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ISO / BSI · ISO 11731:2017 (replaced ISO 11731:1998 and ISO 11731-2:2004)

ISO 11731

Water quality. Enumeration of Legionella

Reference last checked 27 June 2026

What it is
The international standard laboratory culture method for isolating and counting Legionella in water samples - what an accredited lab follows to produce a CFU/L result.
Who it binds
A voluntary international standard (adopted in the UK as BS ISO 11731); the reference method behind your sample results.
What it covers
Sample concentration, culture on selective media, confirmation and enumeration of Legionella, reported as colony-forming units per litre (CFU/L).
Key references
Culture method by membrane filtration and direct plating; CFU/L results are read against the HSG274 action levels.
Edition
ISO 11731:2017 (replaced ISO 11731:1998 and ISO 11731-2:2004)
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Our explainers

Editions and clause references are confirmed against the publisher on the date shown; always check you are reading the current version. Nothing here is legal advice. See the control-parameters reference for the numbers, and all standards.

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"ISO 11731: Water quality. Enumeration of Legionella." Legionella.io, last reviewed 27 June 2026. https://legionella.io/standards/iso-11731/

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