Page 1 of 1: Q: Biostatistics: Post Test Probability

dr_saurabh · Credits: 7235 My Scrapbook

Hiiiii folks,

There's a question in Panda's MCQ Book based on Harrison.

What is the "post test" probability in a positive test with prior probability

of 25 % , sensitivity 90 % & specificity 80% :-

a) 100 b) 80 c) 60 d) 46.

Now Harrison 15/ed gives us a formula ,

post test probability = pretest probability X test sensitivity
________________________________

pretest probability X test sensitivity + ( 1 - dis.

prevelance ) X (1- sensitivity )

Now v have got all the factors given in the ques. except for dis.
prevelance. How do we determine its value ,in absence of it ??

A subsequent ques. given next is :-

What will be the post test probability of C.A.D. in an middle aged man
with typical anginal symptoms who shows 2- 2.5 mm down sloping
ST segment depression in treadmill :-

a) 75 % b) 95 % c) 99% d) 100 %

How do we deduce the answers to such questions ??

manipalguy · Credits: 15905 My Scrapbook

its not (1-sensitivity)
its (1-specificity)
true negative rate is (1- false positive rate)and we all know true negative is specificity...
so false positive rate is (1-specificity)

now u are left with one more variable....that is disease prevalence which is same as pretest probability...
so now feed the values and get the answer...
the answer will be 60 percent!!! did u get it???/

manipalguy · Credits: 15905 My Scrapbook

hope u got the answer!!!

dr_saurabh · Credits: 7235 My Scrapbook

Hiiii ,

Sorry for the late reply, but the values fitted in
perfectly & the answer came out all
right

. Thanks.

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