New criteria for immunofluorescence assay for Q fever diagnosis in Japan

Abstract

A study was made to evaluate the cutoff value of indirect immunofluorescent-antibody (IFA) test for Q fever diagnosis in Japan. We used 346 sera, including 16 from confirmed Q fever cases, 304 from Japanese pneumonia patients, and 26 from negative cases. Thirteen sera from the confirmed Q fever cases with an immunoglobulin M (IgM) titer of > or =1:128 and/or IgG titer of > or =1:256 by the IFA test were positive by both enzyme-linked immunosorbent assay (ELISA) and Western blotting assay (WBA), whereas 298 sera from pneumonia patients and 26 negative sera with an IgM titer of < or =1:16 and an IgG titer of < or =1:32 by the IFA test were negative by both ELISA and WBA. In the proposed "equivocal area," with an IgM titer of > or =1:32 and < or =1:64 and/or an IgG titer of > or =1:64 and < or =1:128, we found 9 sera, 3 from confirmed Q fever cases and 6 from Japanese pneumonia patients, by the IFA test. Three sera from the confirmed Q fever cases and one of the sera from pneumonia patients were IgM and/or IgG positive by both ELISA and WBA. These results suggest that a single cutoff value for the IFA test may cause false-positive and false-negative results. In conclusion, this study showed that an "equivocal area" should be used for the IFA test rather than a single cutoff value and that sera in the equivocal area should be tested by additional serological assays for confirmation.

FIG. 1.

Determination of IgM and IgG titers to phase II C. burnetii in 346 serum samples by immunofluorescence assay. ★, acute-phase sera from confirmed Q fever patients; •, chronic- and convalescent-phase sera from confirmed Q fever patients; ▴, sera from Japanese pneumonia patients; ▪, negative sera.