Likelihood ratio estimation without a gold standard: A case study evaluating a brucellosis c-ELISA in cattle and water buffalo of Trinidad G.T. Fosgate a, * , A.A. Adesiyun b , D.W. Hird c , S.K. Hietala d a Department of Veterinary Integrative Biosciences, College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College Station, TX 77843, United States b Faculty of Medical Sciences, School of Veterinary Medicine, University of the West Indies, Champs Fleurs, Trinidad and Tobago c Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616, United States d California Animal Health and Food Safety Laboratory, Davis, CA 95616, United States Received 17 April 2005; received in revised form 25 November 2005; accepted 27 February 2006 Abstract The likelihood ratio (LR) is a measure of association that quantifies how many more times likely a particular test result is from an infected animal compared to one that is uninfected. They are ratios of conditional probabilities and cannot be interpreted at the individual animal level without information concerning pretest probabilities. Their usefulness is that they can be used to update the prior belief that the individual has the outcome of interest through a modification of Bayes’ theorem. Bayesian analytic techniques can be used for the evaluation of diagnostic tests and estimation of LRs when information concerning a gold standard is not available. As an example, these techniques were applied to the estimation of LRs for a competitive ELISA (c-ELISA) for diagnosis of Brucella abortus infection in cattle and water buffalo in Trinidad. Sera from four herds of cattle (n = 391) and four herds of water buffalo (n = 381) in Trinidad were evaluated for Brucella-specific antibodies using a c-ELISA. On the basis of previous serologic (agglutination) test results in the same animals, iterative simulation modeling was used to classify animals as positive or negative for Brucella infection. LRs were calculated for six categories of the c- ELISA proportion inhibition (PI) results pooled for cattle and water buffalo and yielded the following www.elsevier.com/locate/prevetmed Preventive Veterinary Medicine 75 (2006) 189–205 * Corresponding author. Tel.: +1 979 845 3203; fax: +1 979 847 8981. E-mail address: gfosgate@cvm.tamu.edu (G.T. Fosgate). 0167-5877/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.prevetmed.2006.02.007