Cell Free fetal DNA test (cff DNA)
Offered by various companies – BGI (HK), Qnatal (Quest Diagnostics), Panorama (MedGenome, Natera) Harmony (Ariosa)
What is the cff DNA test?
This test analyses cell free DNA in maternal blood and gives a strong indication of whether the fetus is at high or low risk of having trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome) or trisomy 13 (Patau syndrome).
- The test identifies 99%, but not all, of the fetuses with trisomy 21, 97% of fetuses with trisomy 18 and 92% of fetuses with trisomy 13.
- If you decide to have the test you will be asked to sign a consent form by one of our doctors or an appropriately trained member of our staff. We will then draw a small amount of blood from a vein in your arm. This may cause some discomfort but the sample is usually taken very quickly. Sometimes there can be some bruising after a blood sample is taken.
What is trisomy 21, 18 or 13?
In humans, there are 23 types of chromosomes and most people have a pair of each one of these chromosomes (therefore a total of 46 chromosomes). In trisomy, there are three rather than two of a particular chromosome (total of 47 chromosomes). The most common trisomies are those of chromosomes 21, 18 and 13.
- Trisomy 21 is found in about 1 in 700 births and the risk increases with maternal age. The condition is associated with intellectual disabilities and some physical defects, most commonly heart abnormalities. The life expectancy is about 60 years.
- Trisomies 18 and 13 are found in about 1 in 7,000 births and the risk increases with maternal age. The conditions are associated with severe mental handicap and several physical defects. Most affected individuals die before or soon after birth and they rarely survive beyond the first year of life.
When do I expect to get the results?
- The results from the test will generally be available within two weeks and we will notify you as soon as we receive them by phone and email / letter.
- In about 5% of cases the test does not give a result. This is due to technical problems with the analysis of the sample and does not suggest that there is a problem with the baby. If you want you can have the test repeated (at no cost) and there is a 50% chance that the test will give a result.
What would the results show?
- If the test shows that there is a high risk that the fetus has trisomy 21 or 18 or 13 it does not mean that the fetus definitely has one of these defects. If you want to be certain if the fetus has one of these defects you should have CVS or amnio.
- If the test shows that there is a low risk (less than 1 in 10,000) that the fetus has trisomy 21 or 18 or 13 it is unlikely that the fetus has one of these defects.
Do I need to have any other tests?
- The test does not provide adequate information on other rare chromosomal abnormalities in the current stage. If the scan at 11-13 weeks shows a high nuchal translucency (more than 3.5 mm) or major defects, such as exomphalos, holoprosencephaly, heart abnormalities or megacystis, the risk for some rare chromosomal defects may be high. In such cases you may choose to have CVS or amnio.
- The test does not provide information on physical defects, such as heart or brain abnormalities and spina bifida, or fetal growth. It is therefore advisable that you still have ultrasound scans at 11-13 weeks and at 18-22 weeks to examine the fetal anatomy and at 30-32 weeks to examine the fetal growth.
What is the cost of the cff DNA test?
- The cost of the test depends on the company with which you choose to do the test.
- Currently there is not much difference in the efficacy of results as far as the Trisomies are concerned between the companies.
- However, the report of the “fetal fraction” ie, the amount of fetal DNA in the maternal blood that ensure reliability of the test is reported by some and not by the others
- In addition, some companies offer screening for other genetic disorders that you might choose to do along with the screening for Trisomies
- If the test suggests “high risk” and that the fetus has trisomy 21, 18 or 13 we will carry out CVS or amniocentesis at no extra cost. This amount will be refunded by the company to you.
Who carries out the analysis of your blood for the cffDNA test?
We will send your blood sample along with your personal information (including name, date of birth, gestational age) to the company you have chosen to go with following a thorough counselling regarding various options with one of our doctors, who will extract cell-free DNA from your blood and carry out the test.
There will be no further clinical testing on this blood and your blood sample will be discarded once we have confirmed the results with you. Your blood sample or the extracted DNA samples will not be used for any other purpose.
The following scientific papers provide information that you may find to be useful:
- Implementation of cfDNA testing in early screening for aneuploidies
- cfDNA testing in twin pregnancies
- Combining the nuchal test with the cfDNA test