Culture and other direct detection methods to diagnose human granulocytic anaplasmosis

We sought to assess the performance of 3 laboratory tests on blood specimens for direct detection of Anaplasma phagocytophilum, the cause of human granulocytic anaplasmosis (HGA), in patients tested at a single medical institution in New York State.

Direct tests included microscopic blood smear examination for intragranulocytic inclusions, polymerase chain reaction (PCR), and culture using the HL-60 cell line. The HGA cases testing positive by only 1 direct test were not included, unless HGA was confirmed by acute or convalescent serology using an indirect immunofluorescent assay.

From 1997 to 2009, 71 patients with HGA were diagnosed by at least 1 of the 3 direct test methods. For the subgroup of 55 patients who were tested using all 3 methods, culture was positive for 90.9% (50/55) vs 81.8% (45/55) for PCR vs 63.6% (35/55) for blood smear (P =.002). Most cultures (79.3%) were detected as positive within 1 week of incubation.

Although using culture to detect A phagocytophilum is likely not amenable for implementation in most hospital laboratories, in our experience, culture had the highest yield among the direct tests evaluated.