Use of continuous results to compare ELISAs for the detection of antibodies to non-structural proteins of foot-and-mouth disease virus.

Six tests for detection of antibodies against the non-structural proteins of foot-and-mouth disease virus (FMDV) were compared at an international workshop in Brescia, Italy in 2004 on the basis of dichotomous test results. However, as results from all of these assays were also available on a continuous scale, validation was extended by calculating and subsequently analysing the receiver-operator characteristic (ROC) curves and likelihood ratios (LR) for each test method. For the purposes of these analyses, test results for a total of 1337 sera were selected from the Brescia workshop dataset, 237 sera that had been obtained from cattle exposed to FMDV and 1100 sera obtained from cattle that were not exposed to the virus; sera from "exposed" cattle were considered to be "true positives" and sera from "non-exposed" cattle were considered to be "true negatives". Analysis of ROC curves showed that at specificities of both 99 and 99.5%, the IZS-Brescia and the Ceditest ELISA had significantly better detection rates in exposed cattle than the other ELISAs. The ROC analysis confirms the previous finding that the IZS-Brescia and the Ceditest ELISAs have both better detection rates in exposed cattle combined with a high specificity. The analysis of likelihood ratios provides information that may be very useful in the interpretation of test results, and a working example is presented to show how these likelihood ratios might be used in an objective approach to deciding the true infection status of surveyed populations.