Diagnostic accuracy of digital Brix and serum total protein refractometers in estimating different passive immunity levels in dairy calves.

The aim of this study was to determine the diagnostic accuracy of digital serum total protein (TP) and digital Brix refractometers in estimating different passive immunity levels (<10, <18, <25 mg/mL) in dairy calves. Blood samples were collected from 260 apparently healthy Holstein calves, aged 2-7 days. Serum IgG concentrations were measured using digital Brix and TP refractometers and the radial immunodiffusion (RID) assay, as the gold standard. Data were analyzed by a receiver operating characteristics (ROC) analysis, the area under the ROC curves (AUC) and Cohen's kappa (κ). Optimal thresholds were determined as < 8.4, < 9.0 and < 9.4% for the digital Brix refractometer, and < 5.0, < 5.4 and < 5.8 g/dL for the serum TP refractometer in estimating IgG concentrations of < 10, < 18, < 25 mg/mL, respectively. The sensitivity (Se) and specificity (Sp) of the Brix refractometer were 96.3% and 88.8% for < 8.4% Brix, 97.0% and 83.4% for < 9.0% Brix, and 85.5% and 77.8% for < 9.4% Brix, respectively. The Se and Sp of the serum TP refractometer were 96.3% and 90.1% for < 5.0 g/dL, 91.0% and 89.6% for < 5.4 g/dL, 79.6% and 85.2% for < 5.8 g/dL, respectively. The discriminant ability of the refractometers was moderately accurate in estimating IgG concentrations of < 10 and < 18 mg/mL, and highly accurate in estimating IgG concentrations of < 25 mg/mL. Both refractometers substantially agreed with RID-IgG results and almost perfectly agreed with each other. In conclusion, the digital Brix and digital serum TP refractometers offer a good utility for determining different passive immunity levels in dairy calves.