How do UKAS Legionella testing methods differ?

The main differences between UKAS Legionella testing methods are the analytical principle used, the type of result produced, and the limits placed on interpreting that result. A competent laboratory may use a recognised culture method, a molecular method such as quantitative polymerase chain reaction (qPCR), or a combination of approaches. The method should be selected against the monitoring objective, the characteristics of the water system and the decisions that may follow from the result.

Culture-based testing is the established approach for recovering Legionella organisms from a water sample. The laboratory concentrates the sample where appropriate, applies treatment intended to reduce competing organisms, and places the prepared material onto a selective growth medium. Following incubation, suspected colonies are examined and confirmed using appropriate identification procedures. Results are generally expressed as colony-forming units per litre, indicating the quantity of organisms that were recovered under the test conditions.

A significant feature of culture is that it demonstrates recoverable organisms rather than merely detecting their genetic material. This makes it useful when the question is whether Legionella can be isolated from the sampled water. It can, however, be affected by the condition of the organisms, competing microbial growth, sample quality and the time needed for incubation and confirmation. Injured or stressed organisms may not grow readily, so a negative culture result should be considered alongside the sampling plan, system controls and other available evidence.

Molecular testing, including qPCR, identifies targeted Legionella DNA through amplification and detection. The laboratory extracts genetic material from the sample and uses controls to check that the reaction has operated correctly and that substances in the water have not inhibited the analysis. Molecular testing can provide information without waiting for organisms to grow, but detection of DNA does not by itself establish that the organisms are alive or capable of multiplying.

This distinction affects how a molecular result is used. DNA may remain detectable after disinfection or after organisms have lost viability. A positive qPCR result therefore requires interpretation in relation to system history, treatment activity and the reason for testing. Conversely, a negative result can be affected by the volume examined, uneven distribution of contamination, sampling location and substances that interfere with DNA extraction or amplification. The numerical output must not be treated as directly interchangeable with a culture count unless the laboratory and interpreting professional have established what comparison is valid.

Combined culture and molecular testing can provide complementary information. Culture can indicate recoverable Legionella, while qPCR can identify target DNA that may not produce a culture result. Using both approaches may be appropriate during investigations, after control measures, or where a business needs to understand the difference between evidence of current viability and broader evidence of contamination. It is not automatically the best option for every monitoring programme: additional testing should have a defined purpose and should be interpreted consistently over time.

Sample preparation also varies within a method. Laboratories may use filtration, concentration, enrichment or selective treatments, depending on the sample and the validated procedure. These stages affect the amount of material presented for analysis and can influence recovery. The sample container, preservative requirements, transport conditions, holding time and information supplied to the laboratory are therefore part of the testing process, not administrative details that can be separated from the result.

Sampling strategy is different from analytical method. A technically sound laboratory test cannot compensate for a sample that does not represent the system. The sampling plan should identify relevant sentinel outlets, parts of the system with different temperatures or usage patterns, storage vessels, return loops and other locations indicated by the risk assessment. Whether a sample is taken before or after flushing, and whether outlet fittings or hoses are included, should be recorded because these details alter what the result represents.

UKAS accreditation relates to a laboratory’s competence for specified activities within its accreditation scope. It does not mean that every test offered by a laboratory is accredited, nor that one method is universally preferable. When comparing reports or quotations, check that the particular Legionella test, sample matrix and analytical procedure fall within the stated scope. The report should make the method identifiable and should distinguish accredited work from any subcontracted or non-accredited activity.

Useful information in a UKAS Legionella testing report normally includes:

• the unique sample reference and the date and location of collection;
• the sample type and any relevant outlet, tank, loop or system description;
• the analytical method or standard used;
• the reported result, units and reporting or detection limit;
• any qualification, deviation, inhibition, contamination or sample-condition note that affects interpretation;
• the laboratory’s accreditation details and the scope applicable to the test; and
• the date of analysis and the authorised release of the result.

Reports should not be read as isolated pass-or-fail certificates. The significance of a finding depends on the type of method, the sampling location, the reliability of the sample, the system’s risk assessment and the effectiveness of control measures. A result that prompts action in a healthcare, residential, industrial or commercial setting may need to be assessed differently according to the vulnerable population, system design and exposure routes. The relevant control guidance and the organisation’s written scheme should be used when deciding what investigation or remedial work is required.

When selecting a testing approach, establish what decision the result must support. For routine verification, consistency of method and sampling locations is important so that changes can be compared meaningfully. For an investigation, the priority may be to understand whether recoverable organisms are present, whether DNA remains detectable after treatment, or whether contamination is distributed across particular parts of the system. The laboratory should be able to explain the method’s limitations, the significance of its reporting units and any constraints on comparing results from different methods.

In practice, method selection is best documented within the water safety management process. A risk assessment identifies the system features and exposure risks; the sampling plan converts those findings into representative samples; and the chosen UKAS method supplies evidence within its defined technical limits. This approach prevents a rapid molecular result, a culture result or a combined result from being given more certainty than the method and the sample can support.