help with epidemiology

[dr2b0804]

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Hi! I am studying for step 2 and was wondering how many epidemiology questions everyone had on their tests? Also, if someone could explain how to do the following question, I'd be forever grateful!
A 55 year old guy comes in with a 1cm nodule in his prostate. By common standards, a PSA greater than 4ng/ml is considered abnormal. Using this standard, this test has a sensitivity of 80% and a specificity of 90%. A recently published epidemiologic article found that in a cross-sectional study, 10% of men of this age have prostate cancer. The result on the patient's PSA is 7 ng/ml. What is your 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%

Ahhh! I hate this stuff...it gives me a headache! 😱

 

[barone ROCKS]

i got 47%

i just took step one so i may be in over my head but this is how i got it

using sensitivity and specificity and that 10% have the disease i came up with

tp= 80
tn= 810
fp= 90
fn= 20

and the questions is asking for the positive predictive value which is 80/170=47

hope its right

 

[Idiopathic]

That looks right to me. It is certainly work to get there, but your reasoning is sound.

 

[lotanna]

I got 47% too

Gotta luv Epi 😀

 

[smgilles]

Hi! I am studying for step 2 and was wondering how many epidemiology questions everyone had on their tests? Also, if someone could explain how to do the following question, I'd be forever grateful!
A 55 year old guy comes in with a 1cm nodule in his prostate. By common standards, a PSA greater than 4ng/ml is considered abnormal. Using this standard, this test has a sensitivity of 80% and a specificity of 90%. A recently published epidemiologic article found that in a cross-sectional study, 10% of men of this age have prostate cancer. The result on the patient's PSA is 7 ng/ml. What is your 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%

Ahhh! I hate this stuff...it gives me a headache! 😱

prevalence=p=.10
sensitivity =a=.80
specificity =b=.90

PV+=(p)(a)/(p)(a)+(1-p)(1-b)x100%=(.08)/(.08)+(.09)x100%=47%

 

[lotanna]

PPV is affected by prevalence.

 

[smgilles]

PPV is affected by prevalence.

Is that a question or a statement?

 

[dr2b0804]

That is the right answer....thanks!! 😀

 

[lotanna]

Is that a question or a statement?

statement 😛