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Maternal Serum Screening

Down Syndrome, Trisomy 18 and Neural Tube Defects

Sequential Screening

Slide 16

March 2011

In this schematic representation of the sequential screening test, the first measurements are performed at 11 to 13 weeks of gestation and include an NT measurement by ultrasound and a PAPP-A measurement in serum. Based on the laboratory’s predetermined cutoffs, a screen-positive or screen-negative result is generated. If the screen is positive, meaning that the woman is considered to be at high risk of carrying an affected fetus, the results will be reported immediately to give the woman the option to undergo early genetic counseling and diagnostic testing. If the woman had a negative screen in the first trimester, the results are not provided. A second sample needs to be sent to the lab between 15 and 22 gestational weeks for measurement of AFP, estriol, hCG and inhibin A. A risk calculation using 6 markers is provided on the second trimester test. Based on the set of cutoffs used for sequential screening, the majority of women will complete the second part of the test providing a higher detection rate and a lower false-positive rate.

Sequential Screening


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