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 )[Note 3. Somaticell SCC Test, Idexx Laboratories, Inc., Westbrook, Maine, USA ...] 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),[Note 2. DCC, DeLaval, Tumba, Sweden ...] 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.