Laboratory diagnosis of infections caused by legionellae.

Legionnaires' disease can be diagnosed by detection of antibody to the organism or by direct detection of the bacterium and/or its products using immunological means, culture or a DNA probe. Culture isolation, still the most specific and sensitive test, can be done with sputum samples if proper selective techniques are used. Antibody detection is more suited for epidemiological purposes than individual cases. It has been overused to the exclusion of more specific and rapid methods, such as culture and other means of bacterial detection. Immunofluorescent microscopy remains an important tool in diagnosis, although urinary antigen detection and DNA probes are promising alternative tests. For optimal sensitivity, culture must be performed, regardless of which test is used.