DOWN SYNDROME SCREENING
Combined Screening Test
The combined screening test is also known as the first trimester screening test: it is done during the first trimester (between 11 and 14 weeks), and combines information from the ultrasound examination of the fetus and the blood of the mother. The test can be done in a particular window of pregnancy: when the length of the fetus is between 45mm and 84 mm only. Before and after this window, the test is inaccurate and hence not offered.
Quadruple Screening Test
The quadruple screening test (QST) is preferred over the triple screening test (TST), when it comes to choosing between the two. Essentially, the test consists of testing mother’s blood for hormones produced by the fetus and placenta. In triple test, three hormones were tested – human chorionic gonadotropin (hCG), alpha fetoprotein (AFP) and unconjugated estriol (UE3). Subsequently, a fourth hormone was added to the test – dimeric inhibin A (DIA) and the test became known as quadruple screening test. The advantage of QST over TST, is the higher detection rate while the false alarm rate is lesser. The QST is performed when the fetal head measurement called the BPD (Biparietal Diameter) is between 32 and 52mm, since during this time the hormones tested have a predictable level. Therefore, the hormones level in a particular patient can be easily compared to the expected level for that stage of pregnancy. We offer QST as a screening test for Down syndrome if you have not undergone the combined screening test.
Non-Invasive Prenatal Test
The non-invasive prenatal test (NIPT) is also known as non-invasive screening (NIS), DNA- test, cell free fetal DNA test (cf-fDNA). It has a high sensitivity of 99% for detection of common aneuploidies.
The Genetic Sonogram is a detailed fetal examination through ultrasound looking for 8 subtle clues. In about 50% of Down syndrome fetuses, there may be two or more of these features which will then alert us to offer the definitive test – amniocentesis. Usually, the genetic sonogram is not offered as a standalone screening test for Down Syndrome because of the low detection rate and high false alarm rate. It is combined with the QST, especially when the results of the QST comes in the borderline zone.