued kill in the bottle [1]. Confirm with them the bottle type, the fill volume they need, and their stated time and temperature window for getting the sample back to the bench; choosing a UKAS-accredited lab for Legionella testing covers what to check before you rely on the result [3].
Assemble the kit: the correct sterile, in-date bottles; a cool box with coolant packs; a calibrated thermometer; alcohol wipes or a flame source where post-flush disinfection of the outlet is required; gloves and hand sanitiser; indelible pen and labels; and the sample submission and chain-of-custody form.
The collection sequence at the outlet
Render this as your on-outlet routine. Each step has a point at which you can say it is done correctly before moving on.
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Confirm you are at the right point. Match the outlet to the sampling plan and its unique reference. Done when the outlet, its asset ID and the intended sample type (pre- or post-flush) all agree with the plan.
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Decide and apply outlet preparation by sample type, do not improvise it. A first-draw sample is taken as found, with no flushing, no flaming and no removal of fittings, so it captures what a user would receive. A post-flush sample follows the protocol’s disinfection of the outlet and a run to a stabilised temperature before filling. Done when the preparation matches the planned sample type, not a habit. Check it against the laboratory’s protocol and BS 7592.
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Sanitise your hands and open the bottle aseptically. Remove the cap immediately before filling. Hold it by the outside; never touch the inside of the cap, the bottle neck or the bottle interior, and do not put the cap down. Done when the bottle is open with no surface having contacted skin, clothing, the tap or the ground.
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Fill to the laboratory’s mark, leaving an air gap. Direct the flow into the bottle without splashing the neck. Fill to the volume the lab specifies and leave the headspace they ask for, do not overfill or top up later. For first-draw samples, do not let the water run before collecting; the first water out is the sample. Done when the level sits at the required fill with the air gap intact.
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Re-cap immediately and securely. Replace the cap without touching the thread or rim. Done when the bottle is sealed and upright.
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Label there and then. Write the unique sample reference, the location/outlet ID, the date and the time directly onto the bottle, not from memory back at the van. Done when the label is complete, legible and matches the submission form entry.
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Record the water temperature, as a separate reading, into a separate vessel. Never put a thermometer probe into the sample bottle. Capture the outlet temperature for context using your monitoring thermometer in run-off water. Done when the temperature is logged against the sample reference.
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Place the sample in the cool box. Keep samples cool and out of direct sunlight, and start the clock on the laboratory’s transport window. Done when the bottle is in the cool box with coolant and the time noted.
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Complete the chain of custody. Fill in the submission form: who took the sample, when, where, sample type, temperature, and the handover to the courier or lab. Done when every bottle has a matching, signed form entry.
What makes the sample valid
Treat these as the pass conditions. The bottle is the laboratory’s own neutralised container, in date, filled to their volume with the headspace they require. Aseptic technique held throughout, no contact with cap thread, neck or interior. The label is unambiguous and matches the form. The sample is cooled and reaches the laboratory within their stated time and temperature window; warm or delayed samples can drift and may be rejected or flagged. And the chain of custody is unbroken from outlet to bench [1][3].
If any of those fail, the honest move is to note it and, where practicable, resample. A caveated weak result is worth more than a clean-looking one you cannot defend. The specific volumes, transport timings and acceptance criteria belong to your laboratory’s UKAS schedule and to the relevant UKHSA standard methods, follow those over any number quoted from memory.
FAQ
Do I need to flame or disinfect the tap before sampling?
It depends entirely on the sample type. A post-flush sample, intended to assess the system rather than the outlet, typically involves disinfecting the outlet and running to a stabilised temperature first. A first-draw (pre-flush) sample is taken as found, with no flaming and no flushing, because the aim is to capture exactly what reaches the user. Apply whichever the sampling plan specifies, and verify the technique against BS 7592 and your laboratory’s protocol.
Why must I use the laboratory’s bottles and not my own sterile containers?
Because the laboratory’s bottles usually contain a neutraliser, commonly sodium thiosulphate, that quenches residual disinfectant the moment the water enters. Without it, any chlorine or other biocide in the line keeps killing bacteria inside the bottle on its way to the lab, and the count comes back artificially low. A generic sterile container has no neutraliser, so it cannot give a representative first-draw result.
Can I reuse a sample bottle or top one up if I underfilled it?
No. Bottles are single-use, sterile and pre-dosed, so reuse risks contamination and removes the neutraliser balance. Topping up later breaks the aseptic chain and the timing of collection. If you underfill or suspect contamination, discard that bottle and take a fresh sample in a new container.
Should I measure the water temperature inside the sample bottle?
Never put a probe into the sample bottle, it contaminates the sample and can introduce disinfectant or organisms from the thermometer. Record the outlet temperature separately, in run-off water with your monitoring thermometer, and log it against the sample reference so the laboratory and the duty holder have that context alongside the result.
A note on competent assessment
This is a general field method, not a substitute for your laboratory’s written protocol or a competent person’s site-specific sampling plan. Whether to sample at all, where, when, and which sample type to take are risk-based judgements made under a competent assessment; this procedure only describes how to take the sample cleanly once those decisions are made. It is not legal advice.
Do this before your next round
Phone your laboratory and ask them to confirm, in writing, three things: the exact bottle type for each sample type you take, the fill volume they need, and their maximum transport time and temperature. Tape that summary inside the lid of your cool box. Most invalidated samples fail on one of those three points, not on technique at the tap.
Sources
- BSI, BS 7592:2022, clause 7 (sampling procedures), p.14. https://www.bsigroup.com/
- HSE, HSG274 Part 2 (2024), “Microbiological monitoring”, p.85. https://www.hse.gov.uk/pubns/books/hsg274.htm
- UKHSA, “Legionella species: guidance, data and analysis (standard methods for isolation and enumeration)”. https://www.gov.uk/government/collections/legionnaires-disease-guidance-data-and-analysis
- Legionella Control Association, “Code of Conduct for Service Providers”. https://www.legionellacontrolassociation.co.uk/