A disinfection certificate tells you a contractor did the work. A clearance sample tells you the work actually worked. Those are not the same thing, and the gap between them is where a duty holder gets caught out, signing off a system that still has a problem because the paperwork looked complete.

Remedial cleaning gets a system disinfected. The certificate records what was done. Neither, on its own, confirms the contamination has gone. That confirmation comes from a microbiological clearance sample taken afterwards, interpreted against criteria, and filed against the certificate it verifies. This is the procedure for doing that step properly.

Why a clearance sample exists at all

You disinfect a system because something prompted it: a sample over an action level, visible fouling, a confirmed case, or planned work on a system that has been standing. The disinfection is the intervention. The clearance sample is the evidence that the intervention succeeded, the closing entry that lets you say the system has returned to a satisfactory microbiological condition rather than merely that it was treated [1].

It matters most when someone else did the work. If a contractor cleans, disinfects and certifies in one visit, the certificate is their account of their own work. A clearance sample, ideally with the result going to you as well as to them, is your independent check. Skipping it means trusting that the dose was right, the contact time was held, and nothing reseeded, none of which a certificate proves.

Before you sample: what must already be true

Clearance sampling is not the first thing you do. Get these in place first, or the result will be meaningless:

  • The disinfection is complete and the system has been thoroughly flushed through to remove the disinfectant.
  • Any chemical residual has been allowed to fall back to normal supply levels, and the system has been returned to its routine operating regime, hot water hot, cold water cold, normal use resumed.
  • You hold the disinfection certificate or record, so the clearance result has something to attach to.
  • You have arranged an accredited laboratory and the correct sample bottles before you go near an outlet.

That last point hides a common trap. If any disinfectant residual remains, a sample bottle without a neutraliser will keep killing organisms on its way to the lab and hand you a falsely clean result. Bottles for this purpose should contain an appropriate neutraliser such as sodium thiosulfate. Confirm this with the supplier and against the sampling standard rather than assuming the default bottle is right.

The clearance-sampling sequence, step by step

Run the steps in order. Each has a “done when” line so you can tell a finished step from a half-finished one.

  1. Confirm the system has normalised. Check the disinfectant residual has dropped to background and temperatures and flow are back to normal. Done when the system is operating as it does in routine service, not still in a treated or drained state.

  2. Observe the waiting interval. Do not sample the moment the flush finishes. Guidance commonly allows a short settling period, typically a few days after the system is back in normal use, so that any surviving or reseeding organisms have a realistic chance of showing up rather than being masked by the just-completed treatment. The exact interval should follow recognised guidance and your risk assessment, not convenience. Done when the agreed interval has elapsed with the system in normal use throughout.

  3. Choose representative and previously failing points. Build a sampling plan that includes the outlets or assets that originally failed or prompted the work, plus a representative spread across the system, sentinel near and far points, the calorifier or storage, and any dead-leg or low-use outlet implicated earlier. Done when every point that drove the disinfection is on the list, alongside points that represent the wider system.

  4. Collect each sample correctly to the sampling standard. Take samples in line with BS 7592, clean technique, the right bottle with neutraliser, and pre- and post-flush samples where the plan calls for them, each labelled with location, date, time and temperature [3]. Done when every bottle is correctly filled, labelled and matched to a point on the plan.

  5. Get samples to the laboratory promptly. Keep them cool and within the lab’s stated transport time so the culture is valid. Done when the lab confirms receipt within an acceptable interval and condition.

  6. Interpret against criteria, not against hope. Read the legionella result in cfu/litre against the action levels in current guidance and against what your risk assessment defines as acceptable for that system [2]. Treat a non-detect at the limit of detection as the goal, not just “lower than last time”. Done when every point has a result interpreted against a defined threshold.

  7. Decide and record. If the points clear, record the system as satisfactory and file the result against the disinfection certificate. If they do not, the disinfection has not achieved clearance, investigate and, where indicated, re-treat and re-sample. Done when the outcome and the decision are written down and linked to the certificate.

