What do legionella test results mean for landlords?
Legionella test results show whether bacteria were detected in a water sample and, where applicable, the concentration found. Landlords should interpret the result alongside the property’s water risk assessment, system conditions and sampling points, with unexpected or elevated findings reviewed by a competent person to determine any necessary action.
A legionella test report is an indication of the bacterial conditions at the sampling point and time of collection, not a standalone pass-or-fail certificate for the whole property. Landlords should read the laboratory result together with the water system’s design, usage, maintenance records and current risk assessment.
What the main result terms mean
- Not detected: the laboratory did not find legionella in the portion of sample examined at the test’s reporting limit. This does not prove that the entire water system is free from legionella, because bacteria may be unevenly distributed or present at another outlet, tank or pipe section.
- Detected or quantified: legionella was found, and the report may state the concentration in colony-forming units per litre. A quantified result gives a basis for assessing the significance of the finding and deciding whether further investigation is needed.
- Below the reporting limit: bacteria may not have been detected at a level the method can reliably quantify. This has a different meaning from proving complete absence.
- Unsuitable or invalid sample: the laboratory may be unable to interpret the result reliably if the sample was incorrectly collected, contaminated, delayed in transit or inadequately identified. The report should explain whether a repeat sample is required.
The report should identify the sampling date, outlet or system location, sample type, laboratory method and result units. These details matter because a result from a shower outlet may have a different significance from one taken from a storage cistern, calorifier or return circuit. A sample taken before remedial work cannot be used to demonstrate the condition of the system after that work.
Why a positive result needs context
Finding legionella does not, by itself, show how much risk a tenant faces. The assessment should consider whether the property contains features that allow water to stagnate or remain at favourable growth temperatures, such as infrequently used outlets, redundant pipework, storage tanks, poor circulation or temperature-control problems. It should also consider who may be exposed, including people who may be more vulnerable to serious illness.
One result should therefore be compared with the wider evidence. Relevant information can include earlier laboratory reports, temperature checks, flushing records, cleaning and disinfection records, maintenance faults and changes in occupancy or water use. Repeated findings at the same location may point to a local problem, while results from several connected points may indicate a wider system issue. A single negative sample does not necessarily cancel out a pattern of previous positive results.
What landlords should do after receiving the report
- Check that the report relates to the correct property, outlet and sampling date, and confirm whether the sample was suitable for analysis.
- Record the result with the property’s water risk assessment and water-system management records.
- Ask a competent water hygiene professional to assess any detected or unexpected result, particularly where the concentration is elevated, results are recurring or vulnerable occupants may be exposed.
- Review the relevant control measures, including temperature control, outlet use, flushing, tank condition, dead legs, cleanliness and the condition of associated equipment.
- Determine whether corrective work, further sampling or a wider investigation is appropriate. The decision should be based on the complete risk assessment rather than on the laboratory number alone.
- Document the finding, the reasoning behind the decision, the work completed and any follow-up monitoring required.
Landlords should not attempt to interpret an isolated result as legal proof of compliance. Compliance depends on managing the foreseeable risks associated with the water system and keeping suitable evidence that control measures are being applied. Testing can provide valuable verification, but it does not replace a suitable risk assessment, regular system management or remedial action where defects are identified.
Where a result is unclear, unusually high, inconsistent with other records or associated with suspected exposure, the report should be referred to a suitably competent person. UKAS-accredited laboratory testing can support reliable decision-making, but the laboratory result still needs to be interpreted against the property’s risk profile and the requirements of the relevant water safety arrangements.
A positive legionella result is an environmental finding, not a medical diagnosis. It shows that legionella bacteria were detected in the sampled water, but it does not show that a tenant has been exposed, has developed an illness or contracted Legionnaires’ disease.
The relevant concern is whether water from the system could create contaminated aerosols, such as through showers, spray taps or other outlets. Landlords should therefore avoid presenting a test result as proof of illness or as evidence that a tenant is necessarily in danger. If a result raises a potential exposure concern, the appropriate response is to follow the water risk assessment, obtain competent advice and provide tenants with clear, factual information about any control measures that affect use of the water system. A tenant with health concerns should seek medical advice.
Need help interpreting legionella test results?
If you need a professional review of a legionella test report and its implications for your property, contact Blue Fish Water Management Ltd to discuss the appropriate next step.