Indirect fluorescent antibody test versus enzyme-linked immunosorbent assay and agglutination tests in the serodiagnosis of patients with brucellosis

Anti-Brucella IgG, IgM and IgA in sera from patients with blood culture positive for B. melitensis and controls were measured by indirect fluorescent antibody (IFA) test and the findings compared with those of enzyme-linked immunosorbent assay (ELISA) and microagglutination test (MAT). Brucella melitensis and B. abortus antigens from three vendors (BioMerieux, Wellcome and Oxoid) and from reference strains (Ames, Iowa) were used in IFA and MAT while a whole cell heat-killed B. melitensis antigen was used in ELISA. Statistical analysis showed comparable results when using B. melitensis or B. abortus antigen, in IFA, from the same manufacturer but there were subtle differences among antigens from different manufacturers. Correlation between IFA and ELISA titers was poor, due to differences in the levels of these titers. However, the percentage of sensitivity, specificity, predictive positive, and predictive negative at different titers indicated the most reliable discriminative titers to be as follows: ELISA IgG 1 : 800 (100% for all), IgM 1 : 400 (100%, 93%, 100%, 100%, respectively) and IgA 1 : 200 (95%, 100%, 100%, 94%, respectively); IFA IgG 1 : 320 (95%, 93%, 95%, 93%, respectively) and IgM 1 : 80 (95%, 100%, 100%, 94%, respectively). IFA IgA showed either poor sensitivity or specificity at all titers. These findings and the subjective reading of IFA limit its value in Brucella diagnosis while the MAT showed high false negatives (5%–40%). Thus, ELISA proves to be the most reliable test for the diagnosis of patients with brucellosis.