A rapid antigen test for COVID-19 has a sensitivity of 80% and specificity of 95%. It was first used as a screening tool among household contacts of confirmed cases and later among asymptomatic blood donors. The positive predictive value of the same test will be:

• A Identical in both groups because it depends only on sensitivity and specificity
• B Higher in blood donors because their disease prevalence is lower
• C Higher in household contacts because their disease prevalence is higher ✓
• D Lower in household contacts because false negatives increase with prevalence

Explanation

Sensitivity and specificity are intrinsic properties of a test, but predictive values depend on disease prevalence. When prevalence rises, more true positives enter the pool, so the proportion of positive results that are genuine increases, raising the PPV. Household contacts have far higher prevalence than blood donors, so PPV is higher there. Option A is tempting but wrong precisely because PPV is not an intrinsic property; option D confuses PPV with negative predictive value.

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

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Written and medically reviewed by the StethoPrep medical team.