CareHomeDeskCareHomeDeskTemperature monitoring tells you whether Legionella growth conditions are being controlled. Sampling tells you whether the system is colonised. They are different tests with different purposes -- and both may be required.
L8 ACOP temperature monitoring -- checking that cold water is below 20C and hot water is above 50C at sentinel outlets -- is the routine control measure that confirms the water system is being managed within safe parameters. Microbiological sampling is a separate layer of assurance: it involves taking actual water samples and testing them in a laboratory for the presence of Legionella bacteria. Temperature compliance does not rule out Legionella colonisation. A system can have compliant temperatures and still be colonised, particularly in complex or poorly maintained distribution systems.
HSG274 Part 2 identifies several situations where microbiological sampling should be considered or is required: high-risk premises (immunocompromised residents, complex water systems); following the installation of new water systems or significant modifications; following the commissioning of a new building; as part of an enhanced monitoring regime after a previous positive result; and as part of the investigation of a suspected Legionella case. For care homes housing frail and immunocompromised residents, quarterly Legionella sampling at sentinel outlets is considered best practice by most L8 consultants, even where temperatures are consistently within range.
Sampling must use the ISO 11731 method, which is the international standard for Legionella detection and enumeration in water. Samples must be taken by a trained person using sterile containers with sodium thiosulphate neutraliser (to stop any residual biocide killing the bacteria after collection), and transported to a UKAS-accredited laboratory within the specified time. Pre-flush and post-flush samples are typically taken at each outlet -- the pre-flush sample captures organisms released from the outlet itself, while the post-flush sample indicates the condition of the distribution system. Ask your contractor to confirm the laboratory's UKAS accreditation before instructing work.
UK guidance uses the following thresholds for Legionella counts in water samples: below 100 colony forming units per litre (cfu/L) is considered within an acceptable range; 100 to 1,000 cfu/L indicates a need for further investigation and review of the control regime; above 1,000 cfu/L or any detection of Legionella pneumophila serogroup 1 at any level triggers an immediate response. Results are reported per the ISO 11731 method and take typically 10-14 days to culture. Negative results are reported faster. Do not delay implementing interim control measures while waiting for results if a risk is suspected.
A positive result above the action threshold requires an immediate documented response. Do not wait for a repeat test before acting. The response should include: notifying your L8 consultant and, in the case of a high-level positive, considering notification to your local authority health protection team; implementing superchlorination or thermal flushing of the affected system; reviewing the L8 control measures that may have allowed colonisation to develop; and increasing monitoring frequency until results are consistently clear. Retain all sample certificates, response records and follow-up results as a continuous record in your L8 file.
CareHomeDesk gives you compliance checklists, maintenance logs, and contractor management tools built around exactly this kind of knowledge.