Prevalence changes predictive value, not the test
Sensitivity and specificity are properties of a test: they describe how well it detects disease in people who have it and rules it out in people who do not, and they stay the same across populations. Predictive values are different. The positive predictive value (PPV) is the proportion of people with a positive test who truly have the disease, and it depends strongly on prevalence. When fasting blood glucose testing is offered only to women with obesity or a family history of diabetes, it is used in a group where diabetes is more common. In a higher-prevalence group, a larger share of positive results are true positives, so the PPV rises.
Seeing why with numbers
Imagine a test with the same sensitivity and specificity used in two groups. In a low-prevalence group, true cases are few, so even a small false-positive rate among the many healthy people produces many false positives, and they can outnumber the true positives. In a high-prevalence group, true cases are many and healthy people fewer, so true positives make up a larger share of all positives. The test has not become better; the population has changed. Targeted screening therefore gives a higher yield of true cases per test and fewer false positives needing confirmation.
| Measure | Depends on | Effect of testing a high-risk group |
|---|
| Sensitivity | The test | No change |
| Specificity | The test | No change |
| Positive predictive value | Test and prevalence | Rises |
| Negative predictive value | Test and prevalence | Falls |
| Reliability | The test and its users | No change |
Why the other explanations are wrong
Reliability is the consistency of results on repeated testing; it depends on the test, the equipment, and the skill of the people using it, not on how many people are tested. Saying specificity or sensitivity rises in a high-risk group treats them as if they depended on prevalence, which they do not. Watch out! Questions often pair "high-risk group" with sensitivity or specificity to tempt candidates. Only the predictive values move with prevalence.
Exam takeaway
Sensitivity and specificity belong to the test; predictive values belong to the test in a particular population. For the RHU, targeting fasting blood glucose testing to women with risk factors makes good use of limited supplies and confirmatory tests. Women who are not tested still receive lifestyle counseling and can be screened if risk factors appear or according to program guidelines.