Comparison of the Brucella Standard Agglutination Test with the ELISA IgG and IgM in patients with Brucella bacteremia.

The presumptive diagnosis of Brucellosis is based on a high or rising antibody titer measured by the Brucella Standard Agglutination Test (SAT). This tests does not discriminate between the immunoglobulin classes (IgG and IgM). The purpose of this study was to compare the diagnostic value of SAT with Brucella Enzyme Linked Immunosorbent Assay (ELISA) IgG and IgM tests in patients with Brucella bacteremia. Over a one-year period, we had 68 patients with clinical features suggestive of Brucellosis who had positive blood cultures for Brucella species. Sera were obtained from all of the patients as well as a control group of 70 healthy military personnel who were blood donors and had no symptoms of Brucellosis. Patients and blood donors originated from the same referral population. All the sera were tested by SAT and ELISA. All the 70 controls had a negative SAT. The sensitivity and specificity of the SAT test for the bacteremic patients were 95.6% and 100.0% respectively, while that of the ELISA IgG were 45.6% and 97.1%, and that of the ELISA IgM were 79.1% and 100.0% respectively. The sensitivity and specificity of either IgG or IgM positivity were 94.1% and 97.1% respectively. Assuming that the population prevalence of active Brucellosis in Saudi Arabia (SAT >or=1:320) is 5%, the positive and negative predictive values of SAT were 100% and 99.7% respectively; of ELISA IgG they were 45.2% and 97.1%; and of ELISA IgM they were 100% and 98.9%. When both the ELISA IgG and IgM were combined, the positive and negative predictive values were 63% and 99.6% respectively. In patients with Brucella bactremia, the sensitivity of either ELISA IgM or IgG were lower than SAT, however, combining IgM and IgG had similar sensitivity and specificity to SAT. The positive predictive value of SAT and IgM is satisfactory.