We determined the frequencies of antibodies to Anaplasma phagocytophila, the agent of human granulocytic ehrlichiosis (HGE), in different groups of adults and children from Westchester County, New York. The groups included 159 adult blood donors and 215 children who were seronegative for Borrelia burgdorferi antibodies, 118 adult patients and 57 children who were seropositive for B. burgdorferi antibodies, and 42 adult patients with culture-confirmed erythema migrans. Eighteen (11.3%) of the blood donors and 11 (5.1%) of the B. burgdorferiseronegative children were found to have A. phagocytophila antibodies by indirect immunofluorescent-antibody assay (IFA). Nine of 42 (21.4%) patients with culture-confirmed erythema migrans tested at the baseline visit, 42 of 118 (35.6%) adults, and 3 of 57 (5.3%) children whose sera were reactive for B. burgdorferi antibodies also tested positive for A. phagocytophila antibodies. The geometric mean titer ranged from 219 to 315 for all groups, and the differences in titers among the groups were not statistically significant. Only one-third of the healthy blood donors reactive by IFA were confirmed to be positive by immunoblotting. We conclude that a significant proportion of adults and children without clinical evidence of HGE will test positive for A. phagocytophila antibodies when the conventional cutoff titer of 80 is used in the IFA. This information must be considered in interpretation of test results.Human granulocytic ehrlichiosis (HGE) is an emerging tickborne disease first described in 1994 in the midwestern United States (5, 7). The etiologic agent is an ehrlichial species closely related to Ehrlichia phagocytophila. It is often referred to as the HGE agent and was recently named Anaplasma phagocytophila (11). Since the original report, increasing numbers of cases of infection with this agent have been diagnosed in many of the same geographic areas where Borrelia burgdorferi infection is endemic (1,5,7,8). Both infections are transmitted by the same Ixodes tick vectors (12).Antibody testing by an indirect immunofluorescent-antibody assay (IFA) is the test most commonly used in the laboratory to confirm a diagnosis of HGE. We and others have shown that approximately 90% of acutely infected patients are seropositive by serologic testing during the acute or convalescent phase (Յ30 days postinfection) (3, 7).An antibody titer of Ն64 or Ն80 by IFA is considered a positive result (10). However, few studies exist on the rate of background seropositivity in areas of endemicity, and it is unclear if this cutoff is the most appropriate threshold for seropositivity. Indeed, evidence exists that this titer is too low. A study by Bakken et al. (6) of 475 individuals from Wisconsin without evidence of active tick-borne infection or a prior diagnosis of HGE found a seropositivity rate of 14.9%, and the patients usually had titers Յ320. While such low titers occur at surprisingly high frequencies in individuals in the general population, in our experience patients acutely infected an...