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#1
05-16-2011
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The likelihood that he has the disease?

55 year old man visits his physician with urinary complaints. Examination finds a 1-cm nodule on his prostate gland. Physician orders a PSA serum test. By common standards, a PSA more than 4ng/ml is considered abnormal. Using this standard, this test has a sensitivity of 80% and specificity of 90%. A recent epidemiologic article found that in a cross-sectional study, 10% of men of this age group have prostate cancer. The result on the patient's PSA is 7ng/ml. What is the best estimate of the likelihood that this man actually has prostate cancer?

a) 13%
b) 25%
c) 36%
d) 47%
e) 58%
f) 69%
g) 72%
h) 81%

#2
05-16-2011
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Positive_predictive_value

D. 47%-This question is testing your knowledge on positive predictive value.

See the link which I found it useful!

http://en.wikipedia.org/wiki/Positive_predictive_value
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#3
05-16-2011
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Using Bayes' theorem:

d) 47%

#4
05-16-2011
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Quote:
 Originally Posted by bebix Using Bayes' theorem: d) 47%
Please Explain step by steps... Thanks.
#5
05-16-2011
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Quote:
 Originally Posted by rulz Please Explain step by steps... Thanks.

PPV= true positive/all test positive

In this case, true positive is 8, total test positive is 17, so 8/17, 0.47 (47%)

Look at the link I posted earlier!
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#6
05-16-2011
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Quote:
 Originally Posted by 1TA2B PPV= true positive/all test positive In this case, true positive is 8, total test positive is 17, so 8/17, 0.47 (47%) Look at the link I posted earlier!
yeap, but you plot the number 17 guessing your FP would be 90, when actually its 10...
#7
05-16-2011
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Please explain this better, because if i take the values from the previous question... it should be something like this...

Sensitivy = 80 So i guess 80/80+20 = 0.8 means the 80%

So here my values are: TP = 80 FN = 20

Specifity = 90 so i guess the same...

90/ 90+10 = 0.9 --- 90%

so my TN = 90 FP = 10

So now i have all i need to go for the PPV

PPV = TP / TP+FP

So i set the numbers 80 / 80+ 10 = 80/90 = 0.88 the closest answer is H - 81....

If am wrong please explain why..

Thanks.
#8
05-16-2011
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Quote:
 Originally Posted by rulz yeap, but you plot the number 17 guessing your FP would be 90, when actually its 10...
Don't forget the prevelance is 10%!!!!!!
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#9
05-16-2011
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Quote:
 Originally Posted by 1TA2B PPV= true positive/all test positive In this case, true positive is 8, total test positive is 17, so 8/17, 0.47 (47%) Look at the link I posted earlier!
Can be please be elaborate.. Am not getting how u got TP 8 and total test positive 17..
#10
05-16-2011
 USMLE Forums Guru Steps History: CK Only Posts: 475 Threads: 39 Thanked 353 Times in 169 Posts Reputation: 376

Quote:
 Originally Posted by rulz Please explain this better, because if i take the values from the previous question... it should be something like this... Sensitivy = 80 So i guess 80/80+20 = 0.8 means the 80% So here my values are: TP = 80 FN = 20 Specifity = 90 so i guess the same... 90/ 90+10 = 0.9 --- 90% so my TN = 90 FP = 10 So now i have all i need to go for the PPV PPV = TP / TP+FP So i set the numbers 80 / 80+ 10 = 80/90 = 0.88 the closest answer is H - 81.... If am wrong please explain why.. Thanks.
Taking 10% prevalence into account,

TP is 8, FP is 9, total positive is 17.

Hope you're alright now!
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#11
05-16-2011
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Quote:
 Originally Posted by 1TA2B Don't forget the prevelance is 10%!!!!!!
What's the importance of 10% prevalence here??
#12
05-16-2011
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Dude, im not getting it... i dont know where did you come up with those i number at all, if you just set the equation as you said its fine,, but PPV doesnt run like that unless there's another equation im not aware of...

all i know when PREVALANCE decrease , PPV decrease and NPV increase... so please step by step like a babe ... explain it if you can..

Thanks.
#13
05-16-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,268 Times in 881 Posts Reputation: 3278

Using Bayesītheorem

P(D|+) =[P(+|D) * P(D)] / [P(+|D) * P(D) + P(+|Dī) * P(Dī)]

where:
P(D) = prevalence of disease
P(+|D) = Prob. of test + given you have the disease = Sensitivity
P(Dī) = people without the disease
P(+|Dī) = Prob. of test + given you donīt have the disease = 1 - Specificity
P(D|+) = Prob. of disease given you tested + = PPV

P(D|+) = 0.8 * 0.1 / [ 0.8 * 0.1 + 0.1 * 0.9] = 0.47

Last edited by bebix; 05-16-2011 at 06:41 PM. Reason: change color
 The above post was thanked by: rulz (05-17-2011), thunderbolt (05-17-2011)
#14
05-17-2011
 USMLE Forums Guru Steps History: CK Only Posts: 475 Threads: 39 Thanked 353 Times in 169 Posts Reputation: 376
PPV equation!

Quote:
 Originally Posted by rulz Dude, im not getting it... i dont know where did you come up with those i number at all, if you just set the equation as you said its fine,, but PPV doesnt run like that unless there's another equation im not aware of... all i know when PREVALANCE decrease , PPV decrease and NPV increase... so please step by step like a babe ... explain it if you can.. Thanks.
Here is the equation from the link I posted earlier-same as my fellow-Bebix explained!

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 The above post was thanked by: thunderbolt (05-17-2011)
#15
05-17-2011
 USMLE Forums Newbie Steps History: 1+CK+CS Posts: 9 Threads: 2 Thanked 1 Time in 1 Post Reputation: 11

Quote:
 Originally Posted by bebix Using Bayesītheorem P(D|+) =[P(+|D) * P(D)] / [P(+|D) * P(D) + P(+|Dī) * P(Dī)] where: P(D) = prevalence of disease P(+|D) = Prob. of test + given you have the disease = Sensitivity P(Dī) = people without the disease P(+|Dī) = Prob. of test + given you donīt have the disease = 1 - Specificity P(D|+) = Prob. of disease given you tested + = PPV P(D|+) = 0.8 * 0.1 / [ 0.8 * 0.1 + 0.1 * 0.9] = 0.47
Thank u.. finally understood the concept..
 The above post was thanked by: bebix (05-17-2011)

#16
05-17-2011
 USMLE Forums Newbie Steps History: 1+CK+CS Posts: 9 Threads: 2 Thanked 1 Time in 1 Post Reputation: 11

Quote:
 Originally Posted by 1TA2B Here is the equation from the link I posted earlier-same as my fellow-Bebix explained!
thanx a lot.. got the concept now..

 Tags Biostatistics-Epidemiology, Step-2-Questions

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