Shannon Mikrut
Week 6 Case Study
Case Study: Screening for Antibody to HIV
4/13/2013
1. With this information, by constructing a 2-by-2 table, calculate the predictive-valuepositive and predictive-value negative of the EIA in a hypothetical population of1,000,000 blood donors. Using a separate 2-by-2 table, calculate PVP and PVN for apopulation of 1,000 drug users. Assume that the actual prevalence of HIV antibodyamong blood donors is 0.04% (0.0004) and that of intravenous drug users is 10.0%
(0.10).
2x2 Table for Blood Donors
Present Absent
380 / 19,99220 / 979,608
Positive
Negative
Total: 400 Total: 999,600
2x2 Table for Drug Users
Present Absent
95 / 185 / 882
Total: 100 Total: 900
Blood Donors:
1,000,000 x.0004= 400
400 x.95= 380
999,600 x.98= 979,608
All positive tests= 20,372
All negative tests= 979,628
Drug Users:
1000 x.10= 100
100 x.95= 95
900 x.98= 882
All positive tests= 113
All negative tests= 887
**please see attachment for 2x2 table calculations**
2. Do you think that the EIA is a good screening test for the blood bank? What would yourecommend to the blood bank director about notification of EIA-positive blood donors?
No, I do not think that the EIA is a good screening test for the blood bank. I do not think that it is a good screening measure because of the predictive value positive and the negative calculations. There are so many blood samples for the blood bank to go through and this leaves room for a lot of error, by showing a positive or negative results when it is actually the other way around. I would recommend for the blood bank director to inform EIA-positive people in person and encourage them to see their doctors for further testing. By seeing a medical professional, they can get a second opinion and confirm or reject the original EIA result and also maintain confidentiality.
3. Do you think that the EIA performs well enough to justify informing test-positive clients in the drug abuse clinics that they are positive for HIV?
Yes, I do think that the EIA test performs well enough to justify informing test-positive clients in the drug abuse clinics that they tested positive for HIV. The clinic is likely to test less individuals and the sensitivity of the test kit should be taken into consideration. I would still strongly encourage them to speak with their doctors and obtain further testing to confirm the diagnosis. The kit is not 100% accurate and seeking further testing could help confirm the person’s disease status.
4. If sensitivity and specificity remain constant, what is the relationship of prevalence topredictive-value positive and predictive-value negative?
If the sensitivity and specify remain constant and the prevalence of a disease falls, the predictive value positive falls. One the other hand, the predictive value negative rises. A low positive predictive value would mean that the person who tests positive would have a low probability of having the disease.
5. In terms of sensitivity and specificity, what happens if you raise the cutoff from "A" to "B"?
If I raised the cutoff from A to B, the specificity would improve at the expense of the sensitivity.
6. In terms of sensitivity and specificity, what happens if you lower the cutoff from "A" to"C"?
If I lowered the cutoff from A to C, the sensitivity would improve, but the specificity would not.
7. From what you know now, what is the relationship between sensitivity and specificity of a screening test?
The relationship between sensitivity and specificity focuses on the cutoff for the measurement and improving how sensitive a test is. To improve the specificity, one would want to move the cutoff point toward the direction of the individuals associated with the disease. To improve sensitivity, one would want to move the cutoff measurement farther in the direction of the non-diseased.
8. Where might the blood bank director and the head of drug treatment want the cutoff point to be for each program? Who would probably want a lower cutoff value?
The blood bank director might want the cutoff to be moved farther in the range of the non-diseased. The blood bank director may want to do this because there are such a large number of tests being done and a more sensitive test would be more effective in reducing the amount of incorrect diagnoses (for example: reducing the number of false positive readings). The drug treatment director may want a lower cutoff value. Intravenous drug use is a risk factor for getting HIV.
9. What is the actual antibody prevalence in the population of persons whose blood samples will undergo a second test?
15.2% (380 x.0004=.152 x100= 15.2)
10. Calculate the predictive-value positive of the two sequences of tests: EIA-EIA and
EIA-Western blot. Assume that the sensitivity and specificity of the EIA are 95.0% and
98.0%, respectively. Assume that the sensitivity and specificity of the Western blot are
80.0% and 99.99%, respectively. Also assume that the tests are independent, even
though they may not be (e.g., those with cross-reactive proteins are likely to cross-react
each time).
EIA-EIA
+ / -+ / 27 / 3
- / 2 / 158
EIA Western Blot
+ / -+ / 23 / 0
- / 6 / 161
**please see attached document**
11. Why does the predictive-value positive increase so dramatically with the addition of asecond test? Why is the predictive value positive higher for the EIA-WB sequence thanfor the EIA-EIA sequence?
The predictive value positive increases so dramatically because the second EIA test people who truly have HIV was a more specific number than the first test. The predictive value positive was higher for the EIA-WB sequence because the specificity of the test was high; the specificity was near 100%.
12. What criteria would you consider in evaluating this proposed screening program?
I would consider evaluating the screening criteria, it may have selection bias. The screening program may not be a good representation of all individuals because it was only screening pre-marital couples for HIV. This especially may be a bias screening process because HIV is often prevalent in homosexuals. The selection of pre-marital couples is therefore not a good representation of HIV in the whole state. HIV is often more prevalent in intravenous drug users, this also can contribute to selection bias within the screening criteria.
13. Compute the cost of the screening program. Assume a cost of $50.00 for every initial EIA test ($10.00 lab fee and $40.00 health-care-provider visit) and an additional$100.00 for EIA-positive persons who will need additional testing. What is the cost ofthe screening program in the next year? What is the cost per identifiedantibody-positive person?
Total for EIA initial testing only (not including positive test result people): $2,992,850 (59,857x 50= 2,992,850)
Total for EIA positive persons who need additional testing: $21,450
(143 x150= 21,450)
Total for initial testing: $3,014,300
(2,992,850 +21,450= 3,014,300)
Cost per identified anti-body person: $21,450
(3,014,300/143= 21,450)
14. What is your final recommendation to the Governor?
My recommendation to the governor would be that the benefit may not outweigh the costs. It would be costly to find anti-body positive cases and the screening criteria are too bias. The screening of pre-marital couples does not take into account homosexual couples, who are at a higher risk HIV. This would be time consuming, costly, and not produce the type of results legislators are seeking.
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