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Old 11-10-2012
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ObGyn The predictive value of alpha fetoprotein

A 35-year-old woman at 18 weeks' gestation has an increased serum α-fetoprotein concentration. Which of the following has the greatest influence in determining the predictive value of this test for neural tube defects?


A) Concentration of α-fetoprotein in maternal serum
B) Maternal history of monozygotic twin pregnancies
C) Prevalence of neural tube defects in the population in question
D) Sensitivity of the test
E) Specificity of the test
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Old 11-10-2012
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C.??prevalence is directly proportional to PPV

I think I should go with EE. Increase specificity of the test
Am not sure
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@alice
Which of them is right??
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Old 11-10-2012
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@blade : I just have the answer .. Don't have the explanation ..

Found the question a bit weird too .. Try to reframe the question and see if you can get to it ( I did the reframing after knowing the answer though .. Lol ..)
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answer is E
im guessing you mean the positive predictive value .
this is a question that requires the double hump diagram
you shall see as the specificity increases , the positive predictive value also increases
easy way to remember , P is P and N is N i.e s(P)ecificity is for (P)ositive predictive value , and se(N)sitivity is for (N)egative predictive value


go through the kaplan stats vids , dr daughtry explains this is very simple manner
hope this helps
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actually to think of it , prevalence also makes a difference to the PPV !
man ! now im confused!!
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Default C..final answer

the way i look at it
A..is wrong because there are other causes of increase AFP in maternal serum eg gastrochisis
B and D are not contributory
E..high specifity of the test also gives high PPV but also it does not give reason for the high AFP
C..should be right since increase prevalence of dx gives better PPV
(but i cant reason all these out in a minute in exam )
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c) is correct ans. PREDICTIVE VALUE OF THE TEST(+/-) VARIES WITH THE PREVALENCE OF THE DISEASE. IF THE PREVALENCE OF THE DISEASE Is LOW, EVEN TEST WITH HIGH SENSITIVITY AND HIGH SPECIFICITY WILL HAVE LOW PREDICTIVE VALUES (first aid page 53).
thanks for the question
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Yeah .. The correct answer is C .. As explained by Trupti
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