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Lyme borreliosis is a multisystem inflammatory disorder caused by the tick-borne spirochete Borrelia burgdorferi. Clinical manifestations are protean, involving the skin, joints, peripheral and central nervous systems, and the heart. However, the presentation of
Lyme disease often overlaps with that of other conditions. We describe four patients from a region endemic for
Lyme disease who had elevated levels of antibodies reactive to B burgdorferi and whose signs and symptoms were initially attributed to
Lyme borreliosis but whose subsequent blood cultures established a diagnosis of nonspirochetal subacute bacterial endocarditis. Although immunoblots on serum samples from three of the four patients were consistent with prior infection from B burgdorferi, a positive immunoblot does not establish active infection. Similarly, seropositivity to B burgdorferi only indicates possible exposure to this organism. The occurrence of positive serologies to B burgdorferi in the presence of other diseases can lead to diagnostic confusion.