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So often in veterinary medicine, as in other medical fields, we are looking for diagnostic tests to aid in treatment and to prognosticate for various diseases. Both infectious and non-infectious diseases may be diagnosed by detecting the causative agent, clinical signs, pathologic changes, biochemical changes and/or surrogate evidence of past or present exposure to an agent (antibody). A large number of the tests that we use are able to detect antibody, however, our dilemma is whether that is what is currently producing the clinical signs or is it a past infection. Alternatively, is the reaction we see a cross reaction to some other agent. Seldom do we think about the test in terms of what information it is going to provide basing our recommendations on the prevalence of the disease in your particular practice area. This is commonly referred to in epidemiology circles as the pre-test probability of the disease in question. Prevalence of disease is different for every area, and will differ in the healthy animal versus the sick. In healthy animals we often screen groups to determine exposure or to rule out disease, in other words, prove they are healthy. In this regard, we like a very sensitive test so there are infrequent false negatives, and therefore, we have less trouble believing a negative test result. In the sick animal, we want the test to prove the animal is truly sick (rule-in disease). Therefore, we like to have a very specific test which reduces the number of false positives and subsequently are more inclined to believe a positive test result.
This section's content was compiled by Dr. William Saville, Assistant Professor and Extension Epidemiologist in the Department of Veterinary Preventive Medicine, The Ohio State University, Columbus, OH 43210-1092. Collaborators include Dr. William P. Shulaw, beef cattle and sheep Extension Veterinarian and Dr. Alecia Larew Naugle, Extension Graduate Research Associate. You can contact Dr. Saville via e-mail at saville [dot] 4 [at] osu [dot] edu.