Homework 12 Solutions
1) Compare two different testing strategies. For each, list the sensitivity, specificity, false positive, false negative, positive predictive value, and total cost in both a high and low prevalence setting.
Is the cost of the more expensive one justifiable? Which strategy would you recommend under what circumstances?
High Prevalence (5%) Setting
Therefore (depending on the test chosen),
Se = 99.9% PPV = 98%
Sp = 99.9% Total Cost = $2,341,176.50 (USD)
Low Prevalence (0.5%) Setting
Therefore (depending on the test chosen),
Se = 99.9% PPV = 83%
Sp = 99.9% Total Cost = $252,117.65
The cost of the more expensive one is not justifiable, despite the fact that these are the parts of the world with the greatest prevalence and thus have a very good positive predictive value. For these are also the parts of the world that suffer from a great many other needs and they would benefit more from many much less costly one time health interventions, such as vaccines. Furthermore, even in these locations persons with a positive diagnosis are not likely to receive treatment.
(Many other answers would be acceptable here).
I would recommend doing mass screenings in low-prevalence settings where there is sufficient money to support treatment. In these settings, I would target pockets of higher prevalence to improve the positive predictive value of the test. I would do mass screenings in high-prevalence settings in specific groups, such as sex workers, where diagnosis and treatment would best stem the spread of disease.
(Many answers would be acceptable).
2) In 2002, the US Congress spent $143,800,000.00 on the CDC Global AIDS program. How inexpensive would your screening test have to be (with a sensitivity of 99.9% and specificity of 99.9%) to be able to screen the population of Uganda (25,632,794) which has a prevalence of 5% and not be over budget for the entire CDC Global AIDS program?
The test could not have a kit cost of more than $3 and would have to have not labor cost.