Use of an enzyme-linked immunosorbent assay for detecting bovine viral diarrhoea virus antibodies in individual cow milk samples

Abstract

AIMS: To calculate and verify an adjustment for the test result cut-off when testing milk samples using a bovine viral diarrhoea (BVD) antibody test so that it is a better approximation of the test on serum as the reference. Previous experience has demonstrated that different cut points may be required for results from milk and serum. METHODS: Paired milk and serum samples were collected from 90 cows from three herds in two regions of New Zealand and were tested using a commercially available bovine viral diarrhoea virus (BVD) antibody ELISA. A regression equation with second-order polynomial was developed to estimate a milk sample to positive-control optical density ratio (S/P ratio) value from a serum S/P ratio, and the milk values equivalent to the recommended serum cut points were calculated. The change in milk test result agreement with the result based on serum samples was assessed primarily using weighted Kappa statistics. RESULTS: The new cut points between negative and suspect, and suspect and positive milk samples were defined as S/P ratios of ≥ 0.08 and ≥ 0.13 respectively, compared to cut points for serum samples of ≥ 0.20 and ≥ 0.30, respectively. Using the new cut points increased the level of agreement (weighted Kappa) between serum and milk from 0.58 to 0.82. CLINICAL RELEVANCE: Milk BVD antibody testing can be used to estimate serum BVD antibody status. This is particularly useful for investigations where susceptibility to infection or the incidence of new infections is of interest. CONCLUSIONS: Using new adjusted cut points for milk, ELISA BVD antibody testing resulted in a higher level of agreement with serum samples.