Adapted from EIS case studies

In December 1982, a report in the MMWR described three persons who had developed acquired immunodeficiency syndrome (AIDS) but who had neither of the previously known risk factors for the disease: homosexual/bisexual activity with numerous partners and intravenous drug use. These three persons had previously received whole-blood transfusions. By 1983, widespread recognition of the problem of transfusion-related AIDS led to controversial recommendations that persons in known high-risk groups voluntarily defer from donating blood. In June 1984, after the discovery of the human immunodeficiency virus (HIV), five companies were licensed to produce enzyme-linked immunosorbent assay (EIA, then called ELISA) test kits for detecting HIV antibody. A Food and Drug Administration

(FDA) spokesperson stated that, "...getting this test out to the blood banks is our No. 1 priority...." Blood bank directors were anxiously waiting to start screening blood with the new test until March 2, 1985, the date the first test kit was approved by the FDA.

In order to evaluate whether the EIA test should be used as a screening tool, people were tested using the EIA test and had their blood tested to determine whether the HIV antibody was present or not (the gold standard).

HIV antibody present

HIV antibody absent

Total

EIA test positive

380

19,992

20,372

EIA test negative

20

979,608

979,628

Total

400

979,608

1,000,000

The sensitivity of the EIA test was 95% and the specificity was 98%.

The positive predictive value of EIA is 1.9%. The negative predictive value of EIA is 99.9%

1. Calculate the prevalence of HIV in this population.

2. Why do you think the positive predictive value of this test is so low?