Legionella testing & sampling

What do we sample for Legionella?

Cooling towers are the classic Legionella source and the system Hong Kong regulates, so most of our Legionella sampling is cooling tower water. But the bacteria are also associated with hot and cold water services, calorifiers and spa pools, and the sampling plan follows the risk rather than the habit.

• Cooling towers — total Legionella count and heterotrophic colony count (HCC), the two parameters the EMSD Code of Practice sets an action ladder on
• Hot and cold water services where the risk assessment points there — in hospitals we look after cooling towers, potable and hot water; in hotels, hot water services and pools alongside the plant
• Before-and-after samples around cleaning and disinfection, so the clean is confirmed rather than assumed
• Re-tests after a high result, timed to the Code's resample windows
• Towers another contractor treats — sampling is offered independently of the treatment relationship, including as part of an independent audit

We test HCC alongside Legionella because HCC moves first. A rising heterotrophic count means the water is becoming hospitable — nutrients, biofilm, somewhere for amoebae to live. By the time Legionella is climbing, the HCC has usually been telling you for a while.

Who takes the sample, and why that matters

A Gotech technician takes the sample on site. That is not a detail. A sample drawn from the wrong point, at the wrong time, or handled badly on the way to the laboratory produces a number that is precisely wrong — and a precisely wrong number is more dangerous than no number, because people act on it.

1. Agree the scope. Which systems, which sample points, and why. The question decides the sampling plan.
2. Sample on site. Right point, right method, right handling. Our technician records the sample point, date and time so the result can be read in context later.
3. Deliver to the laboratory for analysis.
4. Report. A formal, dated result with an interpretation — what the number means for your system and what, if anything, to do about it.
5. Act. If a result is out of range we tell you what has to happen next, and we can do the work.

Which laboratory analyses the samples?

Samples are analysed by a HOKLAS-accredited independent laboratory — one of Hong Kong's largest. The laboratory is independent of Gotech, which is the point: the count is an accredited third party's measurement, not our opinion of our own work, and every result comes back to you as a formal dated certificate.

Legionella is cultured, not read off a meter, so it is not an instant answer. We tell you the turnaround when we agree the scope, and if a situation is urgent we say so rather than let you wait on a result that will arrive too late to matter. See laboratory testing for the full range of water tests we arrange.

How do I read a Legionella test result in Hong Kong?

The EMSD Code of Practice for Fresh Water Cooling Towers (2023 edition, Part 2) sets more than one threshold, and each carries a different required response. Read your certificate against this ladder:

The Code also sets a ladder on heterotrophic colony count: below 100,000 cfu/mL, repeat routine monitoring; 100,000 cfu/mL or more, immediate cleaning and disinfection; 5,000,000 cfu/mL or more is an emergency decontamination trigger in its own right.

If EMSD takes the sample rather than us, the same numbers apply with a statutory layer on top: 10 to 1,000 cfu/mL draws an advisory letter, and 1,000 cfu/mL or more draws a nuisance notice under the Public Health and Municipal Services Ordinance, with the building location published. The full rulebook is in the EMSD Code of Practice, explained.

What happens if the Legionella result is positive?

A positive result is not the end of the story; it is the start of a defined sequence. What you do in the first hours matters.

• Do not drain or dose the system before taking advice. You can destroy the evidence you will need and make the re-test harder to interpret.
• Keep the certificate, and note the sample point and date.
• 10 to below 1,000 cfu/mL: investigate the cause, review the programme, carry out immediate on-line disinfection, and resample within 3 to 7 days.
• 1,000 cfu/mL or more: immediate emergency decontamination — cleaning, desludging and disinfection — and a resample within 3 days. If that is where you are now, go straight to emergency Legionella and EMSD notice response.
• Two consecutive samples with no Legionella detected are required before you return to the routine quarterly regime. One clean result does not close the loop.

A high count almost always has a physical cause: biofilm and sludge shielding the organism from the biocide, pH drifting to where chlorine barely works, or a doser that failed unnoticed between visits. A clean without a diagnosis buys a few months. We find the cause as well as treating the symptom — see Legionella control.

How often should a cooling tower be tested for Legionella?

At least quarterly. Under Part 2 of the Code, Legionella and HCC are tested at least every three months while the system is under control. After a high result the interval collapses to days until two consecutive clean samples are in hand. Quarterly is the Code's floor; the interval we set follows the tower's risk and history.

Two other clocks run alongside. The tower must be cleaned, desludged and disinfected at least every 6 months, and since 2011 EMSD has been able to enter premises and sample at random, with no notice. The surveillance is real: in January to June 2026 EMSD took 348 samples across 219 buildings, and 35 of them sat in the 10–1,000 cfu/mL advisory band — roughly one in ten. Your own quarterly result is how you know, before EMSD does, what a random sample would show. See EMSD sampling results, tracked and how often a cooling tower should be cleaned.

Certificates for your compliance file

Every Legionella result comes back as a formal, dated certificate from the accredited laboratory, with our interpretation alongside it. It is written to be filed: a compliance file, a tender submission, a group audit, or an EMSD inspection.

• The Code requires operation and maintenance records to be kept for at least 2 years and to be readily available for inspection on request; inspections also check completed checklists, records and logbook for the past 1 year.
• A nuisance notice cannot be closed without a laboratory re-test on file showing the count has come down.
• Hotels get reports formatted for group compliance files and brand audits; hospitals get documentation to the EMSD Code of Practice at every visit.
• Every Gotech visit, sampling included, ends with a dated written report.

Hiring a contractor does not transfer the owner's responsibility for a cooling tower. What protects an owner is treatment that keeps the water safe and documents that prove it — the certificate is the second half. The printable compliance checklist tracks the sampling and cleaning dates month by month; compliance sets out the rest of the owner's duties.

Frequently asked questions