ACCURACY OF IGM IMMUNOBLOTTING TO CONFIRM THE CLINICAL-DIAGNOSIS OF EARLY LYME-DISEASE

Background: A 2-test approach for the serologic diagnosis of Lyme dise ase has recently been proposed. A positive or equivocal result on a fi rst-stage test leg, an enzyme immunoassay) is followed by a Western im munoblot test. For a sample to be considered seropositive for Lyme dis ease, the immunoblot result must be positive. Objectives: To assess th e accuracy of IgM immunoblotting for detection of early Lyme disease a nd to establish interpretative criteria for a commercially available i mmunoblot assay. Methods: Serum samples from 44 patients with erythema migrans were tested by an IgM immunoblot assay. All patients were cul ture-positive for Borrelia burgdorferi. Serum samples from 2 different control groups were also tested. Interpretative criteria were develop ed using receiver operating characteristic curves. Results: The presen ce of any 2 IgM bands was found to be the optimal criterion for a posi tive test result, and in patients with illness of less than 7 days' du ration, this was significantly more sensitive than the criterion of an y 2 of the 3 specific bands defined by the Centers for Disease Control and Prevention/Association of State and Territorial Public Health Lab oratory Directors Lyme Disease Workgroup (P<.05). Specificity of the c riterion of any 2 bands was 100% for 1 group of controls but only 96% for the more clinically relevant control group; this small difference had a large impact on the positive predictive value in populations at low risk for Lyme disease. Conclusions: Using a commercially available immunoblot test kit, the presence of any 2 IgM bands is proposed as a positive result. The predictive value of a positive IgM immunoblot re sult, however, is poor in patients with minimal clinical evidence for Lyme disease.