Comparison of diagnostic tests for determining the prevalence of failure of passive transfer in New Zealand dairy calves

AIM: To evaluate the level of agreement of three indirect testing methods with concentrations of IgG in serum, and to determine their test characteristics for diagnosing failure of passive transfer (FPT), in dairy calves in New Zealand.METHODS: From 17 July to 30 November 2015, 471 blood samples were collected from dairy calves aged 0-8 days, from 19 different commercial dairy farms in the Waikato region of New Zealand. All serum samples were tested for concentrations of IgG and total protein (TP), and gamma-glutamyl transferase (GGT) activity, and 138 samples were tested using a digital Brix refractometer. The diagnostic tests were assessed for level of agreement with concentrations of IgG using Bland-Altman plots. FPT was defined as concentration of IgG 10.0g/L and receiver operating characteristic curve analysis was used to determine optimal cut-points for correctly predicting FPT. The test characteristics for TP, GGT and Brix for predicting FPT were determined using the optimal cut-points. The effect of age on the relationship between test results and concentrations of IgG was also assessed.RESULTS: Based on the limits of agreement plots, the agreement with concentrations of IgG was greatest for concentrations of TP, followed by Brix, with the lowest being GGT activity. There was an interaction between age at sampling and the association between concentrations of IgG and GGT activity (p<0.001); in calves 5-8 days of age there was a greater increase in concentrations of IgG associated with a unit increase in GGT activity than in calves <5 days of age. The optimal cut-points for diagnosis of FPT were 52g/L for concentrations of TP, 8.8% for Brix, 250 IU/L for GGT activity in calves <5 days old, and 210IU/L for GGT activity in calves 5-8 days old. Accuracy for predicting FPT was greatest for TP (0.95), followed by GGT in calves <5 days old (0.94) and Brix (0.92). Specificities were high for all three tests, however sensitivity varied greatly.CONCLUSION: Measurement of concentrations of TP in serum was the most accurate alternative to measuring concentrations of IgG for diagnosing FPT in non-dehydrated calves.CLINICAL RELEVANCE: Veterinary practitioners in New Zealand now have confidence in using a test that is more accurate and practical than GGT and considerably cheaper than IgG to screen for FPT.