The Lyme immunoblot test is designed only as a confirmatory test, so it is important not to test screen-negative samples. Some antigens on the blot react with non-Lyme antibodies, and the immunoblot can be overinterpreted in the absence of a positive screening test. Current two-tiered serology has a sensitivity of 70% to 100% and specificity > 95% for disseminated Lyme disease; use of an immunoblot without a positive screening test is unwise. While the exact characteristics of current immunoblot tests used alone are not well defined, high false-positive IgM rates have been observed in patients tested without a prior enzyme immunoassay. This recommendation assumes that a patient has had the potential for contact with ticks in an endemic area.