Routine Testing for Legionella

Purpose

Use this document to

1. Help analyze hazards and establish Legionella control measures per ASHRAE Guideline 12
2. Complement existing resources for
   1. Testing
   2. Sampling
   3. Water management programs (WMPs)
3. Support environmental assessment during public health investigations

Testing objectives

Legionella testing may be useful for

Facilities that may see benefit

Certain types of facilities may benefit from routine testing, including those

• That house or treat people at increased risk for Legionnaires' disease
• Unable to meet control limits consistently
• With a history of associated Legionnaires' disease cases
• That want additional information on their WMP performance

Sample collection

Before sampling, consider how results will be used in the broader context of a WMP.

Environmental assessment

Perform an environmental assessment to identify areas with increased risk of Legionella growth and spread. Consider the key factors for Legionella growth when assessing risk:

• Sediment and biofilm
• Temperature
• Water age
• Disinfectant residual

Sampling plan

Create a sampling plan that represents the entire building water system. Sampling location recommendations are included in the device-specific modules of this toolkit.

Sampling guidance

The volume of water you collect may depend on the

• Source type
  • Potable
  • Non-potable
• Water condition
  • Detectable disinfectant residual
  • Visible debris and no detectable disinfectant residual

Typically, a 250 mL sample is sufficient for routine testing. Larger sample volumes and other sample types, such as swabs or ice, may provide additional information for at-risk facilities.

Laboratory considerations

Some test methods may be performed onsite by the user or a qualified technician. Other methods require contracting with a commercial laboratory. For best results, follow instructions from the manufacturer or testing laboratory closely.

Consider the following when selecting a commercial laboratory:

• Accreditation
• Storage capability
• Testing capacity

Accreditation: Is the laboratory accredited to a recognized standard for routine Legionella test methods (such as ISO/IEC 17025)? There are regional, national, and international accrediting bodies.

Storage capability: Is the laboratory willing to save samples and Legionella isolates from samples for additional characterization?

Testing capacity: Can the laboratory perform additional Legionella characterization as needed by the submitter?

Regardless of the test method, understand the performance characteristics of the test the laboratory uses, including:

• Limitations
• Sensitivity
• Specificity

Consider testing for all Legionella species as multiple species are supported by similar environmental conditions.

Test method considerations

Test method may vary by the type of water system and the reason for testing.

The following can vary by lab even when using standard operating proceduresA:

• Plate types
• Result reporting
• Resuspension procedures
• Sample volumes processed

Traditional culture (spread plate)

• Detects viable bacteria
• Detects all Legionella species
• Reports results typically in colony forming units (CFU) per volume with limit of detection ~10 CFU/mL
• Yields isolate for additional characterization
• Reports results typically in 7–14 days
• Is subject to skill, experience, and procedural rigor of the laboratory
• May be preferred for evaluating growth trends

PCR

• Detects Legionella-specific DNA or RNA
• May not differentiate between live and dead bacteria
• Reports results typically in genomic units (GU) which is not directly equivalent to CFU
• Reports results typically in 2–48 hours
• Is useful for negative screening
• May be preferred for evaluating whether remediation was successful

Alternative and novel methods

• Should be validated against a standard method by a third party (e.g., ISO/IEC 13843)
• Should be verified by the laboratory (e.g., per ISO 17025)
• May detect only a subset of Legionella species or serogroups
• Reports results in hours or days
• Reports results in a variety of units or only qualitatively
• May be useful for repeated measurements when quick turnaround time is preferred

Test result considerations

Results of Legionella testing alone:

• Don't provide a measure of health risk
• Aren't predictive of disease

There is no "safe" level or type of Legionella.

Additional considerations for results

Results indicate the presence of Legionella within the sample only, as there's 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)
• Type of Legionella
  • Legionella pneumophila serogroup 1
  • Other species, serogroups, or sequence types

The presence of any Legionella should trigger response activities.

Level of response indicated by performance indicators

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

Continue the program if Legionella growth appears well controlled:

• ≤1 CFU/mL for potable water
• ≤10 CFU/mL for cooling towers

Implement suggested response activities if

• Conditions may allow for Legionella growth
• Growth appears poorly controlled
• Growth appears uncontrolled

Suggested response activities

Follow these steps when Legionella laboratory results aren't indicative of well-controlled growth:

1. Review for potential errors in sample collection, handling, and testing.

2. Examine equipment to confirm that system's in good working order and functioning as intended.

3. Review records to confirm that the WMP was implemented as designed (verification).

4. Review assumptions about operating conditions, such as physical and chemical characteristics of incoming water.

5. Re-evaluate the WMP, including analysis of

• Chemical treatment
• Cleaning
• Hazardous conditions
• Maintenance procedures
• Any other aspects that could affect Legionella testing

6. Adjust WMP as necessary to address any deficiencies identified.

7. Consider whether remedial treatment is needed only after completing prior steps.

8. Re-test: After performing any remediation, wait at least 48 hours after the system returns to normal operating conditions. Then test representative samples to confirm the response's effectiveness.

Considerations for immediate control measures

Consider implementing immediate control measures in facilities with populations at increased risk ifLegionella growth doesn't appear well controlled. These measures can protect people from exposure to water aerosols while implementing the guidance above.

Systems associated with disease

Whenever disease is associated with a water system, always review WMP verification and validation activities. Re-evaluate and revise the WMP if needed.

Are the WMP activities

• Occurring as intended? Verification
• Working as intended and effective for Legionella control? Validation

Resources

General public information: Routine Legionella testing in buildings without cases

Form: Legionella environmental assessment

Sampling: Procedure and potential sampling sites

Toolkit: Developing a water management program to reduce Legionella growth and spread in buildings

Training: Preventing Legionnaires' disease: A training on Legionella water management programs

Guidelines

ASHRAE Guideline 12 [Free read-only preview]

ASHRAE Standard 188 [Free read-only preview]