A new point-of-care rapid test for pulmonary TB is evaluated in a high-prevalence district (disease prevalence 5%). Sensitivity is 90%, specificity is 85%. The Positive Predictive Value (PPV) of this test in this setting is approximately:
- A 45%
- B 90%
- C 24% ✓
- D 64%
Correct answer: C. 24%
Explanation
Using Bayes' theorem with prevalence 5%: True positives = 0.90 × 50 = 45 (per 1000); False positives = (1−0.85) × 950 = 142.5. PPV = 45 / (45 + 142.5) = 45/187.5 ≈ 24%. This illustrates that even with good sensitivity and specificity, PPV is low in moderate-prevalence populations because false positives outnumber true positives. PPV is strongly determined by disease prevalence—the same test deployed in a very high-prevalence (≥20%) population would yield PPV >70%. This is fundamental to the decision of where to implement screening.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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