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#1
06-27-2010
 USMLE Forums Veteran Steps History: 1 + CK Posts: 234 Threads: 63 Thanked 128 Times in 63 Posts Reputation: 158
The likelihood that the patient is having cancer

A 55 yr old man visits his physician for urinary complaints, who finds a 1cm nodule on his prostate gland and orders PSA test. PSY > than 4ng/ml is considered abnormal by common standards. The test's sensitivity is 80%, specificity 90%. It's found that 10% of men of his age have prostate cancer on a cross sectional study. The patient's PSA is 7ng/ml. What is ur best estimate of the likelihood that this man has prostate cancer?

A. 13%
B.25%
C.36%
D.47%
E. 58%
F.69%
G.72%
H.81%

#2
06-28-2010
 USMLE Forums Guru Steps History: --- Posts: 487 Threads: 69 Thanked 296 Times in 133 Posts Reputation: 316

D.47%...??
#3
06-28-2010
 USMLE Forums Veteran Steps History: 1 + CK Posts: 234 Threads: 63 Thanked 128 Times in 63 Posts Reputation: 158

pls explain how u get ur ans. i want understand it . thank u .

#4
06-28-2010
 USMLE Forums Guru Steps History: --- Posts: 487 Threads: 69 Thanked 296 Times in 133 Posts Reputation: 316

prevalance of prostate ca. is 10%...means out of 100, 10 people r having ca.
so we hv TP+FN = 10
AND FP+TN = 90 in our 2*2 table
now test sensitivity is 80%... sensitivity=TP/TP+FN
80/100=TP/10
80/100*10=TP
TP=8
SO,FN=10-8=2
AND specificity 90%....specificity=TN/TN+FP
90/100=TN/90
90/100*90=TN
TN=81
SO,FP=90-81=9

NOW they hv asked about PPV...likelihood that pt with positive test actually hv the disease....PPV=TP/TP+FP
=8/17
=0.47
=47%

Hope this helps.....i dont know how to make table here...n m not able to attach any file...hope someone else help u making a table...
 The above post was thanked by: achistikbenny (06-29-2010), doctorF (06-29-2010)
#5
06-29-2010
 USMLE Forums Veteran Steps History: 1 + CK Posts: 234 Threads: 63 Thanked 128 Times in 63 Posts Reputation: 158

thank u very much .i do understand it now. thank u very much

 Tags Biostatistics-Epidemiology

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