610: Actual rates of recommended diagnostic testing after first trimester screening vs. same-day screening by cell free DNA

To compare actual patient referrals for post-screen diagnostic tests following First Trimester Screening (FTS) vs same-day cell free DNA (cfDNA). Retrospective analysis of all patients with singleton gestations who elected to have concurrent cfDNA at time of routine FTS at our ultrasound unit between January-June 2015. FTS was considered high risk if nuchal translucency (NT) > 3 mm or if risk based on serum screen markers and NT was higher than 1:300 for T21 or 1:150 for T13/18. High risk cfDNA screen included samples with increased risk for T21, T18, T13, 45XO and sex chromosome trisomies or if no results could be reported. Groups were compared for patients gestational and screen characteristics and for rate of high risk results. p<0.05 was considered significant. 125/1467 (8.5%) women had a high risk screen result. FTS was high risk in 68 (4.6%) patients (56xT21, 4xT13/18, 8xNT >3) whereas 63 (4.3%, p=0.65) of women had high-risk cfDNA (1xT21, 4xT13, 2 sex trisomies + 56 with no results). Mean maternal age was 32.1 (21-41) and mean gestational age was 12w4d (+/-4d). The averaged fetal fraction was 7.1% vs 9.3% for all samples (p=0.3). However, more high risk samples (76/125, 60.8%) had fetal fraction under 8% compared to the entire cohort (580/1467, 39.5%, p<0.05). 6 (4.8%) women had both tests positive. For 54/1467 (3.7%) women, cfDNA yielded no results. Redraw was performed in 31 women and complete report was given to 20 of them. Ultimately, 43 (2.9%) women had high risk cfDNA for which diagnostic testing was recommended vs 68 (4.6%) high risk after FTS (p=0.016). Fewer women who choose cfDNA for screening for aneuploidy in first trimester would be offered diagnostic testing after the screen. However, the advantage of cfDNA over FTS in lowering invasive testing when done in first trimester may be less prominent than expected.