Diagnosis of Legionnaires disease is somewhat complicated. L. pneumophila is a fastidious bacterium and is difficult to culture. In addition, since the bacterial cells are not efficiently stained with the Gram stain, other staining techniques, such as the Warthin-Starry silver-precipitate procedure, must be used to visualize this pathogen. A rapid diagnostic test has been developed that detects the presence of Legionella antigen in a patient’s urine; results take less than 1 hour, and the test has high selectivity and specificity (greater than 90%). Unfortunately, the test only works for one serotype of L. pneumophila (type 1, the serotype responsible for most infections). Consequently, isolation and identification of L. pneumophila from sputum remains the defining test for diagnosis.