A sensitive radioimmunoassay for the detection of Brucella abortus antibody is described. The assay, performed in flexible 96-well microplates coated with Brucella abortus antigens, utilizes 125I-labeled staphylococcal protein A to detect antibody to Brucella abortus. The parameters of the assay have been analyzed using well recognized statistical methods. Least squares analysis of antigen concentration, antiserum dilution, antigen by antiserum and replicates within antigen by antiserum, estimated an r2 of 0.98, a coefficient of variation of 3.58 and a standard deviation of 0.10. Results of regression analysis of serum dilution versus antigen concentration (ranging from 635ug/ml to 6.35 ng/ml) indicated that an antigen concentration of 6.35ug/ml was the most efficient for describing antibody variability (r2 = 0.98, with a coefficient of variation = 3.28). Regression analysis also revealed a closer correlation between the radioimmunoassay with complement fixation test (r2 = 0.98) than with the standard tube test (r2 = 0.84). Detection of specific antibody assessed by radioimmunoassay was 4 to 64 fold more sensitive than the standard tube test titer and 16 to 32 fold more sensitive than the complement fixation test titer. The results described here indicate that this radioimmunoassay is very sensitive and may be capable of discriminating between false positives and false negatives, thereby, improving the diagnostic efficiency of Brucella serologic tests.
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