Utility of combined detection of IgG/IgM/IgA vs. separate IgG and IgM antibodies to Anaplasma phagocytophilum in culture and PCR-confirmed patients with human granulocytic anaplasmosis

Summary Sera from 26 patients from New York State with culture and PCR confirmed human granulocytic anaplasmosis (HGA) were used to assess the performance of three different immunofluorescence assays (IFA), combined IgG/IgA/IgM and separate IgM and IgG to detect acute and/or convalescent antibodies to Anaplasma phagocytophilum . Convalescent samples were obtained within 6 weeks of the acute sample. An IFA titer ≥ 80 was considered positive. Overall, 22 acute and 21 convalescent samples were tested by all 3 IFA assays. On the acute phase serum sample, 14/22 (63.6%) were positive using the combined IgG/IgA/IgM assay, 10/22 (45.5%) by IgM, 12/22 (54.5%) by IgG and 13/22 (59.1%) by either IgM or IgG assays. Only 1 of 8 (12.5%) samples testing negative by the combined IgG/IgA/IgM assay was positive by an individual IFA testing (IgM IFA only). Of the 21 cases with a convalescent phase sample, 100% tested positive on the combined IgG/IgA/IgM, 20 (95.2%) on the IgG and 20 (95.2%) on the IgM IFA individual assays; all 21 (100%) tested positive by either the IgM or the IgG individual IFA assays. Titers ≥ 640 were found in the acute phase in 58.3–80%, and in the convalescent phase for 85%–95.2% of the positive samples for the 3 IFA assays assessed. In conclusion, the 3 IFA methods tested had comparable sensitivity on acute and convalescent phase serum samples of patients with HGA when using a cut-off of ≥ 80. Further studies should be conducted to determine the IFA cut-off values that offer the best sensitivity and specificity.