Not as well as you might think!
Legionella urine antigens are 60%-80% sensitive (>99% specific) for detecting L. pneumophila serogroup 1 which accounts for about 70%-80% of Legionnaire’s disease (LD) in the US1; there are at least 15 serogroups.2 So as many as 40% or more LD may be missed by urine antigen testing alone. 2
Urine antigen can be excreted as early 3 days after the onset of symptoms and can persist for >300 days which may present a problem in diagnosing a current illness in patients with recurrent pneumonia. 2 One study reported lowest sensitivity (80%) for antigen testing during days 4 to 7 days of symptoms.3
Other means of looking for Legionella include culture of respiratory samples for L. pneumophila which can detect all types of Legionella species (sensitivity 20%-80%) but has a lengthy turnaround time. Paired antibody testing may also be performed (sensitivity 70%-80%) in undiagnosed cases of severe pneumonia. 1
Take home point: Don’t depend totally on urine antigen testing to rule out LD.
Final fun fact: Did you know that legionellae survive in the aquatic environment by parasitizing free-living protozoa?
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- CDC. Legionellosis: United States, 2000-2009. MMWR 2011;60:1083-86. https://www.cdc.gov/mmwr/preview/mmwrhtml/mm6032a3.htm
- Fields BS, Benson RF, Besser RE. Legionella and Legionnaire’s disease: 25 years of investigation. Clin Micro Rev 2002;15:506-26. https://www.ncbi.nlm.nih.gov/pubmed/12097254
- Kohler RB, Zimmerman SE, Wilson E, et al. Onset and duration of urinary antigen excretion in Legionnaire’s disease. J Clin Microbiol 20:605-7. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC271393