8. Maternal Serum Screening and Down Syndrome screening methods + NIPT Flashcards
What is nuchal translucency?
Thickness of subcutaneous translucency between fetal skin & soft tissue over cervical spine
What is the detection rate for screening based on NT?
75-80% detection rate with 5% false positive rate
What is considered the cut off for NT?
> 3.5mm strongly associated with aneuploidy
> 3.5mm with normal karyotype associated with range of structural malformations & genetic syndromes
How are serum markers interpreted?
Multiples of the median - serum marker concentration in patient divided by median concentration for unaffected pregnancies at the same gestation
What abnormalities are visible at the 18-20 week scan?
Cardiac defects, cleft lip, cystic hygroma, neural tube defect, polydactyly
What serum markers are used in the combined/quadruple tests?
PAPP-A & free BhCG
When are the combined and quadruple tests given?
Combined = 12 weeks
Quadruple = 14-20 weeks
From what point is NIPT feasible and why?
10 weeks onwards
Sufficient level of cffDNA
Why is NIPT not considered diagnostic?
Not reliable enough - very sensitive and specific for T21 (99% sensitivity, 0.1% FP rate) but less so for other aneuploidies
Why might the results of NIPT be inaccurate?
- CPM as cffDNA is derived from placental trophoblasts
- Detection of maternal chromosomal rearrangement
- Detection of maternal ctDNA from cancer
- Shedding of DNA from vanishing twin
What risk is considered ‘screen positive’ in the UK?
Why has this figure been choosen?
> 1:150
Balance between high detection rate and minimising invasive procedures
What fetal fraction is required for a singleton pregnancy? And for twins?
4% single, 8% twins
Why is NIPT less accurate for T13 & T18?
Analytical bias as chr 13 and 18 have high GC content - corrected by with bioinformatic algorithms
When is NIPT offered in the UK?
Following a higher chance result from the combined or quadruple test
Why does a higher chance NIPT result require confirmation?
Result may be due to CPM
