An ELISA test for HIV antibody detection has a sensitivity of 99.5% and specificity of 99.2%. In a screening programme in a population where HIV prevalence is 0.1%, what is the positive predictive value (PPV) of a positive ELISA result?

• A Approximately 55%
• B Approximately 11% ✓
• C Approximately 99%
• D Approximately 80%

Explanation

In a population of 100,000 with 0.1% HIV prevalence: true positives = 100 (HIV+); true negatives = 99,900 (HIV−). True positives detected = 100 × 0.995 = 99.5 ≈ 99. False positives = 99,900 × (1−0.992) = 99,900 × 0.008 = 799.2 ≈ 799. Total positive results = 99 + 799 = 898. PPV = 99/898 ≈ 11%. This illustrates the critical principle that in low-prevalence settings, even a highly sensitive and specific test has a low PPV — the majority of positives are false positives. This is why confirmatory Western blot (or now NAAT/4th generation combination assay with confirmation) is mandatory before diagnosing HIV in a low-prevalence screening programme. PPV increases when testing in high-prevalence populations (targeted testing).

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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