alpha fetoprotein levels BQ'

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alpha fetoprotein levels BQ'

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Believe
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12/03/05 - 07:12 AM

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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cyra
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12/03/05 - 07:42 AM

drk1980
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12/03/05 - 07:47 AM

why do u say D?

Believe
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12/03/05 - 08:30 AM

welcome about the brain pics.

could you please explain your answers cyra? thanks

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mjl1717
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12/03/05 - 09:53 AM

he guessed

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roopashri
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12/03/05 - 12:12 PM

i think C. prevalence of the NTD in the population in Q. (remember prevalence goes up, PPV goes up, NPV goes down)

cyra
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12/03/05 - 12:59 PM

I am sorry...i answered thinking that the question asked about the positive predictive value...for that it would have been the sensitivity...but since the predictive value as a whole is being referred to...i'd agree with roopashri...its C...cos prevalence in part determines the predictive value of a test...

mjl1717
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12/03/05 - 02:12 PM

Briefly: How good is a test at detecting presence or absence of disease-this is sensitivity and specificity respectively.(the abiliy of a test to discover this) {Epidemiologist} Predictive value-the doc or patient wants to know when faced with test results "how likely" it is that a disease is present or absent. Predictive values as stated above vary according to prevalence or underlying probability of disease. Predictive values are not qualities of the test per se bu are a function of the test's characteristics and the setting in which it is being used. As stated above as prev. increaeses PPV increases and NPV decreases. So c is statistically the best answer. Very good q Believe. nod

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GMOB
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12/03/05 - 02:42 PM

Thanks for this question, i thought D but now i understand why it was wrong!

Believe
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12/05/05 - 05:46 PM

#10

yes, it's a good Q, I would have missed it, thanks for the explainations, I read a little bit more, found in my notes...

If Prevalence increaseds:Positive predictive value increases, Negative Predictive value decreases.

Edited by Believe on 12/05/05 - 11:05 PM

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Geroo
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12/05/05 - 10:33 PM

#11

If prevalence increases:
- sensitivity and specificity don't change
- (+PV) increases and (-PV) decreases.
however if the sensitivity increases the prevalence will increase because the test will pick up more people with the disease.

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