BVD CONSULT

Factors affecting sensitivity and specificity of pooled-sample testing for diagnosis of low prevalence infections

aDepartment of Veterinary Population Medicine, College of Veterinary Medicine, University of Minnesota, 1365 Gortner Avenue, St. Paul, MN 55108, United States bDepartment of Medicine and Epidemiology, School of Veterinary Medicine, University of California at Davis, One Shields Avenue, Davis, CA 95616, United States cCalifornia Animal Health and Food Safety Laboratory System, University of California at Davis, One Shields Avenue, Davis, CA 95616, United States dDepartment of Statistics, University of California at Davis, One Shields Avenue, Davis, CA 95616, United States

Abstract

Testing of pooled samples has been proposed as a low-cost alternative for diagnostic screening and surveillance for infectious agents in situations where the prevalence of infection is low and most samples can be expected to test negative. The present study extends our previous work in pooled-sample testing (PST) to evaluate effects of the following factors on the overall PST sensitivity (SEk) and specificity (SPk): dilution (pool size), cross-contamination, and cross-reaction. A probabilistic model, in conjunction with Monte Carlo simulations, was used to calculate SEk and SPk, as applied to detection of bovine viral diarrhea virus (BVDV) persistently infected (PI) animals using RT-PCR. For an average prevalence of BVDV PI of 0.01 and viremia in each animal between 102 and 107 virus particles/mL, the pool size associated with the lowest number of tests, and lowest cost, corresponded to eight samples/pool. However, the least-cost pool size (lowest number of tests) was associated with a SEk of 0.90 (0.75–1), which corresponded to a decrease of 0.04, relative to the assay sensitivity for a single sample. The SPk for the same pool size, considering the effect of detection of BVDV acutely infected animals and cross-contamination as source of false positive results, was 0.90 (0.85–0.95). The effect of a hypothetical cross-reacting agent was to markedly decrease SPk, especially as the prevalence of the cross-reacting agent increased. For a pool size of eight samples and a prevalence of the cross-reacting agent of 0.3, SPk ranged from 0.67 to 0.86, depending on the probability that the assay would detect the cross-reacting agent. The methods presented offer a means of evaluating and understanding the various factors that can influence overall accuracy of PST procedures.

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This article was published in Preventive Veterinary Medicine, 74, Claudia Muñoz-Zanzia, Mark Thurmondb, Sharon Hietalac and Wesley Johnsond, Factors affecting sensitivity and specificity of pooled-sample testing for diagnosis of low prevalence infections, 309-322, Copyright Elsevier 2006.