Performance characteristics of virus neutralization test (VNT) and liquid phase blocking ELISA (LPBE) and their relationship in the cattle immunized with trivalent foot and mouth disease vaccine

Virus neutralization test (VNT) and liquid phase blocking ELISA (LPBE) are accepted tests for screening and as in vitro alternative to challenge in FMD vaccine potency testing. To replace VNT by LPBE for the screening of cattle, the optimized tests need to be first evaluated for their diagnostic performances. To replace it with LPBE in the absence of protection data, the interrelationship between VNT and LPBE have to be established to find out LPBE cut-off titer corresponding to the currently used VNT titers. Accordingly, VNT and LPBE were carried out using known negative (n = 306) and positive samples [Serotype O (n = 43), A ( n = 14) and Asia1 (n = 11)], for the initial screening. The cut-off of < 1.5 log 10 LPBE was comparable with that of < 1.2 log 10 VNT titer for screening. LPBE was comparable to VNT in terms of specificity, sensitivity as shown by ROC curve and least varying (coefficient of variation 7.73% in LPBE vs 24.19% in VNT). Based on linear regression model using 471 bovine sera, the predicted LPBE titers corresponding to the currently used log 10 VNT titers of 1.65, 1.5 and 1.5, were 2.24, 1.87 and 2.00 for O, A and Asia1, respectively. These LPBE titers hence can be used as cut- off titers for classifying cattle as protected or not protected until correlation based on in vivo challenge between protection and antibody titer is established.