fighter1
Forum Newbie
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35 year old at 18weeks gestation, with increased serum AFP. which of the following has the greatest influence in determining the predictive value of this test for neural tube defects
a. concentration of AFP in maternal serum
b. maternal history of monozygotic twin pregnancy
c. prevalence of neural tube defects in the population in question
d. specificity of the test
e sensitivity of the test
please explain why the correct answer is C and not the specificity of the test
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| abdo
Forum Senior
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when the postive predicitive value increases the prevalence increase as the more the ppl who has the disease" the prevalnce" the more when u will do the test it will be postive
in the question the disease is neural tube defects , the best measure to be sure that it is positve is the prevalence of the population , the more ppl having it the more the test will be +ve
twins has no relation to the disease , nor the specificty or sensitivity , and serum alpha feto protein is a good measure but it s not the thing that u can say to the patient i m sure that u have the disease like increaing the prevalence in a society
so know this : increase +ve predicitve value increase prevalence
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| DRFP
Forum Newbie
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I have no idea what you mean above but AFP concentration would have the best predictive value,
"The first step in prenatal screening is measuring the maternal serum AFP between 15 and 20 weeks of gestation. A patient-specific risk is then calculated based on gestational age and AFP level. For example, at 20 weeks of gestation, a maternal serum AFP concentration higher than 1,000 ng/mL would be indicative of an open NTD. Normal AFP concentration in the maternal serum is usually lower than 500 ng/mL."
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| nochoice
Forum Junior
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This question is simply testing the relationship between PV and prevalence.
The PV of a serum AFP level is influenced by the Prevalence of NTDs in foetus' where the AFP is abnormal. For eg, if the AFP is high, it is associated with an NTD (being a TP) in the fetus. There is a higher probability of actually having the defect (being TP) when compared to the general population of pregnant women.
Therefore, the test's PPV increases as the TP increases (as PPV = TP/TP+FP) If the increased AFP was not linked to NTDs, then the TP would be very low as NTDs are rare in the general population. And the PPV would also be low.
Sensitivity tells if someone is sick with a disease, what are the chances that the test will test positive for it. The predictive value of the test is the other way around - whether someone who's already been tested positive ACTUALLY has the disease or not (PPV) or if someone who tests negative is actually disease free (NPV)
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| Ig F
Forum Elite
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Prevalance is directly related to Positive Predictive Value(PPV)....If prevalance goes up,PPV goes up and if prevalances goes down PPV goes down....
But Prevalance is inversely related to Negative Predictive Value(NPV).....so answer is C....
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