When a screening programme is designed and the cut-off value of a test is shifted to the LEFT (towards lower values) in a positively-skewed disease distribution, which of the following BEST describes the consequence?

• A Sensitivity decreases, specificity increases, PPV increases
• B Both sensitivity and specificity increase simultaneously
• C Sensitivity increases, specificity decreases, PPV decreases ✓
• D PPV increases because fewer false positives occur

Explanation

Moving the cut-off to the left (lower threshold) means more individuals are classified as 'test positive'. This captures more true cases (sensitivity increases) but also misclassifies more truly disease-free individuals as positive (false positives increase, specificity decreases). Since PPV = TP/(TP+FP), an increase in false positives at constant prevalence leads to a decreased PPV. This is the fundamental inverse relationship between sensitivity and specificity when adjusting a single threshold — they cannot both increase simultaneously by threshold manipulation alone.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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