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Subject: Legionella Antigen Screen
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Legionellosis refers to two clinical syndromes caused by bacteria of the genus Legionella—Legionnaires disease and Pontiac fever. Legionnaires disease is considered an atypical pneumonia. Legionella pneumophila is responsible for approximately 90% of infections. Most cases are caused by L. pneumophila, serogroup 1. Although a number of prominent clinical manifestations are distinctive for Legionella infection, none of them are pathognomonic or highly specific. Therefore, laboratory testing using specialized tests for Legionella should be considered for all patients hospitalized with community-acquired pneumonia.
Culturing for Legionella species is the single most important laboratory test. Urinary antigen testing is rapid, sensitive, and specific, but it is only useful for the diagnosis of L. pneumophila type 1 infection. The combination of culture of an appropriate respiratory specimen and urinary antigen testing is optimal as a diagnostic approach. Serologic tests are generally far less useful for the diagnosis of an individual patient. Although PCR-based tests exist, to date they do not exceed the sensitivity of culturing the organism.
Normal range: Negative.
In conjunction to culture for the presumptive diagnosis of past or current Legionnaire disease (L. pneumophila serogroup 1), patients suspected of health care–associated pneumonia, patients who have failed outpatient antibiotic therapy, and patients with a travel history within 2 weeks before the onset of illness.
Positive: Presumptive positive for L. pneumophila serogroup 1 antigen in urine, suggesting current or past infection.
Negative: Presumptive negative for L. pneumophila serogroup 1 antigen in urine, suggesting no recent or current infection. Infection due to Legionella cannot be ruled out, since other serogroups and species may cause disease, antigen may not be present in urine in early infection, and the level of antigen present in the urine may be below the detection limit of the test.
There is no single confirmatory laboratory test for Legionnaire disease. Culture results, serology, and antigen detection methods may all be useful, in conjunction with clinical findings, for diagnosis.
The Legionella antigen test will not detect infections caused by other L. pneumophila serogroups and by other Legionella species. Culture is recommended for suspected pneumonia to detect causative agents other than L. pneumophila serogroup 1 and to recover L. pneumophila serogroup 1 when antigen is not detected.
Excretion of Legionella antigen in urine may vary depending on the individual patient. Antigen excretion may begin as early as 3 days after onset of symptoms and persist for up to a year afterward.
A positive Legionella urinary antigen test result can occur due to current or past infection and, therefore, is not definitive for infection without supporting evidence.