Evaluation of 3 refractometers to determine transfer of passive immunity in neonatal beef calves

Background Refractometry is used to assess transfer of passive immunity (TPI), but studies evaluating different refractometers and appropriate thresholds for recommended target immunoglobulin G (IgG) concentrations for beef calves are limited. Objectives To evaluate test performance of digital (DSTP) and optical (OSTP) serum total protein (STP) refractometers and a digital Brix (DBRIX) refractometer for assessment of passive immunity in beef calves. Animals A total of 398 beef calves from 6 herds, 1 to 7 days of age. Methods Serum IgG concentration was estimated by DSTP, OSTP, and DBRIX, and measured by radial immunodiffusion (RID). Correlation coefficients (r) among results were calculated. Optimal STP and Brix thresholds for identification of IgG <10, <16, and <24 g/L were determined using interval likelihood ratios. Refractometer performance and agreement were assessed using areas under the curve (AUC), diagnostic test characteristics, Cohen's kappa (kappa), and Bland-Altman analysis. Results Refractometer results were highly correlated with RID (r = 0.82-0.91) and with each other (r = 0.91-0.95), and overall test performance was excellent (AUC = 0.93-0.99). The STP concentrations of <= 5.1, <= 5.1, and <= 5.7 g/dL and Brix percentages of <= 7.9%, <= 8.3%, and <= 8.7% indicated IgG concentrations <10, <16, and <24 g/L, respectively. Agreement of refractometers with RID was variable (kappa = 0.46-0.80) and among refractometers was substantial (kappa = 0.62-0.89). Conclusions and Clinical Importance All refractometers showed good utility as monitoring tools for assessment of TPI in beef calves.