A screening programme lowers the cut-off blood glucose value used to label a subject 'screen-positive' for diabetes. Compared with the previous cut-off, this change will:
- A Increase sensitivity and decrease specificity ✓
- B Decrease sensitivity and increase specificity
- C Increase both sensitivity and specificity
- D Leave sensitivity unchanged but increase positive predictive value
Correct answer: A. Increase sensitivity and decrease specificity
Explanation
Lowering the cut-off classifies more people as positive, capturing more true diabetics, so sensitivity rises. Simultaneously, more normoglycaemic individuals fall above the lower threshold and become false positives, so specificity falls. Sensitivity and specificity move in opposite directions when a single cut-off is shifted along the test's distribution, which is precisely the trade-off visualized by the ROC curve.
Reference: Gordis Epidemiology, 6th ed.
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