A Simple blood test is the most effective test. Unlike many other genetic disorders such as Cystic Fibrosis and Haemophilia, which require complex and often expensive DNA marker testing, b (Beta)Thalassaemia carriers are the easiest gene disorder carriers to discover as their red blood will be microcytic (smaller in size) and hypochromic (paler in colour). The test is called Haemoglobin Electrophoresis.
TESTS DURING A PREGNANCY THE FINAL INFORMED CHOICE
- The opportunity to test the foetus is first offered to a couple when BOTH partners have been found to be Carriers and they then CHOOSE to test the foetus and exercise an informed choice.
- There are 3 tests available and they must be done at their respective optimum time. These tests are different from the blood test used to determine a Carrier. Chorionic Villus Sampling (CVS) Test is done at 10 weeks of pregnancy. Fetal blood sampling is done between 18 - 20 weeks and the rarely done Amniocentesis is between 14 - 18 weeks.
HOW DO THE TESTS DETERMINE IF THE FOETUS IS AFFECTED? The choice of which test to use is a very carefully considered matter. However, if the mother has presented herself early in a pregnancy, Chorionic Villus Sampling is the best test because of the ease of doing it and its acceptability to the mother as it can be done earlier in the pregnancy.
Chorionic Villus Sampling (CVS) involves taking a small sample of the developing placenta at 10 weeks of pregnancy. The placenta, which is attached to the growing foetus by the umbilical cord, contains the DNA imprint of the baby. The placenta and its position is first assessed by ultra sound. A sample of the placenta is obtained either through the vagina or through the abdomen dependent upon where the placenta is located. The sample of placenta has its DNA then tested to see if the child's Haemoglobin making genes are affected and this determines if the baby has the full blown condition Thalassaemia Major.
Fetal blood sampling involves the taking of a blood sample from the umbilical cord of the foetus in the womb. Fetal blood sampling is used when the foetus has developed more and the pregnancy has advanced to 18 - 20 weeks. An affected baby will have little or no adult Haemoglobin HbA. This test is used when the CVS test cannot get access to the placenta or when the DNA test cannot determine the exact DNA profile of the baby.
Amniocentesis involves taking a small sample of the amniotic fluid that surrounds the baby (between 14 -18 weeks). As there are very few cells from the baby floating about in the amniotic fluid, the test requires culturing and growing of the captured cells in the laboratory and then the DNA of these cells is tested as per the placenta test.
HOW DO THE TESTS COMPARE? EASE OF SAMPLING AND TESTING
The CVS has merits as you can take a sample from a sizeable placenta and have a DNA test made available within 5- 10 days. Fetal blood sampling requires getting access to the small umbilical cord and then measuring the Hb A or DNA- it can be difficult. Amniocentesis requires a cumbersome process as it requires looking for fetal cells in the amniotic fluid and then the culturing of the cells in addition to the DNA test. PSYCHOLOGICAL ASPECTS OF TESTING
It is reported that the earlier a test is done in a pregnancy, the lesser the psychological factors affecting the mother and couple. CVS is done from 10 weeks and is very early in the pregnancy and the mother has not developed a deep bond with the pregnancy- especially when it is not showing and/ or has not been physically demanding. Fetal blood sampling is quite late and will be more demanding psychologically where termination of a pregnancy is a considered option.