- New antenatal “reflex DNA” screening for three chromosomal disorders including Down’s syndrome found to be more accurate, safer, and causing less anxiety to expectant parents
- Current protocol involves initial screening comprising of ultrasound and blood test followed by an invasive procedure to confirm diagnosis; this is more stressful to the woman and not as safe as reflex DNA testing.
protocol of antenatal DNA screening of pregnant women for three serious
chromosomal disorders including Down's
syndrome is found to be safer, more accurate and one that causes less
tension for the expectant parents according to a recent study by scientists at
the Queen Mary's University London. The findings of the study appear in the
journal Genetics in Medicine.
Antenatal Diagnosis of Down's Syndrome - New Reflex DNA Testing ProtocolPresent Protocol
- Currently, pregnant women in the UK are offered a screening test for three chromosomal disorders including Down's syndrome at 10-14 weeks gestation.
- The screening test combines an ultrasound scan and a blood biochemical test.
- After estimation of risk, if these tests throw up the possibility of the woman having an affected pregnancy at an estimated risk of 1 in 800 or more, the woman is recalled and offered a diagnostic test namely amniocentesis or chorionic villus sampling.
- Both the above mentioned tests are invasive and involve insertion of a needle into the womb via the abdominal wall to obtain samples of the amniotic fluid or placental tissue.
Suggested New Reflex DNA Testing Protocol
- In the novel protocol followed in the study, the pregnant woman gave a blood sample around 11 weeks of gestation
- The sample was divided into two parts; one part was used to perform the conventional
- bloodbiochemical test while the second part was held in reserve.
- If the estimated risk was found to be 1 in 800 or more, the reserved sample was automatically retrieved and a DNA test (reflex DNA testing) performed to confirm the diagnosis.
Reflex DNA Testing Protocol - Findings of the StudyThe new protocol was implemented in five UK NHS obstetric units, and more than 22,000 women took part between April 2015 and August 2016; the protocol still continues to be followed.
The three chromosomal disorders screened for included Down's syndrome, Edward's syndrome and Patau's syndrome.
- Out of the 22,812 women screened, 11 percent had reflex DNA testing.
- On reflex DNA testing, it was found that 101 out of the 106 positive results did have one of the three disorders. There were only four false positives, which meant 95 percent accuracy (in comparison to 81 percent with the conventional test alone).
- False positive rate was reduced more than 100 fold from existing 2.42 percent with the combined test to 0.02 percent with the reflex DNA testing.
- Among the pregnancies that tested positive, the women were then offered an invasive diagnostic test; 25 out of 26 women tested were affected by one of these syndromes.
Other Advantages of Reflex DNA TestingReflex DNA testing also reduces the number of women who would need counselling as a result of a positive conventional screening test, and the unnecessary anxiety caused in this group.
Unnecessary invasive diagnostic procedures such as amniocentesis and chorionic villus sampling are avoided with potential risks such as miscarriage.
Adds Professor Wald, "The National Screening Committee is considering a similar proposal, which involves recalling women with a positive initial result for a DNA test, some of whom will be sufficiently anxious to request a diagnostic amniocentesis rather than the DNA test. The reflex method avoids causing worry to this group, and avoids the dilemma in choosing between a DNA screening test and a diagnostic amniocentesis."
Mr Joseph Aquilina, consultant obstetrician at Barts Health NHS Trust who participated in the project, said: "The new reflex DNA approach is transformational. Not only is the screening method better than current practice, but I have more time to devote to other clinical needs, as do the nursing and midwifery staff involved."
- The scientists are also approaching other hospitals in the UK to check if they would be interested in adopting the new protocol.
- They claim that reflex DNA testing could be followed by hospitals worldwide so long as they have access to labs that can perform the DNA analysis.
Chromosomal DisordersThe normal human cell contains 23 pairs of chromosomes. Defects in the number or the structure of the chromosomes result in birth defects and other conditions that may develop during life.
Numerical defects of chromosomes i.e. presence of an extra chromosome or absence of a chromosome (aneuploidy) is the most common form of chromosomal abnormality. Down's syndrome (trisomy 21), Edward's syndrome (trisomy 18) and Patau syndrome (trisomy 13) are all characterized by the presence of an extra chromosome 21, 18 and 13 respectively resulting in 47 chromosomes.
Antenatal screening and diagnosis of these serious disorders gives parents the option to go ahead with or terminate the pregnancy.
- Down Syndrome: Prenatal Risk Assessment and Diagnosis - (http://www.aafp.org/afp/2000/0815/p825.html)