Laboratory diagnosis of Legionnaires' disease: The expanding role of the Legionella urinary antigen test

Abstract

Introduction Legionnaires’ disease is pneumonia, caused by Legionella species. At least 39 Legionella species have been identified, but L. pneumophila causes approximately 90% of the reported cases of Legionnaires’ disease in the U.S. (1). Legionnaires’ disease has been aptly described as under-diagnosed but overtreated (2). The Centers for Disease Control and Prevention in Atlanta, GA (CDC) estimates that 18,000 to 25,000 cases of Legionnaires’ disease occur annually in the U.S. However, these figures underestimate the true incidence of Legionnaires’ disease (I ). Cases of Legionnaires’ disease go unrecognized because it is not possible to clinically distinguish Legionnaires’ disease from other causes of pneumonia, and the diagnostic tests for Legionella are not ordered routinely. The tests currently available for making the diagnosis of Legionnaires’ disease include culture isolation, visualization of the bacterium by direct fluorescent antibody (DFA) staining of respiratory secretions, serum antibody testing, and Legionella urinary antigen testing. The reported sensitivities of these tests vary widely depending on the study. The sensitivity of culture, DFA, serology, and the urinary antigen test has been reported to be between 40 to 80,25 to 75,40 to 75, and 70 to 90%. respectively (3-5). These tests should be performed by experienced microbiologists to achieve optimal results. Since many laboratories have inadequate experience with these tests, specimens for Legionella testing are often sent to a reference laboratory. Those laboratories that do perform Legionella diagnostic testing in-house may not do it well. A College of American Pathologists survey of laboratories showed that only 32% of laboratories successfully identified a pure culture of L. pneumophila (3). At the V.A. Medical Center Special Pathogens Laboratory, Pittsburgh, PA, we have found culture to be the most sensitive diagnostic method. We optimize the recovery of Legionella from culture by using multiple selective media and acid-buffer pretreatment of sputum specimens, but for many smaller laboratories this approach may be impractical (6). For pneumonia caused by L. pneumophifa serogroup 1, the urinary antigen test is a practical alternative to culture for the diagnosis of Legionnaires’ disease. The urinary antigen test has several advantages over culture. For many patients with Legionnaires’ disease, obtaining an adequate sputum specimen is difficult; urine, on the other hand, is easily obtained. The results of urinary antigen testing can be available within hours, whereas culture results require three to five days. The disadvantage of the urinary antigen test is that it is specific for L. pneumophila, serogroup 1 only. Since the vast majority of cases of Legionnaires’ disease are caused by this species and serogroup, this limitation has not been considered a major disadvantage of the test.