drshvetasm
Forum Elite
Topics: 22
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Hey everyone, there is a question in kaplan notes...page 130 in my version which goes as...
A 55yr old man visits his primary care physician wid a complaint of urinary infrequency. p/e: 1cm nodule.PSA > 4ng/dl is considered abnormal with a sensitivity of 80% and specificity of 90%.prevalence of prostate cancer is 10%. This dude has a PSA of 7ng/dl. What is the best estimate of the likelihood of prostate ca in this man??
i got close to the answer with a complicated 10min calculation....but there has to be a formula for such questions which i'm not aware of...or a way to estimate the positive predictive value which is asked in this case.
any simple solutions?? The answer given is 47% with no explanation....can anyone explain how...
Thanx in advance...
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| Ivonne
Forum Guru
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Probably i have the same approach than you:
Total population-------->1000
Total diseased: 100-------->based on the prevalence
If the sensitivity is 80% then the upper part(diseased +)of my 2 per 2 table is 80
Non diseased 900 if specificity is 90% then the upper negative of my table is 90
Then PPV -------> 80/80+90---------->47%
+ _ Total
+ 80 90 170 ---------------> PPV=80/170
- 20 810 830
100 900 1000
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| drshvetasm
Forum Elite
Topics: 22
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so ur saying
PPV = sensitivity
sensitivity+ specificity
or PPv = sensitivity / sensitivity+specificity ??
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| Ivonne
Forum Guru
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No, what i am saying is that we have to make our 4 by 4 table and PPV and NPV are obtained horizontally while sensitivity and specificity are obtained vertically in that table.
Sensitivity---------->80/100-------->80%
Specificity----------->810/900-------->90%
PPV------------------->80/170
NPV------------------->810/830
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| deja_vu
Forum Senior
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PPV=
sensitivity x prevalence
----------------------------------------------------------------------
sensitivity x prevalence + (1 - specificity) (1 - prevalence)
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GOAL: NEURO-2009, Creds: 2007 Grad, Need H1B/J1, Doing MPH, Step1:266, Step2CK:272, CS: Awaiting results, USCE (O'ship) & USLORs: + (Neuro), Trying to get into a clinical research in INR.
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| drshvetasm
Forum Elite
Topics: 22
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thanx deja_vu...ohhhhh....thank God i asked this question...thanxxxxxxxx u guys...didnt know at all bout these formulae.
Ivonne...i think i get u know...thanx to u...i wasnt doing it this way coz i think u r substituting prevalence as TP+FN and i was doing it as TP+FN/TP+FN+TN+FP. but ur way must definitely be correct as u r getting the right answer and i'm getting lost...so ur exam is scheduled for next week right?
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| drbin
Forum Guru
Topics: 27
Posts: 535
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i think if u refer s1 Q book for beh sciences there is such qs with nice explanations. 47 is the answer
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