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#1
11-25-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 790 Threads: 76 Thanked 672 Times in 316 Posts Reputation: 690
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%

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#2
11-25-2012
 USMLE Forums Guru Steps History: 1+CK+CS Posts: 387 Threads: 82 Thanked 612 Times in 234 Posts Reputation: 622

#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
 The above post was thanked by: aknz (11-25-2012), anomali (11-25-2012), chriledi (04-07-2013), Novobiocin (11-25-2012)
#3
11-25-2012
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Quote:
 Originally Posted by heartbeat #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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#4
11-25-2012
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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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#5
11-25-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,867 Threads: 149 Thanked 2,088 Times in 1,047 Posts Reputation: 2108

Quote:
 Originally Posted by anomali 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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#6
11-25-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 790 Threads: 76 Thanked 672 Times in 316 Posts Reputation: 690

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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#7
11-26-2012
 USMLE Forums Scout Steps History: CK Only Posts: 64 Threads: 14 Thanked 74 Times in 22 Posts Reputation: 84
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 %

??????????????
#8
11-26-2012
 USMLE Forums Guru Steps History: 1+CK+CS Posts: 387 Threads: 82 Thanked 612 Times in 234 Posts Reputation: 622

Quote:
 Originally Posted by Novobiocin 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 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 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
#9
11-26-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,867 Threads: 149 Thanked 2,088 Times in 1,047 Posts Reputation: 2108

Quote:
 Originally Posted by anomali 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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#10
11-26-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,867 Threads: 149 Thanked 2,088 Times in 1,047 Posts Reputation: 2108

Quote:
 Originally Posted by hamzarayes 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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 The above post was thanked by: hamzarayes (11-26-2012)
#11
11-26-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 790 Threads: 76 Thanked 672 Times in 316 Posts Reputation: 690

Quote:
 Originally Posted by Novobiocin 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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 Tags Biostatistics-Epidemiology, Step-2-Questions

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