The gold standard for anaplasmosis diagnosis is PCR on whole blood in the first week. IgG antibody titers by IFA in paired serum samples (first week and 2-4 weeks later) are positive also.
Identification of morulae in granulocytic leukocytes and a single positive IFA titer are considered diagnostic. The morulae are deep blue to grey-blue coccobacilli, arranged in spherical groups within the cytoplasm of granulocytes. In a retrospective study of 14 confirmed cases, morulae were identified in 11 (79%) cases before counting 100 granulocytes. The remaining three cases (21%) had morulae identified when 200 granulocytes were examinined. More morulae are seen in anaplasmosis than in erhlichiosis.
Immunohistochemical (IHC) staining in a biopsy or autopsy samples is diagnostic. Culture of A. phagocytophilum from a clinical specimen can be performed in HL60 cell culture.
Clinicians should consider coinfections with Babesia or B. burgdorferi.