Table 3.

ZFY/X digital NASS was performed on the maternal plasma samples to determine the fetal DNA concentration to select the relevant SPRT allelic ratio cutoff points for genotype classification

SampleWellsNo. negative wellsLXYLXLYXYLXY*All DNA
Short DNA
Enrichment, %
X copiesY copiesFetal %X copiesY copiesFetal %(Short–all)/all
M4176P376285057725110949186382328
M3848P384295054626120929186182431
M3960P2,6812,196036121962325072811391261319
M3146P383310057510001756156261819
M2098P2,2711,7680339221342425383011372281433
  • L, extension product generated only by the long amplicon; X, extension product of ZFX; Y, extension product of ZFY.

  • *As these are maternal plasma samples, wells positive for LXY are assumed to contain one copy of LX (i.e., long X DNA molecule) and one copy of Y (i.e., short Y DNA molecule).

  • All DNA Y copies = {−ln(NYLYXYLXY)/N]}N

  • All DNA X copies = {−ln(NXLXXYLXY)/N]}N

  • Short DNA Y copies = {−ln(NYXYLXY)/N]}N

  • Short DNA X copies = {−ln(NXXY)/N]}N

  • where N = the total number of digital PCR wells analyzed and ln is the natural logarithm.