Laboratory Tests for Legionnaires' Disease

A laboratorian working with an environmental sample in a biological safety cabinet. Image from CDC's page.

Clinical laboratories can diagnose Legionella infections with a urinary antigen test (UAT), culture, or molecular methods. The UAT usually detects only one type of Legionella, but infections with other types happen, so CDC recommends pairing tests.

The recommended pair

For diagnosing Legionnaires' disease, CDC prefers both of the following together:

• culture or molecular testing of a lower respiratory specimen — sputum, bronchoalveolar lavage, tracheal aspirate or lung tissue — to detect a range of Legionella types;
• the urinary antigen test for Legionella pneumophila serogroup 1 (Lp1).

Testing a lower respiratory specimen matters because culture and many molecular tests can detect species and serogroups the UAT misses; molecular tests may also be more sensitive than culture once antibiotics have started. Culture also produces isolates that can be compared with environmental samples during an outbreak investigation.

The UAT is the most widely used test. It detects a piece of the bacterium, an antigen, in urine, and can sometimes find infection days to weeks — rarely longer — after treatment.

How accurate are the tests?

Sensitivity depends on the quality and timing of specimen collection and the skill of the person running the test. General ranges:

Strengths and weaknesses

• Nucleic acid amplification (molecular) tests: fast and sensitive, usable on tissue such as lung, and some detect types other than Lp1; they count as confirmatory evidence, but assays vary between laboratories.
• Culture: detects all species and serogroups and yields isolates to compare with environmental samples, counting as confirmatory evidence; but it is slow (up to 14 days), its sensitivity varies widely, it is affected by antibiotic treatment, and it needs specialized media some laboratories lack.
• Urinary antigen: fast and confirmatory, but detects only Lp1 and yields nothing for molecular comparison.
• DFA stain: usable on tissue and may detect other types, but only supportive evidence, technically difficult, and its reagents can be hard to get.
• Serology: may detect other types and is confirmatory for Lp1 (supportive for others), but needs paired blood samples — one at onset to 2 weeks after symptoms start and another 3 to 6 weeks later — so it is too slow for treatment decisions, and a single result cannot indicate Legionnaires' disease.

Collecting and handling specimens

• Collect lower respiratory specimens and urine at the same time.
• Ideally collect respiratory specimens before antibiotics, but never delay treatment to do so; collection can happen after antibiotics start.
• Laboratories sometimes reject lower respiratory specimens for pneumonia work-ups because of too few white blood cells or contamination with other bacteria. They should not reject them for these reasons when testing for Legionnaires' disease, because Legionella can often still be recovered.
• Public health laboratories can forward approved specimens or isolates to CDC or the Legionella Reference Center for specialized testing.

CDC's Environmental Legionella Isolation Techniques Evaluation program lets laboratories test, and document, their proficiency at isolating Legionella from simulated environmental samples.

Sources

• Centers for Disease Control and Prevention: "Laboratory Testing for Legionella."