I'm a healthy 35yo. what is the % of tests coming back as a false + for the down syndrome blood worki?

Cell-free fetal: DNA testing of maternal blood, offered by many centers after 10 wks. gestation to women who'll be 35 or > at delivery, detects > 99% of Down Syndrome, Trisomy 21, > 98% of Edward Syndrome, Trisomy 18 & ~ 65% of Patau Syndrome, Trisomy 13 if done right. If + for any, Karyotyping or, rarely, Chromosomal Microarray is offered on Chorionic Villous Sampling or Amniocentesis for definitive diagnosis