Reference no: EM132564905
1. In the mid-1980s, the first ELISA screening test kits for HIV were sent out to state blood banks. By that time, it was well-established that people could become HIV positive through blood transfusions so keeping HIV positive blood out of the blood banks was critical. These first ELISA (now called EIA) tests had sensitivities of 94.5% and specificities of 97.5%.
a) Construct a 2x2 screening table using the above values when 100 positive samples and 100 negative samples are tested
b) At one state blood bank, two groups were selected for testing the kits. One was comprised of 10,000 blood units donated by healthy donors with no known risk factors for HIV infection. The estimated prevalence of HIV antibodies in such groups was 0.04%. Using the available information construct the 2x2 screening table for these 10,000 blood units. What are the PV+ and PV-?
c) The second group was 1000 units of blood that had been collected from known intravenous drug user (IDUs) who often sold their blood, especially if they had a rare type. It was estimated at this time that 15% of IDUs were HIV positive. Construct a table similar to the one in (b) Calculate the PV+ and PV-
d) What does the information from (b) and (c) say about the importance of the prevalence of a condition to PV+ and PV- values of a screening test
e) The ELISAs were read according to optical density ratios which tested the absorbance of a testing specimen against a known negative control. The cutoff for a positive ODR test was somewhat arbitrary at the time. In the figure below, identify the ODR cutoff (the vertical line) where sensitivity is higher than specificity, where sensitivity is lower than specificity, and where the two measures are about equal
f) In the figure, is the small arrow in A pointing towards FN or F? The left small arrow in B? The right small arrow in B? The small arrow in C?
g) In this situation, keeping HIV positive blood out of the blood banks was clearly of paramount importance. Thus, should the sensitivity or specificity of the ELISA test be focused on when trying to improve it or are both equally important?
h) True or false: If you had a rare blood type, you would be at higher risk of acquiring HIV from a blood transfusion.