Comparison of tests for failure of passive transfer in neonatal calf serum using total protein refractometry and the biuret method

Failure of passive transfer (FPT) is defined as failure to absorb colostral antibodies sufficient to achieve a serum Immunoglobulin G (IgG) concentration >10 g/L. Immunoglobulin G can be measured directly in calf serum using radial immunodiffusion (RID), or indirectly estimated by measuring total protein (TP). Indirect TP measures are usually favoured because of their relatively lower costs. The aim of this work was to compare TP measurements using refractometry and biuret methods against the reference RID test in neonatal dairy calves, and to assess agreement between these indirect measures. Neither the biuret nor the refractometer method provided a high sensitivity for detection of FPT, as defined by RID. There was no systematic difference between the methods in their estimation of TP, although the biuret method was more accurate than the refractometer method when tested against the reference RID test (accuracy = 83.1 % v 69.3 %) and the refractometer was more likely to overestimate the number of calves with FPT. Specificity for the biuret test was 93.9 % compared with the refractometer specificity of 74.4 %. Mean TP as estimated by the biuret method was higher than the mean TP estimated by the refractometer (6.25 g/dL versus 5.52 g/dL), and the Pearson correlation coefficient for the two assays was only moderate, at 0.58. This suggests that the biuret method is preferable to the refractometer for detecting FPT in calves, despite the superior convenience of the refractometer.