In a new fasting plasma glucose assay, the laboratory lowers the diagnostic cut-off value closer to the normal range. Which combination of effects on the test will follow?
- A Sensitivity rises and specificity falls ✓
- B Sensitivity falls and specificity rises
- C Both sensitivity and specificity rise
- D Both sensitivity and specificity fall
Correct answer: A. Sensitivity rises and specificity falls
Explanation
Moving the cut-off toward normal classifies more truly diseased individuals as positive, raising sensitivity, but simultaneously labels more healthy individuals as abnormal, lowering specificity. Sensitivity and specificity are inversely related for a single continuous test and depend on the chosen cut-off. The optimal balance is chosen using the ROC curve, typically the point closest to the top-left corner, and the choice itself depends on whether missing cases or false alarms carries greater cost.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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