Comparison of microbiologic culture, an enzyme-linked immunosorbent assay, and determination of somatic cell count for diagnosing Staphylococcus aureus mastitis in dairy cows.

Results of using microbiologic culture of a single milk sample, determination of somatic cell count (SCC), an ELISA, and a combination of determination of SCC and ELISA to diagnose Staphylococcus aureus mastitis in dairy cattle were compared. Cows were considered to have S aureus intramammary infections if microbiologic culture of at least 2 of 3 consecutive sets of milk samples yielded growth of the organism. Data were analyzed from milk samples collected over a 4-month period from 185 cows in 5 herds. Sensitivity, specificity, and likelihood ratio of a positive test result for microbiologic culture of a single milk sample were 93%, 99%, and 93.0, respectively. Sensitivity, specificity, and likelihood ratio of a positive test result for ELISA were 69%, 61%, and 1.8, respectively, and for determination of SCC, they were 79%, 72%, and 2.9, respectively. Combination of determination of SCC and ELISA had sensitivity, specificity, and likelihood ratio of a positive test result of 80%, 62%, and 3.4, respectively. Results from microbiologic culture of consecutive milk samples were more consistent than results of ELISA performed on consecutive samples. These data suggest that microbiologic culture of a single milk sample is the best of the 3 tests studied for diagnosing S aureus intramammary infection.