Legionella & Water Hygiene · 9 min read

Updated 28 May 2026

Legionella Testing and Sampling: When, How and Reading Results

Pipe Assassin Technical Team — G3 certified, WRAS approved — 10+ years in UK plumbing & water hygiene

For most UK hot and cold water systems, routine microbiological Legionella sampling is not required — temperature control is the primary measure under HSG274. Sampling is justified when temperatures are out of range, after an outbreak or major works, when a non-temperature control method is being used, or in higher-risk premises like healthcare. Done properly, it's a defined procedure with clear pass/fail thresholds.

When sampling is — and isn't — required

The HSE and HSG274 Part 2 are explicit: in a well-run hot and cold water system, microbiological sampling for Legionella is not a routine requirement. Doing it for show, on an arbitrary schedule, isn't part of the compliance picture and can be actively unhelpful (one positive sample then triggers a chain of expensive remedials that the system didn't actually need).

Situation	Sample?
Routine domestic let with combi heating, temperatures in range	No
Routine commercial system, temperatures in range, controls working	No (HSG274 Pt2)
Temperatures out of range; control regime in doubt	Yes — to size up the problem
After major works on the system	Yes — once recommissioned and re-flushed
After a case of Legionnaires' disease linked to the building	Yes — urgent, with public health input
Where temperature control is impractical and a biocide is used instead	Yes — schedule per HSG274 Pt2
Healthcare premises (especially augmented care)	Yes — defined schedules under HTM 04-01
Spa pools, cooling towers	Yes — separate routine schedules under HSG274 Pts 1 & 3

How a compliant sample is taken

Sampling follows BS 7592 and the methods described in HSG274. The short version:

1
Choose the right outlet
Sentinel outlets (nearest and furthest on each loop), calorifier outlets, plus any specific point being investigated.
2
Use the right bottle
Sterile 1-litre bottle containing sodium thiosulphate to neutralise residual chlorine and stop further bacterial change in transit.
3
Take a pre-flush sample
Aseptically — no flame to TMVs or plastic taps. Captures what's actually being delivered to the user.
4
Run the outlet
Record temperature at full flow after 2 minutes (hot ≥50°C, cold <20°C). Disinfect the outlet if a post-flush sample is also being taken.
5
Take a post-flush sample
Captures what is in the system feeding the outlet, distinct from contamination at the outlet itself.
6
Transport cool and fast
Keep samples between 6-18°C, ideally analysed within 24 hours. UKAS-accredited lab only.

Reading CFU/L results

Results come back as CFU/L — colony-forming units per litre. The HSG274 Pt2 action thresholds for hot and cold water systems are:

HSG274 Pt2 action thresholds for hot & cold water systems
Result (CFU/L)	Interpretation	Action
< 100	Satisfactory	System under control. Normal monitoring.
100 - 1,000	Some Legionella present	Review control measures, resample. If repeat or several outlets are positive, treat as out-of-control and act.
> 1,000	Significant contamination	Immediate action: review controls, sample further to map extent, disinfect the system, investigate cause.

A single result is one data point

Don't panic at a single low-positive — but don't ignore it either. The point of the control regime is the temperature, the cleanliness and the absence of stagnation. A positive sample is information that something in the regime isn't working; the fix is almost always upstream of the result.

UKAS-routed sampling and remedial work — water hygiene team in EN2

07956 645 527

Enfield-based, 24/7. G3 certified. WRAS approved. Insured to £5m.

Other useful tests

Total Viable Count (TVC) — broader microbial load at 22°C and 37°C; useful as a general "is the water clean?" indicator on tanks and outlets.
Free chlorine / chlorine dioxide residual — where a chemical regime is in use, the residual is checked at the same points the temperatures would be.
Pseudomonas — relevant in augmented care areas of hospitals.

What about home test kits?

Mail-in kits exist and can be useful as a curiosity check, but for any compliance purpose a sample must be taken to BS 7592 by a competent person and analysed at a UKAS-accredited laboratory. For a landlord or employer, a kit you posted yourself with no chain of custody is not evidence that will stand up.

Ninja Tip

If your risk assessment says you don't need routine sampling, you don't need routine sampling. The right work is the temperature audit, the tank inspection, the dead-leg removal and the records. Sampling is reserved for the situations where it actually answers a question — see the pillar guide for the bigger picture.

Frequently Asked Questions

Do I need to test for Legionella?

Not routinely for most low-risk hot and cold water systems where temperature control is in place and working. HSG274 is clear that temperature control is the primary measure. Sampling is justified when control is in doubt, after an outbreak, after major works, where another control method (e.g. chemical biocide) is used instead of temperature, or in higher-risk settings such as healthcare.

How is a Legionella sample taken?

From a sentinel or representative outlet, into a sterile bottle containing sodium thiosulphate (to neutralise any chlorine). A pre-flush sample is normally taken first (to capture what's in the outlet), then the outlet is run, the temperature recorded, and a post-flush sample taken (to capture what's in the wider system). Samples are kept cool and tested at a UKAS-accredited lab — usually within 24 hours.

What is a safe CFU/L result for Legionella?

For a hot/cold water system, below 100 CFU/L is generally considered satisfactory — keep the system under normal control and review. 100-1,000 CFU/L means review the controls and resample; if confirmed, take corrective action. Over 1,000 CFU/L means immediate action — disinfection of the system and a full review of controls.

Who can take Legionella samples?

A competent person trained in BS 7592 sampling. Samples must be taken aseptically into the correct bottles, recorded against the system schematic, and analysed at a UKAS-accredited laboratory. DIY home test kits are not appropriate for compliance evidence.

Sources & further reading

Guidance only. This article is general information for UK readers, not a substitute for a site-specific assessment by a competent person. Regulations and best practice change — always check the current official guidance and, for compliance work (Legionella risk, unvented cylinders, water regulations), use a suitably qualified professional. Pipe Assassin is an electric-boiler and water-hygiene specialist and is not Gas Safe registered; we do not carry out gas work.

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