SDK
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55 yr old male visit complaint of urinary infrequency.Exam: reveal 1 cm nodule on his prostate gland..PSA result is 7 ng/ml while normal is 4 ng/mlwhich is 80% sensitivety & 90% specificity..A cross sectional study show that 10% of this age group has prostate cancer....man has actual % of prostate cancer is??
a)13%
b)36%
c)47%
d)69%
e)81%
Explain the answer in detail???
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| txgirl
Forum Newbie
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try to answer it.
PSA is positive
Prostate cancer None cancer
PSA+ 80 10
PSA- 20 90
this question is asking positive predictive value, so answer is TP/TP+FN=80/80=10
closeset answer is E.
Am I right?
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| txgirl
Forum Newbie
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80/80+10
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| kingsofke
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Positive perdictive value depends upon actual prevalence of disease .
But i dont know how to correlate ??? any ideas
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| reg
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Hi  ,
Try to calculate the positive predictive value using the positive likelihood ratio:
Positive likelihood ratio= sensitivity divided by 1-specificity, so PLR= 0.8/1-0.9=8. Then,
1-Convert the prevalence (10%) to a ratio: 10% to 1:9
2-Multiply the first part of the ratio (1) by the positive likelihood ratio:8x1:9=8:9
3-Convert the ratio back to a percentage: 8/8+9=0.47 = 47%
Answer C. Let me know if I´m right 
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| kingsofke
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Thanks a million ,Man this concept always haunts . It makes sense. But I could nt follow he last point, uptill that was good, which is Convert the ratio back to a percentage: 8/8+9 (what rule did you appliued here) thanks again
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| kingsofke
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Oh i get it .sorry i was a bit sleepy. 
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| reg
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