WordPress.com



Nicole RuchotzkeDr. DavilaMPH 510Case Study 3Screening for Antibody to the Human Immunodeficiency VirusQuestion 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 Donors1,000,000 x 0.0004= (a + c) = 4001,000,000-400= (b + d) = 996,600400 x 0.95 = (A) = 380 400 – 380= (C) = 20996,600 x 0.98= (D) = 976,608996,600 - 976,608= (B) = 22.992 Screening Test ResultPositiveNegativeTotalPositive38022,99223,372Negative20976,608976,628Total400996,6001,000,000PVP= 380/ 23,372 x 100= 1.63%PVN= 976,608/ 976,628 x 100 = 99.9%1,000 Drug Users1,000 x 0.10= (a + c) = 1001,000 – 100= (b + d) = 900100 x 0.95 = (A) = 95100-95 = 5900 x 0.98= (D) =882900 – 882 = (B) =18Screening Test ResultPositiveNegativeTotalPositive9518113Negative5882887Total1009001000PVP = 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. Part IIQuestion 9: What is the actual antibody prevalence in the population of persons whose blood samples will undergo a second test?PVP= 380/ 23,372 x 100= 1.63%Question 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-EIAPVP= 380/760 x 100 = 50.0%EIA-Western BlotPVP= 320/400 x 100= 80.0%Question 11: Why does the predictive-value positive increase so dramatically with the addition of a second test? Why is the predictive value positive higher for the EIA-WB sequence than for the EIA-EIA sequence? By adding the second test, in increases the likelihood that a positive value is positive. The second test weeds out the false positives only leaving behind the ones that are. The predictive value positive increases dramatically because of specificity of the western blot. The western blot is able to accurately identify false positives more than the EIA due to the increases specificity. Question 12: What criteria would you consider in evaluating this proposed screening program?I would evaluate its ability to screen for the presence of antibodies. In this case, the screening using the western blot is extremely good at determining who has the antibody present. Additional criteria I would consider is the time involved with other screening programs in addition to cost. Some programs may be highly efficient in determining positives and negatives, but maybe extremely expensive, therefore it would need to be determined if it was worth the cost to run the tests. Question 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 of the screening program in the next year? What is the cost per identified antibody-positive person?Cost of Screening Next Year($50.00 x 60,000) + (143 x 100.00) = $3,014,300Cost per Identified Antibody-Positive Person$3,014,300/22= $137,013.64Question 14: What is your final recommendation to the Governor?My recommendation is to stick with the EIA testing. It is cost effective. Whereas in regards to the western blot test, it is not cost effective to perform. The cost outweighs the benefits. In sticking with the EIA, I would suggest having the positive samples be matched with the donors. From there notify them and have them get tested on their own. By having predictive value positive donors be aware of their HIV status, it will keep them from donating blood in the future in addition to reducing the risky behaviors they are involved in. The idea of having those who are to be married tested is a good idea, it would give public health officials an idea of the rate of HIV in the state in addition to potentially leading to who is spreading HIV. ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download