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Sera from 665 apparently healthy dogs were examined for antibodies to the
Lyme disease spirochete Borrelia (B.) burgdorferi by using an ELISA with a whole cell sonicate of B. burgdorferi sensu stricto reference strain B31 (ATCC 35210) as antigen. To discover false positive reactions due to the unsatisfactory specificity of conventional enzyme-linked immunosorbent assays for B. burgdorferi, sera were absorbed in parallel with both B. burgdorferi and a heterologous sorbent consisting of whole cells of Escherichia coli, Salmonella typhimurium, and eight serovars of Leptospira interrogans. The difference between optical densities obtained in the ELISA after serum absorption with the heterologous sorbent and the B. burgdorferi sorbent was defined as a new value, "ODdiff", for ELISA reactivity specific for B. burgdorferi. ELISA results were confirmed by immunoblot studies. By testing unabsorbed sera, 48 of 665 serum samples (7.2%) were considered ELISA positive. 37 of these 48 sera (77.1%) were apparently false positive: here a similar reduction of ELISA reactivity was obtained after absorption with B. burgdorferi antigen and with the heterologous sorbent (ODdiff approximately equal to 0). None of these 37 sera gave immunoblot patterns characteristic for canine B. burgdorferi infection.