Validation of a test for dams carrying foetuses persistently infected withbovine viral-diarrhoea virus based on determination of antibody levels in late pregnancy

Our objective was to estimate, using a generalised linear mixed-model appro ach, the sensitivity and specificity of an indirect ELISA when used to iden tify dams pregnant with persistently bovine viral-diarrhoea virus (BVDV)-in fected foetuses. Cows that had been tested for antibodies to BVDV with a po sitive result during their pregnancy and where the offspring had been teste d for both antibody and virus were identified by accessing the Swedish BVD database and the official pedigree records. The resulting data set consiste d of 2162 cow-calf pairs in 126 herds, of which 281 included virus-positive calves. The sensitivities and specificities at 12 different decision thres holds (corresponding to optical densities (ODs) between 0.5 and 1.6) were e stimated using generalised linear mixed models (binomial error, logit link) , in which the gold standard (the BVDV status of the calf) was included as a covariate. In each model, the dependent variable was the dichotomous test result at the decision threshold in question. There was a significant posi tive interaction between the calf's status and gestational stage in all 12 models - indicating that the sensitivity and specificity at any given decis ion threshold was improved when the the test was performed later in pregnan cy. The test should be applied only when samples have been taken in late ge station - not before the seventh month in pregnancy. If applied during the last months of pregnancy, the point estimate of the sensitivity ranges betw een 0.94 and LO as the decision threshold is moved from 1.0 and downwards t o 0.7. Similarly, the specificity ranges between 0.39 and 0.67 as the decis ion threshold is moved from 0.8 and upwards to I. I. (C) 2001 Elsevier Scie nce B.V. All rights reserved.