Technical note: Evaluation of a commercial on-farm milk leukocyte differential tester to identify subclinical mastitis cases in dairy cows

The objective of this study was to validate the precision and accuracy of a milk leukocyte differential tester to identify subclinical mastitis cases in dairy cows. Milk samples from individual quarters (n = 320) of 80 Holstein cows were aseptically collected and analyzed in this study. Each sample was divided into 2 replicate samples after mixing. One replicate was analyzed for somatic cell count (SCC) using the current gold standard of flow cytometry immediately after milking. The second sample was evaluated using the on-farm milk leukocyte differential tester directly after milking, where total leukocyte count (TLC; cells/mL) was obtained. The SCC and TLC were used to calculate somatic cell score (SCS) and TLC score [TLS = log₂ (TLC/100,000) + 3]. Two subclinical mastitis thresholds were set: >200,000 (low) and >400,000 (high) cells/mL. First, precision was determined between the 2 methods. Total leukocyte count and calculated TLS from the milk leukocyte differential device were compared with the gold standard using correlation and regression coefficient of determination analyses. Correlation coefficients (r) were 0.97 for TLC and SCC and 0.90 for TLS and SCS. The coefficient of determination for regression (R²) was 0.94 for TLC and SCC and 0.80 for TLS and SCS. Slopes of regression for scores and measures were 0.36 [95% confidence interval (CI): 0.35–0.37] and 0.69 (CI: 0.65–0.73), respectively; both were significantly different from 1. Sensitivity, specificity, and diagnostic accuracy were calculated for correct diagnosis of the 2 SCC thresholds using the gold standard as reference. The sensitivity of the on-farm test was 58% (95% CI: 44 to 71%) and 73% (95% CI: 56 to 86%) for the low and high thresholds, respectively. The specificities for the on-farm test were 100% (95% CI: 99 to 100%) and 100% (95% CI: 98 to 100%) for the low and high thresholds, respectively. Subclinical diagnosis accuracies were 93% (95% CI: 89 to 95%) and 96% (95% CI: 92 to 98%) for the low and high thresholds, respectively. The on-farm milk leukocyte differential tester was precise but not overall accurate for total cell counts; it had high specificity and accuracy for diagnosis compared with a standard diagnostic tool. These results suggest that the tested system is a promising technology to detect subclinical mastitis on-farm.