Serologic testing for antibodies is, at present, the only practical means of diagnosing Borrelia burgdorferi infection. However, specific IgM spirochetal antibodies may not be at detectable concentrations in the early stage of Lyme disease (within 6 weeks of onset). Western blot analysis may be used to confirm indirect fluorescent antibody staining or enzyme-linked immunosorbent assay results, but it is laborious and expensive. False-positive reactions caused by cross-reactivity do occur. In patients who have had subclinical Lyme disease and contract another illness that causes similar symptoms, interpretation of serologic test results may be confused by residual antibodies. Therefore, assay results should always be interpreted in conjunction with clinical and epidemiologic findings.