The position of HSE ACOP L8 on Legionella testing HSG274G

The HSE ACOP L8 guidance on the monitoring and control of Legionella is in 4 parts. A general part covers the management of water systems. Plus 3 technical guidance documents. Part 1: The control of legionella bacteria in evaporative cooling systems; part 2 on hot and cold water systems and part 3 on other water systems. The guidelines are based on the international standard ISO 11731. In fact, in the UK the Approved Code of Practice and guidance for the control of Legionella bacteria in water systems (ACOP L8) only supports the use of the method described in this standard, whose use is mandatory in the Legionella tests made for cooling towers

In the new Part 2 guidelines for hot and cold water systems ACOP introduce the use of alternative testing methods in so far as they are validated with the ISO 17994: Water quality – Criteria for establishing equivalence between microbiological methods.

The ISO 17994 describes criteria for establishing statistical equivalence between microbiological methods to compare the results obtained by one method with those results obtained by the reference method (in this case the ISO 11731). Since the ISO 11731 is based on counting CFU (colony-forming units), any other method of detection or quantification of any other Legionella property/units (e.g. the DNA – expressed in GU, Antigen, etc.), will give a different number and therefore cannot be defined as equivalent, even if these methods have been proven to detect Legionella better than the reference method.

In other words: the standard 17994 cannot be used to decide whether a method is valid for the detection of Legionella, or even if this method is better than the reference one.