Prep for USMLE 
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deals
Forum Newbie Topics: 40 Posts: 88 
55 years old man complaint of urinary infrequency ,i cm nodule in prostate gland PSA ordered.By common standard PSA above 4 ng/l considered abnormal. using this standard the test has sensitivity of 80% specificity 90% recently puplished study showed that in cross sectional study 10% of men at this age has prostate cancer. the result of patient PSA is 7 ng/l.what is your best likelihood that this man actually has prostatic cancer ????? 13% 25% 36% 47% 58% 69% 72% 81% i need your help please thanks  
tuti_hami
Forum Junior Topics: 20 Posts: 347 
81% On Jul 05, 2011  12:02 AM, deals responded: please can you explain how did u get this 81% thanks  
deals
Forum Newbie Topics: 40 Posts: 88 
i need to know the steps to calculate this pleaseeeeeeeeee cause in kaplan notes it is written 47% and i think it is 81% but please tell me the steps thanks  
deals
Forum Newbie Topics: 40 Posts: 88 
gosh anybody knows how to calculate this tuti hami can u tell the steps  
motorola
Ex Moderator Topics: 75 Posts: 570 
The question is asking for Positive Predictive Value of the test. The answer is 47%. Formula for PPV is as follows (sensitivity x prevalance) _________________________________________ (sensitivity x prevalance) + (1specificity)(1prevalance) In the above example it would be 0.8 x 0.1 ______________________ 0.8 x 0.1 + (10.9)(10.0.1) Which is 47%. Although the question uses the word likelihood it is not asking for likelihood ratio (LR). LR is the odds that a positive test result would be found in a patient with, versus without, a disease. Formula for LR+ is as follows: Likelihood Ratio Positive (LR+) = Sensitivity / (1  Specificity). See this link for a detailed description with an example:  
motorola
Ex Moderator Topics: 75 Posts: 570 
deals wrote: gosh anybody knows how to calculate this tuti hami can u tell the steps Here you go deals. Hope this helps.  
deals
Forum Newbie Topics: 40 Posts: 88 
thanks motorolla appreciate it  
monterow
Forum Newbie Topics: 0 Posts: 3 
let's make it simple: assume you have a 1000 patients. 10% with disease that should be on the left column Now  the table should be like: 80 90 20 810 100 900 All you have to do, calculate PV+ which is 80/80+90 = 47%  
USMLEbiostats.c
Forum Newbie Topics: 0 Posts: 3 
I hope anyone with a PPV, NPV, sensitivity, specificity, accuracy problem finds this post. motorola is correct but I recommend using monterow technique. In fact, make n=100 (hypothetically) and the math become a little easier. 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 REMEMBER You needed to see that this is a PPV problem. PSA > 4 is considered +; 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 concept biostat question. The question could have been framed to ask about PPV, NPV, sensitivity, specificity, accuracy!!! Hey, so …. what’s accuracy? 10% 47% 58% 69% 72% 81% 89%, 92% or 110%  
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