It is the responsibility of the ordering physician to see that both acute and convalescent specimens are obtained. The physician should arrange for the collection of the convalescent serum 14 days after the acute is collected.
A negative antibody response does NOT rule out infection with Legionella pneumophila serogroups 1-6. False negative results may occur when samples are drawn too early after onset. It may take up to 9 weeks postinfection for seroconversion. False negative results may also occur due to the lack of antibody acquisition. Only 80% of L. pneumophila culture proven infections develop diagnostic changes in antibody titer. A positive antibody response may be due to cross reacting antibody found in patients with non-Legionella infections. Pneumonia and bacteremia caused by Pseudomonas species, Haemophilus species, Enterobacteriaceae, Bordetella species, Chlamydia, Rickettsia, Bacteroides species, M. tuberculosis and other mycobacteria, Citrobacter species, and Leptospirosis have been shown to cause false positive results in Legionella serology. Due to the background prevalence rate in some populations, a single positive serum titer cannot be construed to constitute a L. pneumophila infection. Therefore, paired sera analysis and Legionellaculture or urinary antigen testing should be performed to aid with diagnosis. Cross reactivity may occur with sera with infections due to other Legionella species.