Prenatal Screening NIPT Flashcards
What does NIPT stand for?
non-invasive prenatal testing
what genetic disorder is routinely offered to pregnant women to test for in the UK?
Down syndrome
what do they measure?
fluid space at the back of neck called the nuchal translucency and combine this with some maternal hormon blood measurments
accuracy of this nuchal translucencey down syndrome test?
false pos = 3%
detection rate of 75%
(wright d et al 2010)
what diagnositc tests are there that give yes or no?
amniocentesis
chorionic villus sampling (take from placenta)
gives diagnostic test on baby to tell if neuploid or aneuploid however risk of miscarriage
what is fetal cfDNA?
Originates from trophoblast
Detectable from 5 weeks
5-10% of total cell-free DNA in woman - coming from placenta
Cleared immediately after birth
what are the principles of NIPT?
take a blood sample from woman and assume 10% of cfDNA is fetal
If the baby has a trisomy, there will be a relative excess of that particular chromosome in the materal circulation. That relative excess will be half of whatever the fetal fraction of dna is. So if there is 10% cfDNA from the fetus then need to be able to distinguish 21 copies of 21 from 20 copies of 18.
meta analysis of NIPT showed?
found the test has a greater than 99% sensitivity and greater than 99% specificity for downs syndrome
alternative approach to deal with low fetal fraction?
likelihood ratio- SAFE test
the lower the ff is, the less robust the test is
what does the SAFE test do?
intergrates fetal fraction into risk. the more fetal dna we have the more robustly we decrease the risk for the woman
steps of SAFE
1) 10mL blood sample collected from the expectant mother
2) fully automated DNA extraction
3) library preperation
4) sample prepared for downstream sequencing and then analyzed
5) automated data analysis
advantages of NIPT
pragmatic and effective safe and sensitive cheaper thatn CVS/amnio results within 3-5 days keeps money in the NHS
