Evaluation of an indirect hemagglutination test for Legionella pneumophila serogroups 1 to 4

Parallel testing of 895 sera by indirect hemagglutination and indirect fluorescent-antibody techniques showed 97.3% agreement. Although the indirect hemagglutination technique usually showed more cross-reactivity among serogroups than the indirect fluorescent-antibody technique with Formalin-fixed antigens and a conjugate which detected primarily immunoglobulin G antibodies, heterologous serogroup reactions were significantly lower than homologous serogroup titers and the etiological serogroup could be easily defined. The indirect hemagglutination techniques showed no cross-reactivity with a crude extract of Escherichia coli O13:K92:H4. Since the indirect hemagglutination technique was shown to detect both immunoglobulin M and immunoglobulin G antibodies and was found to be rapid, simple, and inexpensive, it appears to be an excellent alternative to the indirect fluorescent-antibody test for serodiagnosis of legionellosis.