Optimizing performance of a qualitative zinc sulfate turbidity test for passive transfer of immunoglobulin G in calves.

OBJECTIVE: To determine optimal zinc sulfate test solution concentration for detecting failure of passive transfer in calves. ANIMALS: 235 calves (1 to 8 days old) from a calf-rearing operation in central Washington state. PROCEDURE: Zinc sulfate turbidity tests, using 200-, 250-, 300-, 350-, and 400-mg/L test solutions, were performed on calf serum. These increasing concentrations were evaluated for detection of failure of passive transfer. Using 1,000 mg of IgG1/dl as the threshold for adequate passive transfer, sensitivity, specificity, and accuracy of classification were determined by comparing the zinc sulfate test results with serum IgG1 concentration (mg/dl) measured by radial immunodiffusion. RESULTS: The 200-mg/L zinc sulfate turbidity test solution was 100% sensitive; however, specificity was only 25.5%. Increasing concentrations of zinc sulfate test solution up to 350 mg/L improved specificity with either no change or small decreases in sensitivity. CONCLUSION AND CLINICAL RELEVANCE: The endpoint of the traditional 208-mg/L zinc sulfate turbidity test for failure of passive transfer in calves is too high. Increased test solution concentrations improve specificity with only minor adverse effects on sensitivity.