Evaluating the use of serum total solids to predict serum immunoglobulin G in Holstein and Angus crossbred calves

This study evaluated the use of serum total solids (STS) to assess passive immunity in Holstein and Angus cross Holstein calves. The objectives of our study were to (1) determine whether STS and serum IgG differs between Holstein and Angus × Holstein (Angus X) newborn calves, (2) compare the relationship between STS and serum IgG in both breeds, and (3) determine whether the industry-recommended passive immunity STS cutpoint (>62 g/L) for predicting serum IgG concentration ≥25 g/L is appropriate for Angus X calves. This observational study was part of a wider clinical trial in which calves were randomized to colostrum IgG doses ranging from 200 to 350 g. Blood samples were taken from calves at 2 d of age, and the separated serum was tested for STS (g/L) using optical refractometry. The remaining serum was stored frozen at -20°C and tested 2 wk later for IgG concentration (g/L) using radial immunodiffusion. Multivariable linear regression models were used to compare serum IgG and STS between breeds, controlling for colostrum IgG dose and calf sex. Pearson correlation coefficient and linear regression models were used to compare correlations between serum IgG and STS by breed. Area under the curve (AUC) and diagnostic sensitivity (Se) and specificity (Sp) were used to measure the ability of STS to identify calves with serum IgG ≥25 g/L. The number of calves analyzed in the final dataset from each group were 272 Holstein and 119 Angus X. Although serum IgG was similar between breeds, Holstein calves had 3.1 g/L (95% CI: 1.7, 4.5) higher STS than Angus X calves. The AUC of STS for predicting IgG ≥25 g/L was the same for both breed groups (AUC = 0.86). A previously recommended cutpoint of 62 g/L STS had Se and Sp of 51% and 91%, respectively, when predicting serum IgG ≥25 g/L in Holsteins. We found that a STS cutpoint that was 3 g lower (59 g/L) in Angus X calves provided similar performance (Se 55%, Sp 94%). Although colostral IgG absorption is similar between breeds, the relationship between serum IgG and STS differs, likely due to Angus X calves having a lower concentration of non-IgG proteins in serum. Although we acknowledge that the sensitivity of these cutpoints is low, we believe that breed-specific thresholds may be necessary to predict a calf serum IgG ≥25 g/L for Angus X calves, compared with the >62 g/L recommendation proposed for Holsteins in the literature.