Screening for Down Syndrome (Optional)
Down syndrome is a spectrum of abnormalities that generally result from an error during gametogenesis in the ovary that results in the birth of a child with three copies of chromosome 21 (trisomy 21) instead of the normal two copies. The frame below from the National Institutes of Health provides a summary of the syndrome.
Prior to 2014 the most up-to-date screening method during pregnancy was a combined approach during the first trimester that was conducted in two steps during week 11 to 13 during pregnancy.
- Ultrasound (the nuchal translucency screening test) in which US is used to measure a specific region on the back of a baby's neck looking for fluid accumulations that occur with abnormalities.
- Blood tests. Results of US are paired with levels of pregnancy-associated plasma protein-A (PAPP-A) and a hormone known as human chorionic gonadotropin (HCG).
In late 2011 cell-free DNA sequencing (cfDNA testing) of maternal plasma was introduced as a new screening modality in the US. Bianchi et al.
reported on results with the new screening test among 2052 women with singleton pregnancies who were enrolled in the study.
This link below will allow you to listen to a report about the study on National Public Radio (NPR).
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The tables below summarize the evaluations of the "Standard Test for Down syndrome and the results obtained with the newer DNA sequencing technique.
Standard Test
|
|
Down Syndrome |
Not Down |
|
Test + |
5 |
69 |
|
Test - |
0 |
1835 |
|
|
5 |
1904 |
DNA Sequencing
|
|
Down Syndrome |
Not Down |
|
Test + |
5 |
6 |
|
Test - |
0 |
1898 |
|
|
5 |
1904 |
At first glance, the results look pretty similar. Compute the sensitivity, specificity, and positive predictive value of each screening test and comment on the utility of the newer DNA test compared to the previous standard testing.