Laboratory Testing for Legionella

Key points

• Clinical laboratories can provide diagnostic testing for Legionella infections using urinary antigen testing (UAT), culture, or molecular methods.
• UAT typically detects only one type of Legionella, although infections with other types occur.
• CDC recommends Legionella UAT paired with lower respiratory specimen testing by culture or molecular methods for diagnosis of Legionnaires' disease.
• Learn the advantages and disadvantage of each diagnostic test.

Preferred methods

The preferred diagnostic tests for Legionnaires' disease are both of the following paired together:

• Culture or molecular testing of lower respiratory specimens to detect a range of Legionella types
• Legionella UAT for Legionella pneumophila serogroup 1

Lower respiratory specimens include sputum, bronchoalveolar lavage, tracheal aspirate, and lung tissue.

Importance of testing a lower respiratory specimen

Collecting and testing lower respiratory specimens is important to enable detection of all species and serogroups of Legionella. Both culture and many molecular tests can detect Legionella species and serogroups that the UAT doesn't. UAT typically only detects L. pneumophila serogroup 1. Additionally, molecular tests may be more sensitive than culture for detecting Legionella infection after the start of antibiotic treatment.

Culture allows for comparison of clinical and environmental isolates during an outbreak investigation.

Keep ReadingMethods for Legionella Testing and Specimen Collection

Benefits of the urinary antigen test

The UAT is the most commonly used laboratory test for diagnosis of Legionnaires’ disease. It detects a piece of the Legionella bacterium (known as an antigen) in urine. It can detect Legionella infection in some cases for days to weeks (or longer, under rare circumstances) after treatment.

Sensitivity and specificity

Sensitivity varies depending on the following:

• Quality and timing of clinical specimen collection
• Technical skill of the laboratory worker performing the test

The following table provides general ranges for the sensitivity and specificity of each diagnostic test.

Test

Sensitivity (%)

Specificity (%)

Culture

20–80

100

Urinary antigen for L. pneumophila serogroup 1

70–100

95–100

Polymerase Chain Reaction (PCR)

95–99

>99

Direct Fluorescent Antibody (DFA) Stain

25–75

>95

Paired serology

80–90

>99

Advantages and disadvantages

The following table describes advantages and disadvantages for each diagnostic test.

Test

Advantages

Disadvantages

Nucleic Acid Amplification Test

• Can be performed on pathologic specimens (usually lung tissue)
• Rapid and sensitive
• Some tests detect species and serogroups other than Lp1
• Confirmatory laboratory evidence
• Assays vary by laboratory

Culture

• Detects all species and serogroups
• Yields clinical and environmental isolates that can be compared
• Confirmatory laboratory evidence
• Slow (up to 14 days to grow)
• Sensitivity is highly variable
• Affected by appropriate antibiotic treatment
• Requires specialized media, which some laboratories may not have readily available and can be difficult to obtain

Urinary Antigen

• Rapid
• Confirmatory laboratory evidence
• Can only be used to detect L. pneumophila serogroup 1
• Does not allow for molecular comparison to clinical or environmental isolates

DFA

• Can be performed on pathologic specimens (usually lung tissue)
• May detect species and serogroups other than Lp1
• Supportive laboratory evidence
• Technically difficult
• Reagents may be difficult to obtain

Serology

• May detect species and serogroups other than Lp1
• Confirmatory laboratory evidence for Lp1; supportive for other types of Legionella
• Must have paired sera collected at acute onset to 2 weeks after symptoms appear and 3 to 6 weeks later
• Single results cannot be interpreted as indicative of Legionnaires’ disease
• Not timely for clinical decision making

Specimen collection

Best practice is to obtain both lower respiratory specimens and urine concurrently.

It's ideal to obtain the lower respiratory specimens prior to antibiotic administration. However, antibiotic treatment shouldn't be delayed to facilitate this process. Lower respiratory specimen collection can occur after antibiotic administration begins.

Submitting specimens

Public health department laboratory staff can forward approved specimens or isolates to CDC or the Legionella Reference Center for specialized testing.

Keep ReadingSubmitting Specimens for Legionella Testing

Incorrect rejection criteria

Laboratories sometimes reject lower respiratory specimens during a work-up for pneumonia based on specimen quality. Issues can include a lack of white blood cells in the sample or contamination with other bacteria.

However, laboratories shouldn't reject lower respiratory specimens for these reasons when working-up Legionnaires' disease because Legionella can often be recovered

Resources

Environmental Legionella Isolation Techniques Evaluation Program
Laboratories are able to test their proficiency at isolating Legionella from simulated environmental samples. This program issues documentation to those who pass the proficiency test.

Legionella Reference Center quick reference guide
This reference center accepts testing requests from governmental public health or environmental laboratories. It performs Legionella testing on clinical specimens and isolates, as well as environmental isolates and outbreak samples.

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