What a “pass” actually means

A clearance pass is not one good number. It is the previously failing points and the representative points all sitting within the criteria your risk assessment and current guidance set, with samples taken after a proper interval, by correct method, to an accredited lab [2][3]. A single low result from one tap, taken too soon, in the wrong bottle, proves very little. The discipline is in the plan, the timing and the bottle as much as in the figure on the report.

Express results as the lab does, typically counts per litre, against a stated limit of detection. A “not detected” is reported against that detection limit, so a meaningful pass depends on the lab’s analytical reach as well as on your system. Confirm that scope and the reporting units with the laboratory before the job, so you are not surprised by how the result reads.

An illustrative case

Consider a composite scenario. An office building has a calorifier sample over an action level. A contractor attends, cleans and disinfects, and issues a certificate the same day. The facilities manager files it and closes the action, no clearance sample. Three months later the next routine round fails again at the same point. Because there was never a post-disinfection sample, there is no way to know whether the original disinfection failed on the day or the system reseeded afterwards. A clearance sample at the right interval would have separated those two stories and pointed to the real fix. The lesson is transferable: the certificate closes the contractor’s job; the clearance sample closes yours.

A necessary caveat

This is general guidance on the clearance-sampling step, not a substitute for a competent, site-specific judgement. The interval to wait, the points to sample, the criteria to apply, and the response to a failure all depend on your system, the reason for the disinfection and the findings of your written scheme of control, they belong to a competent person assessing your building. Nothing here is legal, medical or design advice.

What to do next

Pull the last disinfection record on your site and look for the clearance result filed with it. If there isn’t one, if the trail ends at the certificate, you have a system that was treated but never verified. Add a clearance-sampling step to your remedial procedure so no future disinfection is signed off without the sample that proves it worked, and agree the interval, points and acceptance criteria with your competent adviser before the next job, not after it.

FAQ

How long after a disinfection should you wait before taking a clearance sample?

Not immediately. A short settling period, commonly a few days after the system is back in normal operation and the disinfectant has cleared, gives any surviving or reseeding organisms a realistic chance to be detected, rather than being suppressed by the treatment you have just finished. Treat the exact interval as set by recognised guidance and your risk assessment, and confirm it against the chlorination guidance you are working to.

Do clearance samples have to come from the same outlets that originally failed?

Those points must be included, but they should not be the only ones. Sampling only the failing tap tells you whether that spot improved; it does not tell you whether the disinfection cleared the system. A sound plan covers the previously failing points plus a representative spread, sentinel near and far outlets, storage or the calorifier, and any implicated dead leg, so the result speaks for the whole system, not one fitting.

What counts as a pass for a post-disinfection clearance sample?

A pass is all sampled points returning legionella counts within the criteria your risk assessment and current guidance define as acceptable, with samples taken after a proper interval, by correct method, and analysed by an accredited laboratory. Aim for a non-detect at the lab’s limit of detection at the points that matter, not merely a lower figure than before. Interpret against the action levels in guidance, not against improvement alone.

What happens if the clearance sample fails?

A failed clearance sample means the disinfection did not achieve clearance, the system is not yet confirmed safe. Rather than repeat the same treatment blindly, investigate why: insufficient dose or contact time, a missed dead leg or asset, recontamination, or an underlying fault such as poor temperature control. Your competent adviser should direct the response, which may include re-cleaning, re-disinfecting and re-sampling, and is recorded against the original certificate.

Sources

  1. HSE, ACoP L8 (2013), “Carrying out a risk assessment”, p.12. https://www.hse.gov.uk/pubns/books/l8.htm
  2. HSE, HSG274 Part 2 (2024), “Cleaning and disinfection”, p.87. https://www.hse.gov.uk/pubns/books/hsg274.htm
  3. BSI, BS 7592:2022, clause 7 (sampling procedures), p.14. https://www.bsigroup.com/
  4. NHS England, “Health Technical Memorandum 04-01: Safe water in healthcare premises”. https://www.england.nhs.uk/publication/safe-water-in-healthcare-premises-htm-04-01/