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  #1  
Old 11-25-2012
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Stats The likelihood of having the disease

A 55 yr old man visits his primary care physician with a complaint of urinary frequency. Examination finds a 1 cm nodule on his prostate gland. The physician orders a prostate-specific antigen(PSA) serum test.By common standards , a PSA level greater than 4 ng/ml is considered abnormal. Using this standard , this test has a sensitivity of 80% and a 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 this patients PSA is 7 ng/ml. What is your best estimate of the likelihood that this man actually has prostate cancer?

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


plz explain your answers.......
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#Prevalence of Ca prostate at age of 55 year= 10%
#Test sensitivity= 80%
#Test specificity= 90%
#TP= (80 * 10)/100= 8 --> for every 10 person with abnormal PSA the test can detect 8
#TN= (90 * 90)/100= 81
#FP= 90-81=9
#FN=10-2=8
#best estimate of the likelihood that this man actually has prostate cancer= PPV
#PPV= (8/8+9)*100= answer D. 47%
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Quote:
Originally Posted by heartbeat View Post
#Prevalence of Ca prostate at age of 55 year= 10%
#Test sensitivity= 80%
#Test specificity= 90%
#TP= (80 * 10)/100= 8 --> for every 10 person with abnormal PSA the test can detect 8
#TN= (90 * 90)/100= 81
#FP= 90-81=9
#FN=10-2=8
#best estimate of the likelihood that this man actually has prostate cancer= PPV
#PPV= (8/8+9)*100= answer D. 47%
Can you explain the formula you used to arrive at TP & TN.
I used the 2X2 table to calculate but if there is a specific formula then it will help.
I suppose you multiplied sensitivity with the prevalence to get the TP and multiplied specificity with 1-prevalence to get the TN.
Am I correct ?
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same here....how did u arrive at those TP and TN formulas..???? dint hear of any such calculations until now.....
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Quote:
Originally Posted by anomali View Post
same here....how did u arrive at those TP and TN formulas..???? dint hear of any such calculations until now.....
Bayes’ Theorem

I figured it out............that's how it is explained in UW stats............but I'll stick with 2X2 table.
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thank u so much novo and heartbeat.......i understood it....

btw novo can u share how to proceed with such questions by 2*2 table....really confused on how to go by that table method....
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Old 11-26-2012
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Default I would answer it a bit differently though I do not know whether it is right or wrong!!

They are asking for the likelihood of getting a true positive result giving this sensitivty and speceficity which does not DEPEND on prevalence of disease unlike PPV which does so.

the LH of +ve test result = sensitivity / 1 - speceficity = 80% / 1 - 90 % = 0.8 / 0.1 = 80 %

??????????????
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Quote:
Originally Posted by Novobiocin View Post
Can you explain the formula you used to arrive at TP & TN.
I used the 2X2 table to calculate but if there is a specific formula then it will help.
I am also used 2X2 table. That what i remembered from step1.
Quote:
Originally Posted by anomali View Post
same here....how did u arrive at those TP and TN formulas..???? dint hear of any such calculations until now.....
Quote:
Originally Posted by Novobiocin View Post
I suppose you multiplied sensitivity with the prevalence to get the TP and multiplied specificity with 1-prevalence to get the TN.
Am I correct ?
Yup
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Quote:
Originally Posted by anomali View Post
thank u so much novo and heartbeat.......i understood it....

btw novo can u share how to proceed with such questions by 2*2 table....really confused on how to go by that table method....
It's really easy but difficult to explain. If you listen to Dr. Daugherty's lecture you will never forget it.
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Quote:
Originally Posted by hamzarayes View Post
They are asking for the 19096 of getting a true positive result giving this sensitivty and speceficity which does not DEPEND on prevalence of disease unlike PPV which does so.

the LH of +ve test result = sensitivity / 1 - speceficity = 80% / 1 - 90 % = 0.8 / 0.1 = 80 %

??????????????
That's positive likelihood ratio, they are asking PPV.
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Quote:
Originally Posted by Novobiocin View Post
It's really easy but difficult to explain. If you listen to Dr. Daugherty's lecture you will never forget it.

true...i guess i forgot his lectures, gotta review again.....
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