Non-Invasive Prenatal Testing


	Non-Invasive Prenatal Testing: Information for Health Professionals

Prenatal screening for Down syndrome using algorithms incorporating maternal age, three or four second trimester biochemical markers, two first trimester markers, and nuchal translucency measurement detects 80-95% of women carrying fetuses with Down syndrome; however, 2-5% of women carrying normal fetuses also have a screen positive result. A new screening method that directly addresses the chromosomal basis for Down syndrome, and certain other chromosome abnormalities, promises much greater sensitivity and specificity than heretofore available.
How does non-invasive prenatal testing (NIPT) work?

During pregnancy, maternal plasma contains DNA from both the fetus and mother. Although only about 10% of this extracellular (cell-free) DNA is fetal, recently developed DNA sequencing methods can quantitate fetal chromosome 21 DNA so precisely that the slight increase when the fetus has Down syndrome can be detected. The same principle permits detection of trisomy 18, trisomy 13, and certain sex chromosome abnormalities.

Is this considered a diagnostic test?

Not quite. While the sensitivity of this test in a high risk individual may be as high as 99% and is highly specific, prenatal chromosome testing, either by amniotic fluid chromosome analysis or chorionic villus sampling (CVS) remains the gold standard. However, the test has a high negative predictive value. Hence, a negative NIPT result can be highly reassuring, particularly for women who wish to avoid invasive prenatal diagnosis.

When should NIPT be considered?

Presently, NIPT is limited to singleton pregnancies at increased risk for aneuploidy:

Positive first trimester, second trimester, integrated, or sequential screening

Sonographic markers suggestive of aneuploidy

Advanced maternal age

A previous pregnancy with aneuploidy

When either parent carries a chromosome translocation conferring increased risk for aneuploidy

What are some other considerations of NIPT?

Sensitivity and specificity may not be as high as are claimed on the basis of relatively small studies
Cost to patients can be higher than traditional aneuploidy screening
Turnaround time of 7-10 days is longer than for first or second trimester screening
NIPT is not appropriate when there is a risk for chromosome abnormalities not detectable by screening, such as when a parent carries certain balanced chromosome translocations
Results of NIPT require confirmation by CVS or amniocentesis

Recommendations for NIPT?

Until more extensive testing on low risk individuals has been completed, NIPT is at present best reserved for patients at high risk for Down syndrome or trisomy 18. NIPT has the potential to eventually replace current aneuploidy screening, resulting in increased detection and far fewer invasive prenatal diagnostic procedures.

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