r753_2000
Forum Elite
Topics: 59
Posts: 255
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Does anyone here know how to solve this problem?
A 55 year old man visits his primary care physician with a complaint of urinary infrequency. Examination finds a 1 cm nodule on his prostate gland. The physician orders a prostate specific antigen serum test. By common standards, a PSY level greater than 4ng/ml is considered abnormal. Using this standard this test has a sensitivity of 80% and specificity of 90%. A recently published epidemiologic article found that in a cross-sectional study, 10% of men of this age have prostate cancer. The result on teh patient's PSA is 7ng/ml. What is yoru best estimate of the likelihood that this man actually has prostate cancer?
a) 13%
b) 25%
c) 36%
d) 47%
e) 58%
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| smile_md
Forum Junior
Topics: 10
Posts: 46
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Positive Predictive Value - 47%
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| ELM
Forum Guru
Topics: 28
Posts: 936
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We discussed about this very q on IM forum, it's under the head of ''It's an Epid Q'' posted by me. Check it out.
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ELM
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| r753_2000
Forum Elite
Topics: 59
Posts: 255
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==>TP+FN=10
sen=80%
==> TP/TP+FN=80/100==>TP=8
so, FN =2
TN+FP=90
specificity is 90%
==>TN/TN+FP=90/100==> TN=81
==>FP=9
[/b]how did you get TN=81 and FP=9???
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| r753_2000
Forum Elite
Topics: 59
Posts: 255
|
==>TP+FN=10
sen=80%
==> TP/TP+FN=80/100==>TP=8
so, FN =2
TN+FP=90
specificity is 90%
==>TN/TN+FP=90/100==> TN=81
==>FP=9
[/b]how did you get TN=81 and FP=9???
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| mash
Forum Fanatic
Topics: 147
Posts: 1,326
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TN/TN+FP=90/100
TN+FP=90
so, TN=90/100(TN+FP) ==> TN=90/100*90 =81
if TN+FP=90 ==> FP=90-81 = 9
___________________
I hear and I forget. I see and I remember. I do and I understand.
--Confucius
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| r753_2000
Forum Elite
Topics: 59
Posts: 255
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Please bear with me here:
how did you get TN+FP=90???
if TN/TN+FP = 90/100, shouldn't TN+FP=100???
or did you account for the prevalence of 10%?
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| mash
Forum Fanatic
Topics: 147
Posts: 1,326
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suppose we screen 100 people,
prev is 10% so, 10 out of 100 are diseased i.e TP+FN=10
==>healthy people are 100-10=90 i.e TN+FP=90
specificity=90%
TN/TN+FP =90/100
so, TN=90/100(TN+FP) ==> TN=90/100*90 =81
___________________
I hear and I forget. I see and I remember. I do and I understand.
--Confucius
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| r753_2000
Forum Elite
Topics: 59
Posts: 255
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thanks so much mash!
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