Amanda J. Kreuder

Background Colostral immunoglobulin G (IgG) concentration is critical to the attainment of adequate transfer of passive immunity in cattle, however, studies comparing available tools for measurement of colostral IgG concentration in beef cattle are limited. Objectives To report the agreement between 3 commercially available tests for evaluating IgG concentration in beef colostrum. Animals Two hundred six beef‐breed cows hospitalized for calving management or dystocia. Methods Retrospective study to assess IgG of whole colostrum measured stall‐side via turbidimetric immunoassay (TI) and brix refractometry (BRIX), compared to fat separated (FS) analysis via single radial‐immunodiffusion (RID; reference standard), TI‐FS and BRIX‐FS. Test performance was assessed using Passing Bablock regression, Bland‐Altman analysis, and area under the curve to determine optimal thresholds. Results Correlation between RID and TI‐FS, BRIX‐FS, or BRIX was similar (Spearman's ρ = 0.717, 0.715, 0.716, respectively) but correlation for TI was poor ( ρ = 0.586). Regression analysis identified a substantial constant (−214.75 [CI: −272.03 to −178.07]) and proportional (13.24 [CI: 11.81‐15.37]) bias between the RID and TI‐FS which was similar for TI. TI‐FS concentrations of 28.47, 38.75, and 50.62 g/L, BRIX‐FS of ≤21.9%, ≤24.0%, and ≤27.4%, and BRIX of ≤21.3%, ≤23.8%, and ≤26.4% indicated IgG concentrations <50, <100, and <150 g/L, respectively; appropriate cutoffs for TI could not be generated. Conclusions and Clinical Importance Both TI and TI‐FS demonstrated a large constant and proportional bias compared to RID; BRIX and BRIX‐FS were well correlated with RID and remain a reliable method for estimation of colostral IgG concentration in beef cattle.