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#1




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 prostatespecific 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 crosssectional 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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Novobiocin (11252012) 
#2




#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= 9081=9 #FN=102=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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#3




Quote:
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 1prevalence to get the TN. Am I correct ?
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anomali (11252012) 
#4




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




Quote:
I figured it out............that's how it is explained in UW stats............but I'll stick with 2X2 table.
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anomali (11252012) 
#6




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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aknz (11262012) 
#7




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




Quote:
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#9




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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anomali (11262012) 
#10




Quote:
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hamzarayes (11262012) 
#11




Quote:
true...i guess i forgot his lectures, gotta review again.....
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BiostatisticsEpidemiology, Step2Questions 
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