A composite or bulk sample is where water from more than one asset is added to a bottle and submitted to the laboratory for testing. Since the 2020 service standards updates the LCA has stated that these samples should NEVER be taken.
The details on composite samples can be found in Information Box 3 of the Legionella Sampling and Testing Service Delivery Standard:
This guidance is in line with BS7592:2022, which makes it clear that the taking of composite or bulk samples is not best practice, stating that ‘Samples should not be combined from multiple outlets’ in both General Principles (section 5) and Sample Types (section 7.1.5).
The LCA position is that bulk or composite samples should never be taken as they can lead to under detection of issues. Misunderstanding how composite samples lower the limit of detection can lead to false confidence that the system controls are effective and the system is free from Legionella.
At best, a set of bulk or composite sample will lead to delay in confirming which assets are positive and impacted by Legionella. At worst, Legionella will remain undetected in the system leading to an infection or an outbreak.
If your client has asked you to undertake composite samples the LCA expects Members, being the specialist competent help, to advise their client that this is poor practice. It does not follow the British Standard (BS7592:2022) and does not comply with the requirements of the LCA Service Standard. The LCA Member should make every effort to make it clear to the client that bulk or composite sampling is poor practice. If the client fails to understand the implications and acknowledge the risk, then the LCA Escalation Procedure should be considered.