The urine antigen test for detection of Legionnaire’s disease (LD) can remain positive for weeks or months after initial infection. So a positive test in a patient with pneumonia may not just be suggestive of an acute infection but also the diagnosis of LD during recent weeks or months (1,2).
In a study of Legionella urine antigen detection as a function of days after onset of symptoms, 11 of 11 (100%) patients tested remained positive after day 14 (1). In the same study, 10 of 23 (43%) patients excreted antigen for 42 days or longer following initiation of therapy, with some patients remaining positive for more than 200 days!
In another study involving 61 patients with Legionella pneumophila pneumonia, 25% excreted Legionella antigen for 60 or more days (2). Longer duration of antigen excretion was significantly associated with immunosuppressed patients in whom the time to resolution of fever was > 72 h.
The long duration of excretion of Legionella antigen in urine following LD is not surprising. Pneumococcal pneumonia has also been associated with prolonged antigen excretion, some for as long as 6 months after diagnosis of pneumonia (3). It is thought that some microbial polysaccharides may be degraded very slowly or not at all by mammalian tissues which could explain their prolonged appearance in the urine long after active infection has resolved (1).
Bonus pearl: Did you know that the sensitivity of Legionella urinary antigen for LD varies from 94% for travel-associated infections to 76%-87% for community-acquired infection, and to as low as ~45% for nosocomially-acquired infections (4)?
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- Kohler RB, Winn WC, Wheat J. Onset and duration of urinary antigen excretion in Legionnaires disease. J Clin Microbiol 1984;20:605-7. https://pubmed.ncbi.nlm.nih.gov/6490846/
- Sopena N, Sabria M, Pedro-Bolet ML, et al. Factors related to persistence of Legionella urinary antigen excretion in patients with legionnaire’s disease. Eur J Clin Microbiol Infect Dis 2002;21:845-48. https://europepmc.org/article/med/12525918
- Andreo F, Prat C, Ruiz-Manzano J, et al. Persistence of Streptococcus pneumoniae urinary antigen excretion after pneumococcal pneumonia. Eur J Clin Microbiol Infect Dis 2009;28:197-201. https://pubmed.ncbi.nlm.nih.gov/18830727/
- Helbig JH, Uldum SA, Bernander S, et al. Clinical utility of urinary antigen detection for diagnosis of community-acquired, travel-associated, and nosocomial Legionnaire’s disease. Clin Microbiol 2003;41:838-40. https://pubmed.ncbi.nlm.nih.gov/12574296/
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