What Do Our Legionella Test Results Mean?

If you’ve received your Legionella test results, you might have questions. What do the Legionella test results mean. Is the water safe with this level of Legionella in the water? What are the implications for your Legionella control plan and water management program (WMP)?

First, let’s be very clear that just having Legionella test results is not the end of the Legionella control process. The CDC makes this very clear by stating, “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.”

With that in mind, here are things to consider when reviewing your Legionella testing data:

• Legionella test results indicate the presence of Legionella within the sample only. There may be variability across water systems.
• Sample handling, transport, and lab processing can affect Legionella test results.
• The presence of any Legionella in your test data should trigger response activities.
• It is possible to have Legionella in the water line but it may not be present in the sample.

Performance Indicators and Suggested Response for Routine Legionella Test Results

Below are general action steps based CDC’s guidelines for Legionella test results. Note that “do nothing” is not an option in any case. It’s important that you consult with Legionella Control Systems and have, at a minimum, a water management program (WMP, also called a defensible water management plan) that will protect your occupants and reduce your liability.

Concentrations of Legionella Indicating Growth Risk

If ≤1 CFU/mL for potable water or if ≤10 CFU/mL for cooling towers, Legionella growth appears well controlled.
Action: Continue Water Management Program (WMP) as provided by Legionella Control Systems

If ≥1 CFU/mL for potable water or if ≥10 CFU/mL for cooling towers, conditions may allow for Legionella growth.
Action: Implement suggested Legionella control response activities as provided by Legionella Control Systems

If ≥10 CFU/mL for potable water or if ≥100 CFU/mL for cooling towers, Legionella growth appears to be uncontrolled.
Action: Implement suggested Legionella control response activities as provided by Legionella Control Systems

Change of Concentrations of Legionella Concentration Indicating Growth Risk

If Legionella concentration remains steady over time, Legionella growth appears to be well controlled.
Action: Implement suggested Legionella control response activities as provided by Legionella Control Systems

If 10-fold increase in concentration for potable water or 100-fold increase in concentration for cooling towers, Legionella growth appears to be poorly controlled.
Action: Implement suggested Legionella control response activities as provided by Legionella Control Systems

If >100-fold increase, Legionella growth appears to be uncontrolled.
Action: Implement suggested Legionella control response activities as provided by Legionella Control Systems

As noted, Legionella Control Systems will recommend the Legionella control response activities that are appropriate for your facility. Learn more about those possible response activities for Legionella control and contact Legionella Control Systems for more information about Legionella testing and Legionella control.