When a screening programme shifts the cut-off point for a test to include more borderline cases (lowering the threshold), which of the following changes is expected?

• A Sensitivity decreases and specificity increases
• B Sensitivity increases, specificity decreases, and PPV decreases ✓
• C Both sensitivity and specificity increase simultaneously
• D Specificity increases and false positive rate decreases

Explanation

Lowering the screening threshold means more individuals test positive. This increases sensitivity (fewer true disease cases are missed, reducing false negatives) but decreases specificity (more healthy individuals test positive, increasing false positives). Because more false positives are generated, the positive predictive value (PPV) falls. This trade-off is captured by the ROC curve, and the optimal cut-off is chosen based on the relative costs of false positives versus false negatives for the disease in question.

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

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