Water Management Program Performance Indicators

Test Results for Water Management Program Performance

Results of Legionella testing alone do not provide a measure of health risk and are not predictive of disease. There is no “safe” level or type of Legionella. Additional considerations for results are:

  • Results indicate the presence of Legionella within the sample only, as there is variability across water systems.
  • Sample handling, transport, and lab processing can affect results.
  • Results have been interpreted based on concentration (e.g., CFU/mL), extent of colonization (e.g., % positive), and type of Legionella (e.g., Legionella pneumophila serogroup 1 vs other species, serogroups, or sequence types).
  • The presence of any Legionella should trigger response activities.

Performance Indicators and Suggested Response for Routine Legionella Test Results

If test results are expressed in units other than CFU/mL, consult the testing laboratory or test manufacturer for appropriate result interpretation.

  • If ≤1 CFU/mL for potable water or If ≤10 CFU/mL for cooling towers, Legionella growth appears well controlled
    • Continue Program
  • If >1 CFU/mL for potable water or if >10 CFU/mL for cooling towers, conditions may allow for Legionella growth
    • Implement Suggested Response Activities
  • If 10 to 100-fold increase for potable water or cooling towers, Legionella growth appears to be poorly controlled
    • Implement Suggested Response Activities
  • If >100-fold increase, Legionella growth appears to be uncontrolled
    • Implement Suggested Response Activities

                                                                                                  This page last updated 10/20/2021.