JS
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The blood supply used by a hospital for Transfusion discovered Hep C infected.To protect future blood supply protected from contamination,new donors required to screen for hepC.The test used for screening has sensitivity of 95% & a specificity of 90%.If this test is used on a sample of donor in which 10% are known to have HepC.What is the chance that a donor who test neg. is actually free from Dis.?
a) 90%
b) 95%
c) 85%
d) 40%
e) 50%
f) 99%
Please,write the expl. too.How did you solve this question?
Thanks.
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| mjl1717
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1)I played with #s using the square with TP &FP on top and FN and TN on the bottom.
2)Then I wrote out formulas for sensitivity, specificitiy,PPV and NPV.
3)Figured your q referred to NPV and I think 90% is the only # that fits?
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| mash
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NPV--99%
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| mjl1717
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can you explain that a little?
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| JS
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Hi mash,
You got the ans. right ( 99% ).Can you please explain its calcualtion ?
Thanks.
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| mash
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prevalence is 10%
suppose 1000 people r screened, TP+FN =100
sensitivity is TP/TP+FN =95%==>TP =95
==>FN=(100-95)=5
TN +FP =900
specificity=TN/TN+FP=90%==>TN=810
==>FP=(900-810)=90
NPV=TN/TN+FN=810/(810+5)=99%
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I hear and I forget. I see and I remember. I do and I understand.
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| JS
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Topics: 16
Posts: 95
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Thankyou mash,
I really appreciate it D
JS
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| mjl1717
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THANKS SO MUCH MASH! 
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