EDAPT Global Population Health Practice Exam

Sensitivity and specificity are intrinsic properties of a diagnostic test. They measure how well the test identifies those with the disease (sensitivity) and those without the disease (specificity), independent of how common the disease is in the population. Sensitivity is the proportion of true positives among all people who truly have the disease, while specificity is the proportion of true negatives among all people who are truly disease-free. Because these calculations are based on the disease and non-disease groups separately, changing how prevalent the disease is in the population doesn’t change these rates. What prevalence does affect are predictive values: positive predictive value and negative predictive value. PPV increases with higher prevalence (more of the positives are true positives in a higher-prevalence setting), while NPV decreases with higher prevalence. In short, prevalence does not alter sensitivity or specificity, though it does influence how we interpret test results in terms of expected probability that a given positive or negative result reflects true disease.