Figure 1. Routine Legionella testing: A multifactorial approach to performance indicator interpretation
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Figure 1. Routine Legionella testing: A multifactorial approach to performance indicator interpretation*°Δ
This figure has four separate sections that represent different factors of consideration for routine Legionella test results.
The first section relates to the way in which concentration indicates that Legionella growth appears uncontrolled, poorly controlled, or well controlled. Of note, examples are provided along a continuum with a gradient. Concentrations expressed as CFU/mL are for test results generated by traditional spread plate culture methods. If other test methods are used, consult testing lab or manufacturer’s instructions for appropriate interpretation.
The detection of greater than or equal to 10 CFU/mL in potable water or the detection of greater than or equal to 100 CFU/mL in non-potable water indicates that Legionella growth appears uncontrolled.
The detection of 1.0-9.9 CFU/mL in potable water or the detection of 10-99 CFU/mL in non-potable water indicates that Legionella growth appears poorly controlled.
The detection of any to 0.9 CFU/mL in potable water or the detection of any to 9 CFU/mL in non-potable water indicates that Legionella growth appears well controlled.
No Legionella detected also indicates that Legionella growth appears well controlled, and is a stronger indication that Legionella is well controlled than detection of any Legionella. No Legionella detected in multiple rounds of testing is a stronger indicator that Legionella is well controlled than if no Legionella is detected in a single round of testing. No Legionella detected in multiple rounds of testing with methods that detect viable and non-viable bacteria of any Legionella species is a stronger indicator that Legionella is well controlled than if no Legionella is detected in multiple rounds of testing with methods that are restricted to detection of a single species or only viable Legionella.
The second section relates to the way in which concentration change over time indicates that Legionella growth appears uncontrolled, poorly controlled, or well controlled. Of note, examples are provided along a continuum with a gradient.
A 100-fold or greater increase in concentration (for example, an increase from 0.05 CFU/mL to 5 CFU/mL) indicates that Legionella growth appears uncontrolled.
A 10-fold increase in concentration (for example, an increase from 0.05 CFU/mL to 0.5 CFU/mL) indicates that Legionella growth appears poorly controlled.
Steady Legionella concentration (for example, 0.5 CFU/mL for two consecutive sampling rounds) indicates that Legionella growth appears well controlled.
No Legionella detected also indicates that Legionella growth appears well controlled, and is a stronger indication that Legionella is well controlled than detection of any Legionella. No Legionella detected in multiple rounds of testing is a stronger indicator that Legionella is well controlled than if no Legionella is detected in a single round of testing. No Legionella detected in multiple rounds of testing with methods that detect viable and non-viable bacteria of any Legionella species is a stronger indicator that Legionella is well controlled than if no Legionella is detected in multiple rounds of testing with methods that are restricted to detection of a single species or only viable Legionella.
The third section relates to the way in which extent of colonization indicates that Legionella growth appears uncontrolled, poorly controlled, or well controlled. Of note, examples are provided along a continuum with a gradient.
Detection in multiple locations and a common source location‡ that serves multiple areas indicates that Legionella growth appears uncontrolled. Common source locations include water heaters, hot water returns, storage tanks, and cooling tower basins. Similarly, detection across many locations within a water system indicates that Legionella growth appears uncontrolled.
Detection in a common source location or detection in more than one location within a water system indicates that Legionella growth appears poorly controlled.
Detection in a few of many tested locations within a water system indicates that Legionella growth appears well controlled.
No Legionella detected also indicates that Legionella growth appears well controlled, and is a stronger indication that Legionella is well controlled than detection of any Legionella. No Legionella detected in multiple rounds of testing is a stronger indicator that Legionella is well controlled than if no Legionella is detected in a single round of testing. No Legionella detected in multiple rounds of testing with methods that detect viable and non-viable bacteria of any Legionella species is a stronger indicator that Legionella is well controlled than if no Legionella is detected in multiple rounds of testing with methods that are restricted to detection of a single species or only viable Legionella.
The fourth section relates to the way in which type of Legionella, such as species and serogroup, are associated with Legionnaires’ disease.¥ If a facility has a history of associated LD cases, then sequencing isolates obtained during routine testing may provide performance indicators regarding outbreak strain persistence (if that strain is detected.)
Legionella pneumophila serogroup 1 (Lp1), non-Lp1 Legionella pneumophila, and the presence of multiple different Legionella species or serogroups are highly associated with Legionnaires’ disease.
Any non-pneumophila Legionella species, including “blue-white” fluorescent Legionella species are less associated with Legionnaires’ disease.
* This figure is intended for use during routine testing only. Test results are performance indicators and are not a measure of risk of human illness. This figure is not intended for use if a building or device is associated with Legionnaires’ disease (LD) cases or an outbreak.
° See Routine testing for Legionella for guidance regarding suggested response activities. Comparable results may lead to different suggested response activities when other factors are considered (for example, if there is evidence of poorly controlled growth at a healthcare facility.)
Δ Considering the type of Legionella identified along with other performance indicators provides a clearer picture of water system control than the results of any single indicator. For example, facility owners and operators may consider implementing immediate interventions for a healthcare facility with: A. detectable but less than 10 colony-forming units per milliliter (CFU/mL†), B. non0Lp1 Legionella pneumophila, C. observed at steady concentrations, but D. detected at multiple locations including a central water heater.