When presented with a typical erythema migrans rash in a patient from a Lyme disease–endemic area, the early diagnosis of Lyme disease is relatively straightforward. Not all patients with Lyme disease present with a classic rash, however, and some rashes are misinterpreted as erythema migrans, especially in areas with limited clinical experience in the treatment of Lyme disease. In the past several years, a number of advances have been made in laboratory techniques to confirm early Lyme disease; these techniques improve on the spotty sensitivity and specificity that troubled earlier testing methods. Nowakowski and associates compared the sensitivity of different laboratory techniques for early diagnosis of Lyme disease in adults with erythema migrans.

They used the newest refinements available, including large-volume blood culture, two-stage serologic testing, conventional “nested” polymerase chain reaction (PCR), and a newer, quantitative PCR technique. Study participants were enrolled from a center specializing in treatment of Lyme disease, located in a Lyme disease–endemic area of New York. A total of 47 patients with clinically diagnosed Lyme disease (per surveillance criteria of the Centers for Disease Control and Prevention) and erythema migrans were evaluated by all four laboratory methods. Large-volume blood culture entailed an 18 mL specimen, and skin biopsy for PCR testing was done on a specimen obtained at the advancing border of the erythema migrans rash. Two-step serologic testing involved an initial enzyme-linked immunosorbent assay antibody screen, followed by a confirmatory immunoblot assay on acute- and convalescent-phase blood samples.

Quantitative PCR of a skin biopsy was the most sensitive test (81 percent) for Lyme disease, followed by the convalescent stage of the two-step serologic test (66 percent), conventional “nested” PCR (64 percent), large-volume blood culture (45 percent), and acute-stage serologic test (41 percent). All four testing methods were negative in about 6 percent of test subjects. Skin biopsies were more likely to be positive by PCR when samples were obtained soon after the onset of the erythema migrans rash.

The authors conclude that quantitative PCR of a skin biopsy obtained early in the course of the erythema migrans rash is the most sensitive laboratory method to confirm Lyme disease. They also note that such testing is usually not needed when patients with the characteristic rash present from a Lyme disease–endemic area.