Definition: Diagnostic testing isn't perfect. It is effectively impossible to design a diagnostic test that perfectly identifies both those people who have a disease and those people who don't. This is why diagnostic testing regimens often involve multiple tests -- where an initial test result is not considered accurate until it has been double-checked by a confirmatory test.
In general, when it is important to not cause false positive test results -- such as when testing for highly stigmatized conditions such as HIV -- initial testing is designed to definitively identify people who do not have a disease. The second round of confirmatory testing is then performed to check whether the people who initially tested positive actually are.
Designing a test regimen in this way allows the confirmatory test, which is often more expensive than the initial test, to be run only on a high prevalence population, which improves the positive predictive value of the test. It also potentially allows doctors and scientists to save money without putting anyone at risk.
Also Known As: second round test, follow-up test
Examples: When doctors are testing for syphilis, they are supposed to initially test using a VDRL or other non-treponemal test and then use treponemal tests for follow-up. When financial considerations lead to this testing being done in the reverse order, it can lead to significant problems with false positives.