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#1
11-10-2010
 USMLE Forums Addict Steps History: Not yet Posts: 114 Threads: 23 Thanked 60 Times in 40 Posts Reputation: 80
The percentage of test positive and have the disease?

You are working with rare diseases. You have laboratory test which will confirm a disease that affects 1 in 100 000 in general population. Test have sensitivity of 99% and specificity of 95%. How many percent of those who have test positive in the end has the disease?.

a) 0,02%
b) 0,2%
c) 2%
d) 20%

#2
08-25-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,259 Times in 881 Posts Reputation: 3269

I think the answer is a) 0.02%
#3
08-25-2011
 USMLE Forums Guru Steps History: 1+CK+CS+3 Posts: 406 Threads: 30 Thanked 140 Times in 108 Posts Reputation: 150

Quote:
 Originally Posted by bebix I think the answer is a) 0.02%
#4
08-25-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,259 Times in 881 Posts Reputation: 3269

Quote:
 Originally Posted by fin-84 You are working with rare diseases. You have laboratory test which will confirm a disease that affects 1 in 100 000 in general population. Test have sensitivity of 99% and specificity of 95%. How many percent of those who have test positive in the end has the disease?. a) 0,02% b) 0,2% c) 2% d) 20%
Prevalence = 1/100.000

Sensitivity = P(+|D) = 0.99
Specificity = P(-|Dī) = 0.95

How many percent of those who have test positive in the end has the disease? = P(D|+) in %

So, 1 every 100.000 ==> 100 every 10.000.000

............D.........Dī
test +...99....... 499995..... 500094
test -....1........xxxx
Total .100..... 9999900...10.000.000

D = Disease
Dī= Disease free

P(D|+) = 99/500094 = 0.0002

in % = 0.0002*100 = 0.02%
 The above post was thanked by: drfchakera (08-27-2011), pass7 (08-30-2011)

 Tags Biostatistics-Epidemiology, Step-1-Questions

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