Obstetrics & Gynaecology

safeT21 is a non-invasive prenatal DNA test designed to screen for foetal chromosomal abnormalities, including Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13). During pregnancy, foetal DNA circulates in the mother's bloodstream. The safeT21express™ test analyses the quantity of DNA from chromosomes 21, 18, 13, and others in the mother’s blood sample. 

• An abnormally increased level of chromosome 21 DNA molecules in the maternal blood suggests Down syndrome, with a sensitivity of 99.65% and a false-positive rate of 0.03%.
• An abnormally elevated levels of chromosome 18 DNA indicate trisomy 18, with 99.9% sensitivity and a false-positive rate of 0.01%.
• For chromosome 13 DNA, abnormally high amounts suggest trisomy 13, with a sensitivity of 99.9% and a false-positive rate of 0.02%. 

The test can also detect additional chromosomal abnormalities, including sex chromosome conditions (e.g., 45 XO, 47 XXY, 47 XXX, and 47 XYY) and several microdeletion syndromes. These include 1p36 deletion syndrome, 2q33.1 deletion syndrome, 22q11 deletion syndrome (e.g., DiGeorge syndrome), Angelman syndrome, Cri-du-Chat syndrome, Langer-Giedion syndrome, and Prader-Willi syndrome. 

The accuracy of the safeT21express™ test depends on the presence of a sufficient quantity of foetal DNA in the maternal plasma sample. 

What procedures are involved in this test? 

1. An ultrasound examination to measure the size of the foetus and determinate the gestation
2. Blood taking from pregnant women for assessment 

What does it mean if the test result shows positive? 

A positive result does not 100% confirm that the baby has a chromosomal abnormality. There is the chance of a false positive result. Positive test results should therefore be confirmed by amniocentesis or chorionic villus sampling. 

What does it mean if the test result shows negative? 

A negative result means that the chance of the baby having Down syndrome, trisomy 18, trisomy 13, sex chromosomal abnormalities or common microdeletion syndrome is very low. However, it should also be noted that safe T21 remains a screening test. It does not completely exclude the possibility of a baby having these chromosome abnormalities. Invasive testing (e.g. amniocentesis or chorionic villus sampling) may sometimes be necessary to confirm the diagnosis.

The Quadruple Test is a non-invasive screening test for Down syndrome with a detection rate of around 83% and a false positive rate of 5%. The test is carried out between 16–19+6 weeks pregnancy, using biochemical markers (alpha foetal protein AFP, unconjugated estriol uE3, free beta human chorionic gonadotrophin b-HCG, Inhibin A) and maternal age for risk calculation. (This test is not available at HKSH)