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Old 05-19-2014
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Star BioStat Question for Today

I found this rather challenging... trying to improve in my weakness.. biostat!!
lets see how you do this guys

Myasthenia gravis is an autoimmune disorder
that affects approximately 3 in 100,000 people.
Individuals with mysasthenia gravis classically
present with complaints of muscle weakness
and fatigue secondary to the formation of autoantibodies
directed against the acetylcholine
receptors at neuromuscular junctions.
The most accurate method of diagnosis involves the
detection of these autoantibodies. On average,
this test is approximately 80% sensitive and
90% specifi c. If an individual has a positive test
for autoantibodies against the acetylcholine receptor,
what is the approximate post-test probability
of having this disease, assuming a pre-test
probability of 50%?


(A) 80%
(B) 85%
(C) 89%
(D) 90%
(E) 95%
(F) 99%
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Old 05-19-2014
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Is it C-89%?
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Old 05-19-2014
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probability of being actual positive ,when the test is +ve= Positive predictive value=TP/TP+FN.
sensitivity=80% so TP=80, FN=20
specificity=90% so TN=90, FP=10
so with PPV formula,i got 80/90..am I right?
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Default

Quote:
Originally Posted by MEDICINE DOC View Post
probability of being actual positive ,when the test is +ve= Positive predictive value=TP/TP+FN.
sensitivity=80% so TP=80, FN=20
specificity=90% so TN=90, FP=10
so with PPV formula,i got 80/90..am I right?
yes you DID!!!
so let me ask you this... the pretest thingy porcentage is just a DISTRACTOR???
i know that pretest prob and prevalence DO have an effect on PPV, so why here is not affected?? please explain this, thanks.
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Old 05-19-2014
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I take PPV as literally " Probability of ACTUALLY having the disease,IF the test is POSITIVE"
so when it said the test is positive and is asking the chances of the person actually having the disease,its just asking the PPV value,we can derive the TP,FP,FN,TN values with the given sensitivity and specificity..all other values are just distractors I guess.
so remember..probability of ACTUALLY having the disease when the test is POSITIVE=PPV

Probability of ACTUALLY NOT having the disease when the test is NEGATIVE=NPV

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Old 05-19-2014
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Default

Quote:
Originally Posted by MEDICINE DOC View Post
I take PPV as literally " Probability of ACTUALLY having the disease,IF the test is POSITIVE"
so when it said the test is positive and is asking the chances of the person actually having the disease,its just asking the PPV value,we can derive the TP,FP,FN,TN values with the given sensitivity and specificity..all other values are just distractors I guess.
so remember..probability of ACTUALLY having the disease when the test is POSITIVE=PPV

Probability of ACTUALLY NOT having the disease when the test is NEGATIVE=NPV

thank you
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