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Legionella Testing – Indicative Sampling – What is it?

When you take legionella samples, it’s normally a single location in a single bottle and that’s what our laboratories test.  But you can also undertake “indicative sampling” whereby you put up to 4 separate locations in a single bottle.

Is it allowed?

The bible document for this is HSG274 Legionnaires’ disease – Part 2: The control of legionella bacteria in hot and cold water systems. The first sentence of Microbiological Monitoring (page 40, para 2.119) says the following: ” Microbiological monitoring of domestic hot and cold water supplied from the mains is not usually required, unless the risk assessment or monitoring indicates there is a problem”.  So for many properties with simples systems and correct engineering, no sampling is necessary.  Whilst this is true, most property owners prefer to a belt & braces approach. So they do all the correct engineering and temperature monitoring – but still want to be reassured by a laboratory test. This is where indicative sampling come into play.

How do you choose which outlets to mix?

For most properties there are usually three areas where legionella are found in hot water supplies.

  • Production/Storage – this is usually your calorifier\water heater. If the temperatures are too low then legionella can survive.
  • Distribution – this is the pipework which extends from the production/storage all the way up to [but not including] the actual outlet.
  • Outlets – these are the hot taps, showers etc

The table below explains where it is appropriate to use bulked locations for indicative sampling.


Hot Water Source

What Outlets to Sample

What it means if NO legionella are found

This is the unit which produces your hot water.

It will usually be a calorifier, hot water cylinder, combination boiler or water heater.

The best places are drain valves or a sampling point on the flow pipework just above the stored water.

If there is no sampling point, the nearest DIRECT fed outlet can be used.

A maximum of 4 locations are permitted per sample bottle so this means you can bulk 2 or 3 units which are linked together.

Your hot water production is clear at the time of sampling.



There could still be legionella in the distribution pipework or taps (or both).


Distribution System

What Outlets to Sample

What it means if NO legionella are found

The pipework which carries the hot water around the building. Up to 4 direct fed outlets, which means outlets which are not  showers or fed by TMVs or thermostatic taps.

The outlets should encompass those nearest and furthest from the hot water source..

Examples would normally include taps in rooms such as Kitchen/Laundry/Sluice.

It’s unlikely there are any living legionella bacteria from the source of your hot water (calorifier, water heater etc) through your distribution pipework up to the point of entry into Shower mixers, TMVs or thermostatic taps.


It is possible that there may be legionella bacteria living downstream from TMVs, Thermostatic taps or Showers mixers.



What Outlets to Sample

What it means if NO legionella are found

These could be Showers, Taps fed by TMVs, Thermostatic Taps or even Rarely Used Outlets. It’s best to stick with one type of outlet, ie bulk 4 showers into one sample bottle or 4 rarely used outlets into one, etc. It indicates that your maintenance regime for cleaning showers or for flushing rarely used outlets is managing the legionella risk.

  • Don’t mix hot water samples with cold water samples.
  • Don’t mix multiple hot water systems together, keep them separate otherwise any positive results would not be particularly useful.

What if only 1 o f the 4 locations is positive?

It’s quite feasible that just one location out of the 4 is positive, so will the laboratory pick up this small amount of legionella.  For non-healthcare properties, concentrations of legionella below 100 cfu/litre are deemed low risk (or system under control). Consequently if only one location has this minimum level of legionella – it will be diluted down to 25cfu/litre by the other “clear” sample locations. That’s not a problem, our advanced screening methods can detect concentration below 25cfu/litre.

If there is a POSITIVE, will I have to resample again?

The results from the indicative sampling will highlight where the problem is, ie

  1. Production/Storage
  2. Distribution
  3. Outlets

With that knowledge, you can investigate to establish which legionella measures are failing and rectify them, then re-sample. If the legionella count is high, more immediate steps will be required such as a disinfection.

Indicative Sampling – what’s the conclusion

In most cases individual sampling is required so that there is no ambiguity with the result. This is especially true if they may need to be used as evidence in a court of law.  But there are numerous instances where indicative sampling saves both time and money. If you would like to learn more, contact AquaCert on  FREEPHONE 0800 048 1737