for Down syndrome
PerkinElmer Labs/NTD's First Trimester Screen | Fß test reliably assesses a mother's risk for having a baby with Down syndrome, trisomy 18 and 13 in the first trimester of pregnancy. Performed between 11 weeks, 1 day and 13 weeks, 6 days gestation, the First Trimester Screen |Fß uses data from a blood test (biochemistry) and an ultrasound examination (biophysical) which yields a risk assessment that physicians may present to their patients during the first trimester. The First Trimester Screen | Fß is now better than ever, with the addition of alpha fetoprotein (AFP) which increases the detection rate for Trisomy 21 (please select the PDF to the right for more information).
A maternal blood sample is analyzed for three biochemical markers: alpha fetoprotein (AFP), free beta human chorionic gonadotropin (free beta hCG) and pregnancy associated plasma protein-A (PAPP-A).
The ultrasound examination confirms the fetus' gestational age and measures the nuchal translucency (NT), or the fluid accumulation behind the fetus' neck. These combined biochemical and biophysical markers yield the most sensitive screening results possible at the earliest point during the pregnancy, achieving a 93% detection rate at a 5% false positive rate for Down syndrome (and a 95% detection rate at a 0.3% false positive for trisomy 18 and 13). When a Fetal Nasal Bone Assessment is added to the protocol, the detection rate for Down syndrome increases to 96% at a false positive rate of just 2%.
The use of free beta hCG in screening for Down syndrome and other chromosomal abnormalities during the first trimester has been demonstrated to be more effective than other markers, including intact or total hCG. Intact/total hCG, while an effective second trimester marker, is not considered effective during the first trimester. Laboratories that lack the technology to measure free beta hCG have attempted to use total hCG as part of their first trimester screening. This practice results in significantly higher false positive rates for Down syndrome as compared to protocols that measure free beta hCG.
The ACMG (American College of Medical Genetics) published technical guidelines on prenatal screening in September 2009. These guidelienes state:
"Before 11 weeks, free beta hCG is discriminatory but hCG is not. Between 11 and 13 gestational weeks, free beta hCG is univariately a more discriminatory Down syndrome screening marker than hCG."
DISCLAIMER: This test is a part of a lab service offering provided by NTD. This test was developed and its performance characteristics determined by NTD. It has not been cleared or approved by the U.S. Food and Drug Administration. The methods and performance characteristics have been reviewed and approved by the New York State Department of Health. The test results do not eliminate the possibility that this pregnancy may be associated with birth defects or pregnancy complications including preeclampsia, pre-term delivery and low birth weight. The multiple of the median and risk results provided in the test report are dependent on the accuracy of the demographic and ultrasound information provided. The ordering physician should ensure that the ultrasound information has been obtained from a sonographer who is credentialed by and participating in a uterine artery Doppler quality review program such as FMF. NTD assumes no responsibility for ensuring that the ultrasound information has been obtained by a properly credentialed sonographer, including verification or updates to credentialing status.