EVALUATION OF A PROTOTYPE ONLINE ELECTRICAL-CONDUCTIVITY SYSTEM FOR DETECTION OF SUBCLINICAL MASTITIS

A prototype on-line system for measurement of electrical conductivity of quarter milk was evaluated for accuracy in detection of subclinical mastitis compared with that of bacteriological culture and SCC of sam pled quarters. Because of the low quarter prevalence of mastitis, quar ters were sampled conditionally from the signals of the on-line system . All signaled quarters and a random selection of the nonsignaled quar ters were sampled. To calculate sensitivity and specificity, the total number of nonsignaled quarters was extrapolated. The system identifie d correctly 18 out of 23 subclinical quarters and 521 out of 555 healt hy quarters. Quarter prevalence was about 1%. Predictive value of a po sitive test (35%) and the predictive value of a negative test (99%) we re calculated, as well as sensitivity (25%) and specificity (99%), aft er extrapolation of the total number of nonsignaled quarters. Because of repeated measurements, sensitivity may be underestimated. When sign aled quarters were defined by repeated signals within 14 d, predictive value positive increased to 48%. The prototype on-line system did not detect subclinical mastitis very accurately because of suboptimal tes t characteristics.