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?

Liked this post? Download the app on your smart phone and sign up below to  catch future pearls right into your inbox, all for free!

References

1. CDC. Legionellosis: United States, 2000-2009. MMWR 2011;60:1083-86. https://www.cdc.gov/mmwr/preview/mmwrhtml/mm6032a3.htm
2. 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  
3. 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