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Maternal Serum Screening

Down Syndrome, Trisomy 18 and Neural Tube Defects



Second Trimester Screening

Slide 13

March 2011

In contrast to first trimester screening, second trimester screening provides risk estimates for Down syndrome, trisomy 18 and also neural tube defects. For Down syndrome, a screen-positive or negative interpretation and a numeric risk is provided based on the cut-off of equal or higher than 1 in 270. Using this cutoff, the detection rate for Down syndrome is approximately 81% and the false-positive rate is approximately 5%. For trisomy 18, risk estimates of 1 in 100 or higher are reported as screen positives. The detection rate for trisomy 18 is 80% with a false-positive rate of less than 1% for the second trimester screening.  For neural tube defects, only alpha-fetoprotein is used for the risk assessment. An AFP multiple of the median (or MOM) equal to or higher than 2.5 is reported as screen positive. Second trimester screening has a detection rate for neural tube defect of approximately 80% with a false-positive rate of 1% to 3%.

Second Trimester Screening

 


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