What type of sample should be collected to assess the Legionella level in the water supply?

Collecting a post-flush sample is the recommended practice for assessing Legionella levels in the water supply. This approach involves taking a water sample after flushing the system, which can provide a clearer picture of the Legionella concentration present in the plumbing system under typical conditions of use.

The rationale behind this method is that pre-flush samples can be influenced by stagnant water that may accumulate in the pipes, potentially leading to false positives or elevated results that don't accurately reflect the water quality during normal consumption. Conversely, mid-flush samples may not capture the true state of Legionella levels, as they only provide a snapshot of the water as it’s being flushed, rather than after the system has been thoroughly cycled.

By taking a sample after the flush, you ensure that the water has been running long enough to remove any stagnant water and allows for a more accurate representation of the Legionella concentration that would be encountered during regular use. This method aligns with best practices in water quality assessment and is crucial for effective Legionella management in plumbing systems.