Assignment Overview
Infection with the human immunodeficiency virus (HIV) is routinely diagnosed by detecting the presence of specific antibodies in the patient’s serum. Although the presence of the virus itself can be now be detected (i.e. polymerase chain reaction [PCR] to measure plasma HIV RNA, or “viral load”), these tests remain expensive and require laboratory techniques that are not routinely available. The diagnosis of HIV infection begins with an enzyme immunoassay (EIA). The optical density (OD) of the patient’s EIA is compared to a control specimen (OD ratio). If the OD ratio is – above the established cutoff for that control sample on repeat testing the specimen is termed “repeatedly reactive”. The EIA is relatively sensitive, fast, simple and inexpensive which makes it an appropriate screening test. However, if one examines the ODs for a large group of samples from patients with and without true HIV infection you can see that there is some overlap in their EIA results if a value of A is used for the cutoff:

Hypothetical distribution of OD ratios for patients with and without HIV infection.
Hypothetical distribution of OD ratios for patients with and without HIV infection.
Patients with HIV infection are depicted with blue, thick line,those without HlV with red, thin line.
Where a cutoff is drawn to determine a diagnostic test result may be somewhat arbitrary.
Case Assignment
Answer these questions:
1. What would be the impact on this test’s sensitivity if you moved the cutoff for a positive result from A to B? What about specificity? What would happen to sensitivity and specificity if you moved the cutoff from A to C? Where would you put the cutoff for this test? What is the relationship between sensitivity and specificity for any given test?
2. Where would the director of the Blood Bank who is screening donated blood for HIV antibody want to put the cutoff? What would be the potential problem caused by this decision? Where would an investigator enrolling high-risk patients in a clinical trial for an experimental, potentially toxic antiretroviral draw the cutoff? What would be the potential problem caused by this decision? In your answer please detail possible ethical and legal issues of possible concern.