Question 2: One factor that can influence the feasibility of a screening program is the yield, i.e. the number of cases detected. This can be estimated from the positive predictive value. Yield is very dependent on the prevalence of the disease in the population being tested.

1. The following two scenarios illustrate the consequences of HIV testing using a test that is 100% sensitive and 99.5% specific.The only thing that differs in these scenarios is the prevalence of undiagnosed HIV.

Scenario 1: 100,000 female blood donors in whom the prevalence of HIV (true disease status - aka gold standard) is 0.01%. Calculate the positive predictive value. Show the 2 X 2 table. (4 points)

Scenario 2: 100,000 males in a clinic for sexually transmitted infections in whom the prevalence of HIV is 4%. Calculate the positive predictive value. Show the 2 X 2 table. (4 points)

1. Which population would you choose for the administration of our screening program? (2 points) Why? (2 points)