North American Veterinary Licensing Examination (NAVLE) Practice Test

How does a decrease in the prevalence of a disease affect the predictive value positive (PVP) of the best serologic test for that disease?

1. PVP depends on the number tested, not prevalence

2. PVP is affected by specificity, not prevalence

3. PVP increases as prevalence decreases

4. PVP decreases as prevalence decreases

The correct answer is: PVP decreases as prevalence decreases

A decrease in the prevalence of a disease leads to a decrease in the predictive value positive (PVP) of a serologic test used for that disease. PVP is the probability that a positive test result accurately reflects the presence of the disease. It is significantly influenced by the prevalence of the disease in the population being studied. When the prevalence of a disease is high, a positive test result is more likely to correspond to an actual case of the disease, thereby increasing the PVP. Conversely, when prevalence decreases, the number of true cases within the population diminishes, which means that even if the test retains its sensitivity and specificity, the likelihood that a positive result indicates a true positive case decreases. In a scenario where prevalence is low, the ratio of true positives to false positives declines, since even a specific test will produce positive results that do not correspond to actual disease cases. As more individuals who do not have the disease are tested, the likelihood of encountering false positives increases, ultimately reducing PVP. This understanding highlights the importance of considering both the characteristics of the diagnostic test (sensitivity and specificity) and the epidemiological context (prevalence) when interpreting test results.