Clinical utility of a monoclonal direct fluorescent reagent specific for Legionella pneumophila: comparative study with other reagents.

Twenty-four lower respiratory tract samples taken from patients with culture-confirmed Legionella pneumophila infection were examined with three different direct immunofluorescent antisera to L. pneumophila, as were 29 samples from similar sources taken from patients without Legionnaires disease. The reagents studied were Genetic Systems Corp. (GS) monoclonal L. pneumophila conjugate, which reacts with all known serogroups of L. pneumophila, BioDx polyvalent L. pneumophila serogroups 1 through 6 conjugate, and Centers for Disease Control polyvalent pool A L. pneumophila serogroups 1 through 4 conjugate. The specimens had been frozen at -70 degrees C for 0.5 to 5 years. Randomization was used in coding the samples, which were stained and read by an independent observer. All three conjugates correctly identified all positive and negative samples. No difference was noted among the conjugates in the absolute numbers of fluorescent L. pneumophila bacteria per sample. The GS conjugate had a much cleaner background than did the other two reagents. Mean staining intensity scores were 3.4, 3.9, and 3.7 for the GS, BioDx, and Centers for Disease Control conjugates, respectively. This study demonstrates that the diagnostic efficiency of all three conjugates is equivalent. Since the GS conjugate is easier to read, does not cross-react with non-L. pneumophila bacteria, and reacts with serogroups 1 through 10 of L. pneumophila, it appears to be preferable for use in diagnostic testing on nonhistopathologically processed specimens.