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#1




The likelihood that he has the disease?
55 year old man visits his physician with urinary complaints. Examination finds a 1cm 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 crosssectional 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




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




Using Bayes' theorem:
d) 47% 
#4




Please Explain step by steps... Thanks.

#5




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




yeap, but you plot the number 17 guessing your FP would be 90, when actually its 10...

#7




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




Don't forget the prevelance is 10%!!!!!!
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#9




Can be please be elaborate.. Am not getting how u got TP 8 and total test positive 17..

#10




Quote:
TP is 8, FP is 9, total positive is 17. Hope you're alright now!
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#11




What's the importance of 10% prevalence here??

#12




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




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; 05162011 at 06:41 PM. Reason: change color 
The above post was thanked by:  
rulz (05172011), thunderbolt (05172011) 
#14




PPV equation!
Quote:
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#15




Quote:

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bebix (05172011) 
#16




thanx a lot.. got the concept now..

Tags 
BiostatisticsEpidemiology, Step2Questions 
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