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Author  7 Posts  
tompat
Forum Newbie Topics: 64 Posts: 561 
55 yr old man visits his physician with c/o urinary infrequency. on examination 1cm nodule is found on his prostate gland.PSA serum test result is 7 ng/ml.a PSA level of >4ng/ml is abnormal.using this standard this test has a sensitivity of 80% and a specificity of 90%. a recently published epidemiological article found that in a cross sectional study, 10% of men of this age have prostate cancer. what is ur 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%  
galeen
Forum Guru Topics: 56 Posts: 578 
if i had to guess it has to be f/g/h so i would pick H and move on!! no time to calculate during real exam.  
tompat
Forum Newbie Topics: 64 Posts: 561 
none of those that u selected is the ans  
galeen
Forum Guru Topics: 56 Posts: 578 
then move to the next q wats the answer?  
tompat
Forum Newbie Topics: 64 Posts: 561 
it's D. THEY ARE asking about the positive predictive value of the test, and u can calculate the ans in 1 min . its not that long calculation.  
doctorforever
Forum Junior Topics: 91 Posts: 276 
how do you calculate PPV here? should not answer be H?  
USMLEbiostats.c
Forum Newbie Topics: 0 Posts: 3 
When a question is framed like this, I advise that you pick a nice round number for "n." Like, n=100. Now, glue the pieces together as shown below (it is easy if you practice a little).
DECOMPOSITION Prevalence=10% so the number of all those truly diseased is … 10 and those truly healthy are 90. Of the 10 truly diseased: Sensitivity is 80% so TP=8; FN=2. Of the 90 truly healthy: Specificity is 90% so TN=81 (90*0.9); FP=9 (9081). Now you have the values to calc PPV: 8 / (8+9) = 0.47
REMEMBER: You needed to see that this is a PPV problem. PSA > 4 is considered + (like, PSA > 4 is the "gold standard"); the patient had PSA=7; so, what’s the probability that a person w/ a + test result is truly +? That’s PPV.
This is an EXCELLENT high yield biostat question. The question could have been framed to ask PPV, NPV, sensitivity, specificity, accuracy!!! Hey, so …. what’s accuracy? 10% 47% 58% 69% 72% 81% 89%, 92% or 110% Edited by USMLEbiostats.c on Jul 21, 2012  12:31 PM  
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