ELISA Cut-off Point for the Diagnosis of Human Brucellosis; a Comparison with Serum Agglutination Test

Background: Brucellosis is a world-wide disease, which has a diverse clinical manifestation, and its diagnosis has to be proven by laboratory data. Serum agglutination test (SAT) is the most-widely used test for diagnosing brucellosis. The enzyme linked immunosorbent assay (ELISA) can also determine specific antibody classes against brucella. It is a sensitive, simple and rapid test, which could be an acceptable alternative to SAT with fewer limitations, however, like any other new test it should be further evaluated and standardized for various populations. This study was planned to determine an optimal cut-off point, for ELISA which would offer maximum sensitivity and specificity for the test when compared to SAT. Methods: Four hundred and seven patients with fever and other compatible symptoms of brucellosis were enrolled in the study. Serum agglutination test, 2-Mercaptoethanol test, and ELISA were performed on their sera. Results: The cut-off point of 53 IU/ml of ELISA-IgG yielded the maximal sensitivity and specificity comparing to the other levels of ELISA-IgG, and was considered the best cut off-point of ELISA-IgG to diagnose acute brucellosis. At this cut-off, the sensitivity, specificity, positive predictive value, negative predictive value, positive likelihood ratio, and negative likelihood ratio were 84.09%, 85.38%, 62.20, 94.90, 5.75, 0.18, respectively. Conclusion: The best cut-off point of ELISA-IgG is 53 IU/ml, which yields the maximal sensitivity and specificity to diagnose acute brucellosis.