Evaluation of diagnostic tests for the detection of classical swine fever in the field without a gold standard

Knowledge of the sensitivity of diagnostic tests for infectious diseases un der field conditions can be used to design a surveillance program that incr eases the effectiveness of the control policy. In this study, the sensitivi ty of tests for the detection of classical swine fever (CSF) virus (CSFV) u nder field conditions was estimated without knowledge of the true disease s tatus of the animals tested. During the CSF epidemic of 1997-1998 in The Ne therlands, tonsil samples from pigs of CSF suspect farms were collected for laboratory diagnosis of CSF These specimens were tested in a fluorescence antibody test (FAT1) for the presence of CSFV antigen. When at least 1 spec imen in a particular sample series from a farm was positive, this farm was declared CSFV infected. Specimens of that series, either FAT1 negative (98) or FAT1 positive (127), were subsequently tested again (FAT2). After that, a suspension was made of the remaining tissue, and this suspension was eva luated with a virus isolation test. In total, 225 tonsil specimens were exa mined. A statistical model was formulated, and the sensitivity of the 3 tes ts and the prevalence of positive specimens in the sample were estimated by the method of maximum likelihood. The sensitivity of the FAT1, the test th at was used for confirmation of CSFV infection in a pig herd, was approxima tely 78% (95% confidence interval [CI] = 62-92%). The effectiveness of the selection process of animals on the farm by the veterinarian was estimated to be 77% (64-87%). The sensitivity of the combination of FAT1 and FAT2 (60 %) indicates that at least 5 animals should be selected on a CSF-suspect fa rm to gain a detection probability for CSFV of 99%.