A screening test for colorectal cancer using faecal immunochemical test (FIT) is applied to 10,000 asymptomatic adults ≥50 years. The test has sensitivity 80% and specificity 96%. Disease prevalence is 1%. The Positive Predictive Value (PPV) of this test in this population is approximately:

• A 80%
• B 96%
• C 4%
• D 17% ✓

Explanation

In 10,000 adults with 1% prevalence: 100 have disease, 9,900 do not. True positives = 80% × 100 = 80. False positives = 4% × 9,900 = 396. PPV = TP/(TP+FP) = 80/(80+396) = 80/476 ≈ 16.8% ≈ 17%. This illustrates that even a high-specificity test has poor PPV when prevalence is low — a critical concept in population screening. For every true positive, approximately 5 false positives undergo unnecessary follow-up colonoscopy.

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

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