DrVirgo
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142. The standard test to detect a certain sexually transmitted disease has a sensitivity of 90% and a specificity of 80%. IF a recent cross-sectional study found that 1 out of every 10 people aged 15-19 had this disease, the positive predictive value of this screening test for people aged 15-19 would be estimated as
A. 21%
B. 33%
C. 50%
D. 67%
E. 85%
(Please explain your calculations and formulas)
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| robin082006
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B
......................Disease Non-disease Total
Positive--------9 18 27
Negative-------1 72 65
Total-----------10 90 100
PPV=9/27=33%
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| DrVirgo
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You are right.
Correct answer is 33%.
Can you please explain how you got other numbers in the table?
Thanks.
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| robin082006
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Firstly, you write down number of diseases and non-diseases based on prevalence, E.g. prevalence 20%, disease is 20 and non-disease is 80, for easy calculation, total should be 100.
Then based on Sensitivity and Specificity, write down number of true positive and true negative.
False Negative and false positive then are calculated by substracting from total disease-true positive and total non-disease-true negative, respectively.
then calculate PPV = true positive/total positive
NPV=true negative/total negative
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| DrVirgo
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Thanks Robin. Got it! 
(Total of Negatives should be 73)
-And NPV would be 72/73?
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| equinoxe
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Thank you robbin, for both examples and explication. I was wrong all the way. http://www.prep4usmle.com/forum/thread/40765
........dis+....dis-
test+...9.......18 27
test-...1.......72 73
sensitivity 90
specificity 80
prevalence 10
population 100
diseased 10
TP = .9 x 10 = 9
Fn = 10 - 9 = 1
TN = .8 x 90 = 72
fp = 90 - 72 = 18
Pv+ = 9/27 = 1/3 = 33%
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