Detection of low serum immunoglobulin concentrations in clinically ill calves.

The sensitivity, specificity, and accuracy of classification of 4 tests for failure of passive transfer (FPT) were examined in clinically ill neonatal calves. Comparisons were made with serum IgG1 concentrations determined by radial immunodiffusion. Serum samples were obtained from 27 clinically ill calves < or = 21 days of age. The results of 4 commonly used assays, the sodium sulfite turbidity test, the zinc sulfate turbidity test, refractometry, and the serum gamma-glutamyl transferase (GGT) activity test, were compared with radial immunodiffusion determinations of serum IgG1 concentration. Serum GGT activity using a 50 IU/L threshold resulted in correct classification of the highest percentage of calves (93%) with regard to their passive transfer status. The sodium sulfite test with a 1+ end point and refractometry using a 5.5 g/dL end point resulted in correct classification of 85% of the calves studied. When using the sodium sulfite test, the 2+ and 3+ test end points had lower specificity, 0.58 and 0.00, respectively, than the 1+ end point. This loss in specificity resulted in misclassification of calves with adequate serum immunoglobulin concentrations as having FPT. The zinc sulfate turbidity test was inadequately specific (0.33) and resulted in misclassification of 33% of calves.