Enzyme-Linked Immunosorbent Assay and Microbiologic Culture for Diagnosis of Staphylococcus aureus Intramammary Infection in Cows.

Recent reports have indicated that the relative sensitivity and specificity of the ELISA test for detection of intramammary infection of cows with Staphylococcus aureus is not as high as originally reported. It has been suggested that antibodies measured by enzyme-linked immunosorbent assay (ELISA) more closely reflect previous infection status rather than current infection status, and that the delay in antibody formation following infection and the persistence of antibodies after elimination of infection may be responsible for some of the discrepancy observed between ELISA and bacterial culture results conducted on the same milk sample. This study ( n = 209 cows) was undertaken to determine if an ELISA for S. aureus intramammary infection more closely reflects previous infection status than it does current infection status, and to ascertain whether correction of this time-delay factor substantially improves calculated values of ELISA relative sensitivity and specificity. Receiver-operator curves were constructed to compare different time-related definitions of microbiologic culture results used for comparison with ELISA results. A greater degree of curvature in receiver-operator curves indicated that ELISA results did more closely reflect culture results performed on milk samples taken 1 and 3 weeks previously. Insignificant improvement in sensitivity and specificity occurred when the database was limited to cows ( n = 140) with milk production greater than 13.6 kg/day. However, values of sensitivity were all less than or equal to 90%, and values of specificity were all less than 54%.