In this Research Communication we report the measurement of total blood calcium concentration with a water hardness test kit (WHTK) in dairy cattle and compare the results to a reference method. The study was carried out on 107 Holstein Friesian cows from three commercial dairy farms in Sardinia (Italy). Blood samples were collected ≤7 d from calving and total serum calcium concentration was determined using a biochemical analyzer and a WHTK. The relationship between the laboratory results and the WHTK results was investigated using Spearman's rank correlation test and simple linear regression was determined. Sensitivity, specificity, negative and positive predictive values, intra and inter-assay coefficient of variation were also calculated. The test values were highly correlated with the laboratory values (R2 = 0.72, P < 0.001). Sensitivity and specificity were respectively 88 and 83% for the WHTK and intra and inter-assay coefficients of variation were 7.3 and 11.3% respectively. The WHTK was identified as a potential on-farm tool for monitoring early postpartum Ca concentrations at herd level.
