Evaluation of Digital and Optical Refractometers for Assessing Failure of Transfer of Passive Immunity in Dairy Calves

Background: Failure of transfer of passive immunity (FTPI) is the underlying predisposing risk factor for most early losses in dairy calves. Refractometers, either optical or digital, can be used to assess FTPI as a part of calf health monitoring program on dairy operations. Objectives: To evaluate the performance of and differences between digital Brix and optical refractometers for assessing FTPI in dairy calves. Animals: Two hundred Holstein calves from 1 to 11 days of age. Methods: A cross-sectional study was designed to measure serum IgG concentration by radial immunodiffusion (RID) assay, digital Brix and optical refractometers. The correlation coefficients (r) between the 2 refractometers were plotted against each other and against the measured IgG concentration from RID. The Se, Sp, and accuracy of digital Brix and optical refractometers for assessing FTPI using previously recommended cut-offs were calculated. A receiver operating characteristic curve was created and used to identify the optimal cut-off for this dataset. Results: The RID IgG concentration was positively correlated with digital Brix (r = 0.79) and optical (r = 0.74) refractometers. The best combination of Se (85.5%), Sp (82.8%), and accuracy (83.5%) of digital Brix refractometer was at 8.3%Brix. For optical refractometer, the best combination of Se (80%), Sp (80.7%), and accuracy (80.5%) was at 5.5 g/dL. Conclusions and Clinical Importance: Both refractometers exhibited utility in assessing FTPI in dairy calves.