Allison Erickson

Case Study: Screening for Antibody to the Human Immunodeficiency Virus

1. Please see below for full computation and 2x2 tables.

2.

Given the results from the previous question, I would say that the EIA is not a good test for the blood bank because of the predictive value positive and negative calculations. With such a large number of blood samples to go through, there can be room for error by showing a positive or negative result when it is actually the other way around. I would recommend to the blood bank director to notify the EIA-positive individuals in person and inform them to be tested again by their doctor. By encouraging a second test by a medical professional, they can assume confidentiality, get a more accurate result, and, if needed, start seeking medical treatment.

3.I think that the EIA performs well enough to justify informing those individuals in the drug abuse

clinics that they are positive for HIV. Considering the fact that there are probably less individuals to test at the clinics compared to a blood bank and taking the sensitivity of the test kits into consideration, there is enough evidence that would support telling those individuals they are HIV positive. However, because the sensitivity is not 100 percent, there would need to be encouragement to seek out another test to be absolutely positive. In any case, by informing the individual they are HIV positive, which may ignite a sense of urgency to begin making behavior changes.

4.If the prevalence of a disease falls, then the predictive-value positive falls while the predictive-value

negative rises. A low positive predictive-value, in a clinical setting, would imply that the individual’s positive screening test would have a low probability of actually having the disease.

5. If we raise the cutoff from “A” to “B,” the specificity would improve at the expense of the sensitivity.

6. If we lower the cutoff from “A” to “C,” the sensitivity would improve, but the specificity would not improve.

Allison Erickson

7.

The relationship between specificity and sensitivity is that as one focuses on improving how sensitive a test is, the cut off for the measurement should be moved farther into the range of the non-diseased. For one to improve the specificity, the cutoff point should be moved toward the individuals associated with the disease.

8.

The blood bank director would probably want the cutoff point to be moved farther in the range of the non-diseased because with a high number of tests being done, there would be a need for a more sensitive test. In the case of the drug treatment program, the cutoff point would be moved closer in the range of those typically associated with the disease. I would suggest that way because intravenous drug use is a major risk factor for getting HIV, therefore, having a more specific test would be helpful. In this case, the drug treatment program would want a lower cutoff value.

9. 15.2% --- Please see below for full computations.

10. Please see below for full computations.

11.

The predictive-value positive increases so dramatically for the second EIA test because the people who actually have the condition was a more specific number than the first test. The predictive-value positive is high for the EIA-WB sequence because the specificity of the test was so high, almost 100 percent.

12.I would consider the criteria that the screening may have selection bias. Because the screening

program is just looking at the HIV rates among premarital couples, there can be bias in that it may not fully represent a true account for all individuals who are HIV positive in the state. In the same context, because HIV is high in homosexual men and intravenous drug users, the selection of premarital couples is not offering a clear understanding of HIV in the whole state. Additionally, there can be push-back when it comes to requiring a screening, rather than offering it voluntarily.

Allison Erickson

13. Total for just the initial EIA test for all being tested, not including positive test result individuals: $2,992,850

Total for EIA-positive persons who will need additional testing: $21,450

Total for all initial testing: $3,014,300

Cost per identified antibody-positive person: $21,450

(Please see below for computations)

14.My final recommendation to the Governor is that it will cost a lot of money, probably tax payer

money, to find cases of that are antibody-positive people. Also, because HIV is not high in heterosexual couples, compared to homosexual couples or intravenous drug users, spending this much money and investing lots of time would not give positive result for what the legislators would be looking for with the new program. Essentially, the benefit does not outweigh the cost and time.

Allison Erickson

Computations and Answers