A technician runs an ELISA for HIV antibody detection. The OD reading of the patient sample is 2.8 times the cut-off value. The positive predictive value of this test in a low-prevalence population is MOST dependent on which parameter?
- A Test sensitivity
- B Test specificity
- C Disease prevalence in the tested population ✓
- D The magnitude of the OD ratio
Correct answer: C. Disease prevalence in the tested population
Explanation
Positive predictive value (PPV) is most influenced by disease prevalence in the tested population; even a highly specific test yields many false positives in a low-prevalence setting, reducing PPV. This is why reactive ELISA for HIV in voluntary blood bank donors (low prevalence) must be confirmed by Western blot. Sensitivity primarily affects negative predictive value. Specificity impacts PPV but less dramatically than prevalence at very low base rates. The OD magnitude is not used in standard binary PPV calculations.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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