A programme manager considers reducing the cut-off value for a screening test (e.g., lowering blood glucose threshold for diabetes screening). The predictable effect on test performance is:

• A Sensitivity increases, specificity decreases, and false positive rate increases ✓
• B Both sensitivity and specificity increase simultaneously
• C Specificity increases, sensitivity decreases
• D PPV increases because more true cases are detected

Explanation

Lowering the cut-off (threshold) for a continuous test marker shifts the decision boundary so that more individuals are classified as positive. This captures more true cases (increases sensitivity / decreases false negatives) but also classifies more healthy individuals as positive (decreases specificity / increases false positive rate). The SENS-SPEC trade-off is inversely related on the ROC curve. PPV may actually fall when false positives increase dramatically in a low-prevalence population. This is a key decision-making concept in programme design — e.g., cancer screening prioritises high sensitivity even at the cost of specificity.

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

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Written and medically reviewed by the StethoPrep medical team.