Positivity for Legionella should be calculated based on results from which samples?

The correct response emphasizes the importance of focusing on samples collected specifically from the domestic water system. This is crucial because Legionella bacteria are typically associated with man-made water systems, particularly those that supply water for showering, heating, or other domestic uses.

By concentrating on samples from the plumbing system, you can acquire a more accurate assessment of potential Legionella presence in the environments where individuals may be exposed, such as faucets, showerheads, and other domestic water outlets. This approach aligns with risk management strategies designed to mitigate exposure in areas where the bacteria can proliferate and spread.

While samples from any water source may provide information about water quality or contamination in a broader context, the focus for Legionella assessments needs to be on systems that could lead to human exposure. This is why other options—like non-potable water sources, specific recreational facilities like hot tubs and cooling towers, or broader analyses of all water sources—do not directly address the specific risks associated with domestic water systems. Such alternatives might not yield relevant data pertinent to typical environments where Legionella exposure occurs.