Antibodies to Anaplasma phagocytophilum in Patients with Human Granulocytic Anaplasmosis Confirmed by Both Polymerase Chain Reaction and Culture

Sera from patients from a single medical institution in New York State with human granulocytic anaplasmosis established by a positive polymerase chain reaction test (PCR) for Anaplasma phagocytophilum were used to assess the performance of serologic testing. All cases were also confirmed by culture in order to eliminate any false positive PCR samples. A nested PCR was performed targeting the heat shock operon of A. phagocytophilum. Culture was done using the HL-60 promyelocytic cell line. Serologic testing was performed to detect IgG/A/M using an indirect immunofluorescence assay that incorporated a human isolate of A. phagocytophilum as the source of the antigen. From 1997-2009, 38 human granulocytic anaplasmosis patients were evaluated. On the baseline serum sample 21 (55.3%; 95% CI: 38.3% to 71.4%) had a positive serologic test; 7 samples (33.1%) were positive at a titer of 80-320 and 14 samples (66.7%) at a titer of at least 640. Sixteen (94.1%) of the 17 with a negative baseline test had follow-up testing performed. All 16 tested positive on a convalescent phase serum sample obtained from 6 to 45 days later. PCR testing is the most commonly used direct diagnostic test to diagnose human granulocytic anaplasmosis. Our findings demonstrate that only approximately 55% of the PCR and culture positive cases were also seropositive on blood samples obtained at the same time point, indicating that serologic testing performed at the time of presentation has limited sensitivity. However, all of the 16 evaluable seronegative patients developed antibodies to A. phagocytophilum during convalescence.