Screening Test Positive Down’s syndrome  T able 1: T


Yes No


erms used to describe the accuracy of a screening test.


Sensitivity = (a/a+b) x 100 ie the percentage of fetuses with Down’s syndrome that test positive for it


Specificity = (d/c+d) x 100 ie the percentage of fetuses without Down’s syndrome that test negative for it


Positive predictive value= (a/a+c) x 100 ie the percentage of those who test positive who actually have the condition


Negative predictive value = (d/b+d) x 100 ie the percentage of those who test negative who don’t have the condition


A (true positives) C (false positives)


Negative


B (false negatives) D (true negatives)


Quadruple test


Combined screening ffDNA


 T


Sensitivity 85.1 %8 77-84%8 100%1,3


able 2: The accuracy of different screening tests for Down’s syndrome.


Specificity 91.5%8 96%8


100%1,3


False positive rate Detection rate 5%2 3%1


75%2 90%1


<1%3,9 99%3,9


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