What is the role of urinary antigen testing in the diagnosis of Legionella infection?

Updated: Nov 13, 2018
  • Author: Mobeen H Rathore, MD, CPE, FAAP, FIDSA; Chief Editor: Russell W Steele, MD  more...
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Answer

Diagnosis cannot be excluded when one or more of the following results are negative (a combination of tests increases the probability of confirming the diagnosis):

  • Urinary antigen test

    • This is the preferred initial test for Legionnaires disease.

    • The urine antigen test is a rapid, relatively inexpensive, and practical test for the detection of L pneumophila antigen excreted in the urine or present in pleural fluid.

    • The primary disadvantage of urinary antigen testing is that it detects only L pneumophila serogroup 1. However, because this serogroup causes most cases of Legionnaires disease, the test is recommended strongly as part of the workup.

    • Urine antigen testing has 70% sensitivity and approaches 100% specificity. [16]

    • Sensitivity improves if urine samples are concentrated by ultrafiltration and obtained within 7 days of the onset of pneumonia.

    • Test results may remain positive for weeks, even after appropriate antibiotic therapy.


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