What factor can be indicated by comparing pre-flush and post-flush results in Legionella testing?

Comparing pre-flush and post-flush results in Legionella testing provides valuable insights into the impact of stagnation on Legionella levels within a plumbing system. Stagnation occurs when water sits idle in pipes for extended periods, which can create conditions conducive to the growth of Legionella bacteria. By measuring Legionella levels before and after flushing the system, one can identify how much the bacteria concentration changes as a result of the flushing process.

If there is a significant reduction in Legionella levels post-flush, it indicates that stagnant water, which may have harbored higher concentrations of bacteria, was effectively removed. This comparison serves as a practical assessment of how stagnation contributes to Legionella proliferation and emphasizes the importance of regular flushing or water management strategies in controlling bacterial levels.

The other factors presented do not encapsulate the main relevance of comparing pre-flush and post-flush results as accurately as the impact of stagnation. For instance, the effect of hot water on testing may influence Legionella survival or growth but wouldn't be directly measurable through this comparison. Similarly, a reduction in total water usage or the effectiveness of water purification systems does not directly involve stagnation in the same context and purpose as the stated comparison does in assessing bacterial levels.