Immunoglobulin M antibody titers in the diagnosis of Legionnaires disease

The purpose of this study was to determine whether measurement of immunoglobulin M (IgM) antibodies against Legionella pneumophila serogroup 1 can aid in the diagnosis of Legionnaires disease. On the basis of measurements of antibody levels in 1,942 control sera, we used an IgM titer of 1:256, observed in 2.3% of the controls, as presumptive evidence of Legionnaires disease. Measurement of IgM titers permitted us to presumptively or definitively diagnose Legionnaires disease in 13 of 34 patients earlier than we would have if only IgG titers had been measured. Of the 13 patients, 5 were diagnosed serologically only by IgM antibody determination. IgM titers were presumptively diagnostic in week 1 of clinical symptoms in 4 of the 13 patients. We conclude that conjugates used for antilegionella indirect fluorescent-antibody tests should be capable of detecting IgM antibodies so that the value of serological results in diagnosing and managing Legionnaires disease will be maximized.