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 kaplan biostat q  



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Author7 Posts
  #1

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%




  #2

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.


  #3

none of those that u selected is the ans


  #4

then move to the next q wink

wats the answer?


  #5

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.


  #6

how do you calculate PPV here? should not answer be H?


  #7

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 (90-81). 

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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