1. What is mid-trimester biochemical screening?
- Mid-trimester biochemical screening is a screening test performed between 16-20 weeks of gestation for Downs syndrome;
- The risks are calculated based on the age of the pregnant women and the hormonal levels in the mother’s blood
- Depending the number of hormones checked, the biochemical screening tests might be called differently:
- (i) Alfra-fetoprotein (AFP) and Human chorionic gonadotropin (hCG)- Double test
- (ii) AFP, HCG and Unconjugated estriol (uE3)- Triple test
- (iii) AFP, HCG, uE3 and Inhibin A- Quadruple test or Quad screen
- The more hormones are used, the slightly higher detection rates will result but there is also higher cost involved
2. How is the test performed?
- Taking blood from the mother for the hormonal levels
- Computer software is used to calculate the risks
3. How accurate is the test?
- The detection rates depend on the number of hormones used;
- It is ~60% for Double test, 66% for Triple test and 80% for Quadruple test;
- There is a false positive rate of 5%
4. What if the test results are abnormal?
- Not all women with abnormal results have abnormal babies;
- It is important to discuss with a specialist in this field to determine the next appropriate option, whether it is NIPT or amniocentesis
5. What other abnormalities can this test screen?
- A low serum AFP happens in pregnancies with Downs and Edwards syndrome;
- A high serum AFP (> 2.5MoM) is associated with a higher chance of the following conditions:
- (i) Fetal structural malformation such as open neural tube defect and anterior abdominal wall defect;
- (ii) Bleeding during pregnancy;
- (iii) Hypertensive disorder in pregnancy
6. How is this test compared with other antenatal screening tests for Downs syndrome?
|
|
OSCAR |
Mid-trimester (16-20 weeks) biochemical screening |
NIPT |
|
Detection rate |
90% |
60-80% |
99.6% |
|
False positive rate |
5% |
5% |
0.03% |
