mph-510-case-study-3-week-6 (1)

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Nicole Ruchotzke Dr. Davila MPH 510 Case Study 3 Screening for Antibody to the Human Immunodeficiency Virus Question 1 : With this information, by constructing a 2-by-2 table, calculate the predictive-value positive and predictive-value negative of the EIA in a hypothetical population of 1,000,000 blood donors. Using a separate 2-by-2 table, calculate PVP and PVN for a population of 1,000 drug users. Assume that the actual prevalence of HIV antibody among blood donors is 0.04% (0.0004) and that of intravenous drug users is 10.0% (0.10). 1,000,000 Blood Donors 1,000,000 x 0.0004= (a + c) = 400 1,000,000-400= (b + d) = 996,600 400 x 0.95 = (A) = 380 400 – 380= (C) = 20 996,600 x 0.98= (D) = 976,608 996,600 - 976,608= (B) = 22.992 Screening Test Result Positive Negative Total Positive 380 22,992 23,372 Negative 20 976,608 976,628 Total 400 996,600 1,000,00 0 PVP= 380/ 23,372 x 100= 1.63% PVN= 976,608/ 976,628 x 100 = 99.9% 1,000 Drug Users 1,000 x 0.10= (a + c) = 100 1,000 – 100= (b + d) = 900 100 x 0.95 = (A) = 95 100-95 = 5 900 x 0.98= (D) = 882 900 – 882 = (B) = 18 Screening Test Result Positive Negative Total Positive 95 18 113 Negative 5 882 887 Total 100 900 1000

PVP = 95/ 113 x 100 = 84.1% PVN= 882/ 887 x 100 = 99.4% Question 2 : Do you think that the EIA is a good screening test for the blood bank? What would you recommend to the blood bank director about notification of EIA-positive blood donors? I don’t think that the EIA is a good screening test for the blood bank. The PVP value for the blood bank is low, therefore, an individual here would have a low probability of having the disease according to Friis (2014, p. 480). I would recommend further testing on the individuals who tested positive, before throwing out their blood. There is a good chance, since the PVP value was so low, these people do not have HIV, and therefore, it would be a waste of their blood. Question 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? I think the EIA performs well enough to justify informing test-positive clients in drug abuse clinic they are positive for HIV, due to the high PVP value. Prevalence of the disease increases as the PVP value rises. Question 4 : If sensitivity and specificity remain constant, what is the relationship of prevalence to predictive-value positive and predictive-value negative? Sensitivity and specificity are unaffected by the prevalence of a disease. Therefore, when the prevalence falls so does the PVP. However, in the opposite happens for PVN, the value rises. Question 5 : In terms of sensitivity and specificity, what happens if you raise the cutoff from "A" to "B"? By moving the cutoff point from “A” to “B,” the “specificity will increase at the expense of sensitivity,” according to Friis (2014, p. 483). Question 6 : In terms of sensitivity and specificity, what happens if you lower the cutoff from "A" to "C"? When the cutoff is lowered from “A” to “C,” sensitivity would increase and specificity would decrease leading to more people without antibodies being classified with antibodies. Question 7: From what you know now, what is the relationship between sensitivity and specificity of a screening test. In order to minimize the number of people classified as diseased or non-diseased, sensitivity and specificity need to be balanced. Question 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? They would want the cutoff to be at a hypothetical “A” for both groups. Therefore, the sensitivity and the specificity would be equal among the groups. Those in the blood donor group would probably prefer a lower cutoff value in order to increase the number of those screened positive.

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