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Maternal Serum Screening

Down Syndrome, Trisomy 18 and Neural Tube Defects



Sequential Screening

Slide 17

March 2011

Calculation of the risk estimate for Down syndrome is based on 2 cutoffs. In the first trimester, risk estimate values of greater than or equal to 1 in 50 are interpreted as positive and a report will be issued. If the risk is below 1 in 50, a report will not be issued until the second sample is received in the second trimester. In the second trimester, the cutoff used for Down syndrome is equal to or higher than 1 in 270. Using this approach, the overall detection rate for Down syndrome is approximately 90% with a false-positive rate of approximately 4%.

Trisomy 18 risk estimates greater than or equal to 1 in 100 are reported as positive in the first part of the screening when the Down syndrome risk is also increased. If the risk is below 1 in100, the woman will continue to the second part of the screening. In the second trimester, the cutoff used for trisomy 18 is also 1 in 100. Using this approach, the overall detection rate for trisomy 18 is approximately 90% with a false-positive rate of less than 0.5%.

Neural tube defect calculation is the same as the second trimester screening. A multiple of the median (or MOM) equal or higher than 2.5 is reported as screen positive. The detection rate in second trimester screening for neural tube defect is approximately 80% with a false-positive rate of approximately 1 to 3%.

Sequential Screening

 


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