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.