False Positive Lyme Disease IgM Immunoblots in Children.

In our cross-sectional sample of 7289 serologic tests for Lyme disease, we identified 167 instances of a positive IgM immunoblot but a negative IgG immunoblot test result. Considering that only 71% (95% CI 64%-78%) of patients had Lyme disease, a positive IgM immunoblot alone should be interpreted with caution to avoid over-diagnosis of Lyme disease.