A rapid diagnostic test for malaria has sensitivity of 90% and specificity of 80%. In a low-endemicity district the prevalence of malaria is 20%. What is the Positive Predictive Value (PPV) of the test in this district?
- A 62.5%
- B 52.9% ✓
- C 73.2%
- D 81.8%
Explanation
Using a hypothetical population of 1000 with 20% prevalence: 200 true cases and 800 non-cases. True Positives (TP) = 0.90 × 200 = 180; False Positives (FP) = 0.20 × 800 = 160. PPV = TP / (TP + FP) = 180 / (180 + 160) = 180/340 = 52.9%. Despite good sensitivity and specificity, PPV is only 52.9% in a low-prevalence setting — illustrating that PPV depends heavily on disease prevalence. The same test in a 50% prevalence population would give PPV = (0.9×500)/(0.9×500 + 0.2×500) = 450/550 = 81.8%, demonstrating why screening high-risk populations improves positive predictive value.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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