What a legionella test tells you, and what it does not

Testing water for legionella means taking a sample from a defined point in a system, at a defined moment, and culturing it in a laboratory to see what grows. Everything useful about the result, and every limitation of it, follows from that sentence.

A result describes one point at one moment

A building water system is not homogeneous. Temperature varies by floor and by branch, flow varies with occupancy, and the biofilm that matters lives on surfaces rather than in the water passing over them. A negative result from a well-flushed tap on the ground floor tells you very little about a rarely used outlet on the top floor.

This is why a sampling plan is a technical document rather than a list of taps, and why the plan is worth more scrutiny than the laboratory. Where to sample is covered under sampling and analysis.

Pre-flush and post-flush answer different questions

A sample taken before running the tap reflects what is sitting in that outlet and its immediate fitting: the shower head, the flexible hose, the tap body. A sample taken after flushing reflects what is arriving from the system behind it.

Those are two different findings with two different remedies. One points at the outlet, the other at the plant. Which one a laboratory report describes should be recorded on the report, and if it is not, the report is ambiguous.

Culture takes days, and that is not a flaw

The reference method grows the organism, and growing takes time. Faster methods exist and are genuinely useful for screening or for tracking a trend, but they measure something different from a colony count and they are not interchangeable with it.

If a decision depends on the number, it depends on culture. If the question is whether something has changed since last month, a faster method may answer it sooner. The trade-off is set out in a later article in this series.

What the number is compared against varies

Action levels and thresholds are not universal. They come from the guidance or standard a facility operates under, from a state or local requirement, and sometimes from an insurer. The same count can require different responses in two buildings on the same street.

Anyone who quotes you a single universal action level, including a website, is simplifying something that has real consequences. What applies to your facility is established as part of the water management programme, before sampling rather than after the result arrives.

What we will not do with a result

We report what was found, where, and by which method. We do not tell you whether anybody is at risk, because that is a clinical judgement and we are not clinicians. If a result concerns you, it goes to your local health department and to whoever is responsible for occupant health at the facility.

If you are worried about a person rather than a building, stop reading and contact a doctor. That is the correct next step and it is not the one this website is for.