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Maternal Serum Screening

Down Syndrome, Trisomy 18 and Neural Tube Defects

First Trimester Serum Screening

Slide 10

March 2011

For first trimester serum screening, a screen-positive or screen-negative result is based on the laboratory-specific cutoffs. For the first trimester, Down syndrome risk calculation is based on a cutoff of 1 in 230. A screen-positive interpretation is provided if the calculated risk is equal to or higher than 1 in 230. Using this cutoff, the Down syndrome detection rate is 85% and the false-positive rate is 5%.

Keep in mind that in practice, both the detection rate and false-positive rate increase with age, thus detection and positive rates will vary depending on the age distribution of the screening population. If the screened population is older, the detection rate and the false-positive rate will be higher.

For trisomy 18, risk estimates of 1 in 100 or higher are reported as screen positives. In the first trimester, the detection rate for trisomy 18 is approximately 80% and the false-positive rate is less than 1.0%.

First Trimester Serum Screening


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