Calculating the positive predictive value of a diagnostic test

Will a diagnostic test have the same predictive value regardless of how it is used? My intuitive answer is “It should”, but hey, this is why we do statistics before implementing large screening programs. The predictive value of a test is different when the test is used in a high-risk population compared to when it is used in a low-risk population. This means that the positive predictive value of a test differs if the test is used as a screening test versus when it is used as a confirmatory test, or used in two populations with different prevalence. Sensitivity, specificity and accuracy are important principles to consider when evaluating how and when diagnostic tests should be used, such as the mammography screening program, or when evaluating the differences in policies regarding COVID testing during the early and late stages of the pandemic.