When a lab quote lands on your desk offering culture, PCR, or both, the question underneath it is simple: which result actually answers what you need to know about your water? The two methods are not rival brands of the same test. They measure different things, on different timescales, and a result that reassures you under one can be ambiguous under the other.
Culture grows whatever live Legionella was in the sample and counts it. PCR looks for Legionella DNA and finds it fast - including DNA from bacteria that are already dead. Get that one distinction straight and the choice between them, and when to use each, becomes much easier.
What each method is really measuring
Legionella testing in a laboratory splits, broadly, into two approaches.
Culture is the long-standing reference method. A sample is incubated on a growth medium, any viable Legionella forms visible colonies, and the result is reported as colony-forming units per litre (cfu/L) [4]. Because it grows living organisms, a culture count is the closest thing to a direct measure of how much live Legionella was present in that sample. The trade-off is time: colonies take days to develop, so a result commonly takes around ten days or more to come back. (For what actually happens to your sample once it reaches the bench, see Inside the lab: how Legionella culture testing works.)
PCR takes a different route. Instead of growing the organism it amplifies and detects Legionella DNA, so it can confirm presence - and often the species - within a day or two [4]. Speed is the obvious appeal. The catch is that DNA lingers after cells die, so PCR on its own cannot tell you whether the Legionella it found was alive and able to cause harm. A positive PCR straight after a disinfection may simply be reading the genetic debris of the bacteria you have just killed.
Where sampling fits at all
One thing to settle before you compare methods: for most hot and cold water systems, routine sampling is not the default control. HSE guidance treats testing as something driven by the risk assessment rather than a fixed calendar task, and is clear that it is not a routine requirement for these systems [1][2]. Sampling earns its place when you cannot reliably hold your control parameters, after significant works, or during an investigation. Get that order wrong and you end up paying for tests that prove very little.
How the sample is taken matters as much as the method that analyses it. BS 7592 sets out the code of practice for sampling for Legionella - where, when and how to take samples so the result reflects the system rather than the person holding the bottle [3]. A pre-flush versus a post-flush sample, the volume drawn, and whether a neutraliser was used can all change the number that comes back.
Culture vs PCR at a glance
The table below is the quick comparison. Treat the turnaround times as general guidance rather than a promise - confirm them with whichever laboratory you use.
| Decision axis | Culture | PCR |
|---|---|---|
| What it detects | Live, growable Legionella | Legionella DNA (live or dead cells) |
| Typical result unit | Colony-forming units per litre (cfu/L) | Gene copies / genomic units per litre |
| Turnaround | Slower - commonly around ten days or more | Faster - often within a day or two |
| What a positive proves | Viable bacteria were present, and at what level | DNA was present; viability not confirmed |
| Best suited to | Routine verification, defensible evidence | Rapid screening, investigation, ruling areas out |
| Main limitation | Slow; some strains are hard to culture | Cannot confirm the organism is alive |
Which to choose, and when
For routine verification - confirming that a system you already control is staying controlled - culture remains the workhorse. It gives a quantified, defensible count, and because it measures viable organisms it maps directly onto the question a duty holder gets asked: was there live Legionella, and how much? Make sure the analysis is done by a laboratory accredited to a recognised standard; HSE guidance points duty holders towards accredited testing, and accreditation is one of the things a competent provider should be able to evidence on request [1]. (The Legionella Control Association Code of Conduct explained covers how to sense-check that a provider is genuinely competent and not just cheap.)
PCR comes into its own when speed changes a decision. After remedial cleaning and disinfection, a fast negative can support a quicker, evidence-led return to service while you wait for confirmatory culture. During an investigation - a suspected case, an unexplained pattern across a building - PCR can screen many outlets quickly and show you where to concentrate the slower, costlier culture work. Used that way, the two methods are complementary rather than competing: PCR narrows the field, culture confirms it.
Where PCR causes trouble is when it is treated as a like-for-like replacement for culture. A PCR-positive, culture-negative result is genuinely common after disinfection, and it is easy to misread as a failure when it may be the opposite. Decide in advance how you will act on each combination of results, and write that decision into your written scheme before any samples are taken.
None of this substitutes for a competent, site-specific assessment. The right method, the sampling points, the action levels and what a given result obliges you to do all depend on your particular system and who uses it. A number on a lab report means nothing until someone competent decides what it proves and what it should trigger.
Your next step
Before you commission a single test, open your written scheme and answer one question: what result, from which method, at which outlet, would actually change what you do next? If you can answer that, you already know which test to buy and why. If you cannot, the gap is in your control scheme, not your laboratory - close that first, then decide whether culture, PCR or both will give you the proof you are missing.
FAQ
Does a fast PCR result mean I can skip culture?
Not usually. PCR confirms Legionella DNA is present, and it does so quickly, but it cannot tell you whether that DNA came from live bacteria. For verification that stands up to scrutiny, culture is still the reference method [4]. PCR is best used alongside culture to move faster, not instead of it.
Why did one outlet come back PCR-positive but culture-negative?
This happens often, especially after a disinfection. PCR detects DNA from dead cells as well as live ones, so it can flag the genetic traces of bacteria your treatment has already killed [4]. Read the two results together, and against your temperature and control data, rather than reacting to the PCR figure on its own.
Do I have to test my water for Legionella at all?
For most hot and cold water systems, routine sampling is not required. HSE treats it as something your risk assessment decides on rather than a default task, and it tends to be warranted where you cannot consistently meet your control parameters, after major works, or during an investigation [1][2].
Sources
[1] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm [2] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [3] BSI, “BS 7592:2022 - Sampling for Legionella bacteria in water systems. Code of practice”. https://knowledge.bsigroup.com/products/bs-7592-sampling-for-i-legionella-i-bacteria-in-water-systems-code-of-practice-1 [4] CDC, “Laboratory Testing for Legionella”. https://www.cdc.gov/legionella/php/laboratories/index.html