A COMPARISON OF INDIRECT METHODS FOR DIAGNOSIS OF SUBCLINICAL INTRAMAMMARY INFECTION IN LACTATING DAIRY-COWS .2. THE DISCRIMINATIVE ABILITYOF 8 PARAMETERS IN FOREMILK FROM INDIVIDUAL QUARTERS AND COWS

The ability of eight parameters to discriminate between infected and u ninfected udder quarters, or cows, was assessed, using samples taken a t monthly intervals from 121 cows within three herds. The somatic cell count correctly classified 80% of quarters and 75% of cows, while ove r 70% of quarters, and 67% of cows, were correctly classified by the s odium concentration and by the N-acetyl-beta-D-glucosaminidase (NAGase ) activity of the milk. The remaining parameters each classified corre ctly fewer than 70% of quarters, and 60% of cows, as being infected or uninfected. The critical threshold value for each parameter varied be tween herds, with the greatest between-herd variation shown by somatic cell count, ranging from 85,000 to 375,000 cells/ml for quarters and between 73,000 and 344,000 for cows. Critical threshold values increas ed during lactation for somatic cell count and sodium concentration, b ut tended to decrease for NAGase activity. For identification of cows, the specificity of the somatic cell count was slightly lower than tha t of the NAGase activity or the sodium concentration, but the sensitiv ity of the somatic cell count was markedly higher than that of any oth er parameter. The discriminative ability of the somatic cell count sho wed little variation between herds or during lactation provided that t he relevant critical threshold figure was used.