Interlaboratory trial on TSE rapid tests for the control of the Italian scrapie surveillance network

Scrapie, a neurodegenerative disease of sheep and goats and one of several transmissible spongiform encephalopathies (TSEs) has been subject to mandatory active surveillance in EU through rapid testing since 2002. Regulation EC/999/2001 on TSE surveillance requires that each Member State's National Reference Laboratory for TSE periodically verifies diagnostic standards and methods by comparative testing. In 2007 the Italian Reference Centre carried out the first ring trial for classical scrapie on a set of 21 negative and 9 positive homogenated brainstems, the latter consisting of three replicates of an ARQ/ARQ scrapie positive sample diluted 1:10, 1:20 and 1:50. The purpose of the study was to verify the interlaboratory agreement in term of Cohen's kappa (k) of the rapid systems currently used by the 25 national rapid test laboratories (RTLs) [laboratories: Biorad TeSeE (17 laboratories), Enfer TSE version 2.0 (4 laboratories), Idexx Herd chek BSE-scrapie antigen kit (3 laboratories) and Prionics check LIA SR (1 laboratory)]. Our results show that the agreement among the laboratories using the same rapid test varied between 0.84 and 1, while the estimated overall agreement among the 25 laboratories was very good (k-combined=0.87, 95% confidence interval 0.85-0.89). Nevertheless, as regards differences in analytical sensitivity among the rapid tests in use, under-recognition of pre-clinical scrapie cases by lower performing systems must be expected.