Principles of diagnostic testing of animal populations

Along with almost everything else in the swine industry, the way veterinarians apply and interpret diagnostic tests has evolved as production systems have changed. Valid interpretation of diagnostic results requires integration of clinical, epidemiological and laboratory observations, and consideration of potential sources of error. Beyond their traditional application in outbreak investigation and disease diagnosis, diagnostic tests are increasingly used to quantify disease in defined populations in order to support decisions regarding population health management. The optimal strategy for population testing, including the tests selected and the numbers of animals sampled, is a function of the purpose of testing (Table 1). Fundamental objectives are to establish the disease status (positive or negative) of populations (groups, sites, studs, pods, companies, regions, countries), or to quantify disease trends in time or space. Demographic changes (particularly increased herd sizes) and persistent problems with PRRS have altered the needs for, and feasibility of, more intensive use of herd level diagnostics. Veterinarians must have a sound theoretical understanding of the relationships between diagnostic test performance, sampling theory, and disease prevalence that underpin population testing. This paper reviews some core principles that are essential for veterinarians to be able to meet these challenges. More detailed discussions of diagnostic testing of swine populations, and approaches to disease surveillance in swine are available elsewhere.