Which expression gives the apparent prevalence?

• A. p(D+) = (a+b)/n
• B. p(T+) = (a+c)/n
• C. p(D+) = (a+c)/n
• D. p(T+) = (a+c)/n

Answer: (D)

Apparent prevalence is the proportion of people who test positive for the disease, based on the test results themselves, not on confirmed disease status. In the common 2x2 setup, a represents those who are truly diseased and test positive (D+ and T+), and c represents those who are not diseased but still test positive (D- and T+). The total number who test positive is a + c, and dividing by the total tested, n, gives p(T+) = (a + c)/n. This is exactly why the correct expression is the apparent prevalence.

If you were measuring disease prevalence (not just who tests positive), you’d count all D+ individuals across test outcomes (a + b, where b is D+ and T-), which is a different concept.