An epidemiologist lowers the blood sugar cut-off value used to classify individuals as screen-positive for diabetes in a community programme. What is the expected consequence of lowering the cut-off?

• A Sensitivity increases and specificity decreases ✓
• B Sensitivity decreases and specificity increases
• C Both sensitivity and specificity increase
• D Both sensitivity and specificity decrease

Explanation

Sensitivity and specificity move in opposite directions as the cut-off changes because they depend on how the test values overlap between the healthy and diseased populations. Lowering the cut-off labels more people as positive, so fewer true cases are missed, raising sensitivity, while more healthy people are wrongly labelled positive, lowering specificity. Raising the cut-off would produce the reverse trade-off. This trade-off is visualised along the ROC curve.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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