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  #1  
Old 09-27-2011
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Default Percent chance of a test being abnormal

A medical student accidentally sticks his finger after drawing blood on a woman, who is having elective surgery for removal of a benign breast mass. As part of the normal routine in handling accidental needle sticks, a baseline liver profile is ordered. The profile consists of a total bilirubin, serum aspartate aminotransferase (AST), serum alanine aminotransferase (ALT), serum alkaline phosphatase, and serum γ-glutamyltransferase. What is the approximate percent chance of one of these five tests having a value outside the normal reference interval for the test?
A. 10%
B. 13%
C. 23%
D. 35%
E. 47%

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  #2  
Old 09-27-2011
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Default A?

i have no idea, A?
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Quote:
Originally Posted by mayursn39 View Post
A medical student accidentally sticks his finger after drawing blood on a woman, who is having elective surgery for removal of a benign breast mass. As part of the normal routine in handling accidental needle sticks, a baseline liver profile is ordered. The profile consists of a total bilirubin, serum aspartate aminotransferase (AST), serum alanine aminotransferase (ALT), serum alkaline phosphatase, and serum γ-glutamyltransferase. What is the approximate percent chance of one of these five tests having a value outside the normal reference interval for the test?
A. 10%
B. 13%
C. 23%
D. 35%
E. 47%



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i just read from goljan that for each test 5% of normal people will have a result outside of normal. the more test you order the higher the chances of a Fp result. if you order two tests the chance of getting a FP result on one of the test is 10%. in this question you ordered 5 test so it should be 25%, i think... Why is there no 25% in the options. pls can you post the correct answer. thanks
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Old 09-27-2011
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c - 23 %

Normal referance range- mean +/- 2sd - 95 % distribution
So
Chance of having all 5 tests to be normal - 0.95*0.95*0.95*0.95*0.95 = 0.87
And chance of having 1 test outside of normal range = 1-0.87 = 0.23
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Old 09-27-2011
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jaimin ,your explanation was more credible until you made the subtraction error. isnt 1-0.87=0.13?

Most of us suck in maths, that's why we are all here.Dont take it personally, even i could have done the same.

i will stick with answer A.

I honestly wouldnt have time to a math in the exam and examiners are not expecting us to do it too.
So its usually 10%(lowest) or 47%(highest).
moreover needlestick injury related HIV,HBV,HCV is <2%.
Viral hepatitis should be elevating the liver enzymes in case of a transmission,but still the probablity is low, but definitely not 47%.
If you have hepatitis you shall most likely have AST,ALT elevated.
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Quote:
Originally Posted by hadrons123 View Post
jaimin ,your explanation was more credible until you made the subtraction error. isnt 1-0.87=0.13?

Most of us suck in maths, that's why we are all here.Dont take it personally, even i could have done the same.

i will stick with answer A.

I honestly wouldnt have time to a math in the exam and examiners are not expecting us to do it too.
So its usually 10%(lowest) or 47%(highest).
moreover needlestick injury related HIV,HBV,HCV is <2%.
Viral hepatitis should be elevating the liver enzymes in case of a transmission,but still the probablity is low, but definitely not 47%.
If you have hepatitis you shall most likely have AST,ALT elevated.

ya ,i know ..i am not gud at maths ..
But my answer is correct.


C- 23 %

corrected calculation- 0.95*0.95*0.95*0.95*0.95 = 0.77
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Old 09-27-2011
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now it sounds plausible.
I hope you are correct.
I learned some lesson.thank you.
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Default i have a quite different outlook

Quote:
Originally Posted by hadrons123 View Post
jaimin ,your explanation was more credible until you made the subtraction error. isnt 1-0.87=0.13?

Most of us suck in maths, that's why we are all here.Dont take it personally, even i could have done the same.

i will stick with answer A.

I honestly wouldnt have time to a math in the exam and examiners are not expecting us to do it too.
So its usually 10%(lowest) or 47%(highest).
moreover needlestick injury related HIV,HBV,HCV is <2%.
Viral hepatitis should be elevating the liver enzymes in case of a transmission,but still the probablity is low, but definitely not 47%.
If you have hepatitis you shall most likely have AST,ALT elevated.
what i think this question is saying is that, this guy pricked himself, now before giving PEP (just like in PEP for HIV) you want to do his assay. not that you want to find out the chances of his recent prick injury having caused liver damage already. so they are assuming that hs is a regular guy with normal liver profile what are the chances that his result will be abnormal not because of the prick but because a certain percentage of the population will have abnormal results and yet be normal. this is my interpretation, i might be wrong, but it is too soon after his prickto expect liver enzymes to be e levated secondary to the prick, if they are elevated they are probably a false positive elevation. they are asking us to calculate the false positive probabililty in the general population. still i dont get how the other poster arrived at 13%
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well 13 % numbers are wrong calculation.so ignore it.
The right ones are there in the following posts.

i think its a routine post exposure evaluation of blood borne diseases in needlstick injuries and the question asked here is the probablity of at least single lab test turning outside the normal range among 5 of them.
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Old 09-29-2011
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Correct Answer Answer is C

Most normal ranges are established by adding and subtracting 2 standard deviations (SD) from the mean of the test, which encompasses 95% of the normal population. Therefore, 5% of the normal population will be outside out of the normal range (outlier). The likelihood of an outlier increases as the number of tests ordered increases. The likelihood of an outlier = 100 − (0.95n  100), where n is the number of tests ordered. In this patient, five tests were ordered: 100 − (0.955  100) = 100 − (0.77  100) = 23% chance of an outlier in one of the five tests
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