Evaluation and Comparison of 2 On‐Farm Tests for Estimating Somatic Cell Count in Quarter Milk Samples from Lactating Dairy Cattle

BACKGROUND The somatic cell count (SCC) is commonly used to monitor udder health and diagnose subclinical intramammary infection (IMI) in dairy cattle. HYPOTHESIS The Somaticell test (ST) and California mastitis test (CMT) are clinically useful cow-side tests for diagnosing subclinical IMI. ANIMALS One hundred and eleven dairy cows at dry-off and 92 cows within 4-7 days postcalving. METHODS Quarter foremilk samples were obtained and analyzed with a DeLaval cell counter (DCC, reference method), ST, and CMT. The ST was run in a simulated cow-side manner using milk at 37°C instead of 0-8°C as recommended by the manufacturer. Test performance for diagnosing IMI (DCC SCC >200,000 cells/mL) was evaluated by calculating the area under the receiver operating characteristic curve (AUC) and the kappa coefficient (κ) at the optimal cut-point for each test. The effect of milk/reagent temperature also was evaluated. RESULTS Compared to the reference method, the ST run in a simulated cow-side manner had an AUC = 0.68 and κ = 0.24 at dry-off, and AUC = 0.74 and κ = 0.40 in fresh cows. The CMT performed much better than the ST in diagnosing subclinical IMI with AUC = 0.88 and κ = 0.77 at dry-off, and AUC = 0.87 and κ = 0.76 in fresh cows. The measured ST value decreased with increasing temperature of the milk/reagent mixture. CONCLUSIONS/CLINICAL IMPORTANCE The ST is optimized for use on milk at 0-8°C and is therefore designed for on-farm use on refrigerated milk samples. The ST is not suited for use as a cow-side screening test for IMI because the milk temperature exceeds the recommended range for the test.