Screen-positive rate in cell free DNA screening for trisomy 21.

OBJECTIVE: To assess whether the fetal fraction (FF) has an impact on the screen-positive rate (SPR) in cell-free DNA (cfDNA) screening for trisomy 21. METHODS: Retrospective analysis based on samples analyzed using the Harmony® Prenatal Test (Roche Inc). Due to the size of the data set, we focused on the SPR, which was stratified according to the maternal age, weight, gestational age, and FF distribution. RESULTS: The study cohort consisted of 364,881 patients, including 2614 with a high-risk-result. Median maternal and gestational ages were 34.6 years and 12.4 weeks. FF was dependent on maternal age, weight, and gestational age. SPR was 0.72% and it was independent of maternal weight but was dependent on maternal age. There was a positive but weak association between the FF and the SPR until the FF reached 20.0% (OR p = 1.021, p < 0.001, Nagelkerkes r2 = 0.001). This group included 357,800 pregnancies or 98.1% of the study population. In the group of pregnancies with a FF > 20%, the association was stronger (OR 1.099, p < 0.001, Nagelkerkes r2 = 0.042). CONCLUSION: The SPR in cfDNA screening for trisomy 21 was relatively constant up to a FF of about 20%.

Screen-positive rate in cell-free DNA screening for microdeletion 22q11.2.

OBJECTIVE: To examine the impact of the fetal fraction (FF) on the screen-positive rate in screening for microdeletion 22q11.2. METHODS: This study is based on samples that were analyzed using the Harmony® Prenatal Test (Roche Inc). The study cohort comprised samples from women with singleton pregnancies who were at least 16 years old and at least at 11 weeks' gestation. Logistic regression analysis was used to determine significant covariates that carry an impact on the screen-positive rate. RESULTS: The study population consisted of 52,019 pregnancies, including 309 pregnancies with a high-risk result for microdeletion 22q11.2. Thus, the overall screen-positive rate was 0.59%. In the low-risk group, the FF was 10.1%, and in the high-risk group, it was 7.3%. Regression analysis indicated a strong correlation between the FF and the screen-positive rate. In the cases with an FF of <11.0%, the screen-positive rate was 0.92%, while it was 0.13% in the group with a higher FF. CONCLUSION: The screen-positive rate depends on the FF. In order to keep the rate low, we recommend restricting the analysis to samples with a FF of 11% and more.

Confirmation rate of cell free DNA screening for sex chromosomal abnormalities according to the method of confirmatory testing.

OBJECTIVE: To examine the positive predictive value (PPV) of cfDNA screening for sex chromosome aneuploidies (SCA) in a large series of over 90 000 patients. METHODS: Retrospective study based on samples that were sent to Cenata, a private laboratory which uses the Harmony Prenatal Test. The SCA high-risk results were stratified according to the method of diagnostic testing and according to karyotype result. RESULTS: The study population consisted of 144 cases. The CfDNA test indicated monosomy X, XXX, XXY, and XYY in 62, 37, 40, and 5 cases, respectively. The overall PPV was 38.9% (30.9-47.4), 29.0% (18.2-42.9) for monosomy X, 29.7% (15.9-47.9) for 47,XXX, 57.5% (40.9-73.0) for 47,XXY, and 80.0% (28.4-99.5) for 47,XYY). A total of 112 (77.8%) women with a high-risk result for SCAs opted for prenatal karyotyping. In this group, there were significant differences in the PPV if the karyotype was assessed by amniocentesis or by CVS: 29.5% vs 50.0%. This significant difference was driven by the monosomy X result which shows a significantly higher PPV in CVS (54.6% (23.4-83.3) vs 17.1% (6.6-33.6)). For the other SCAs, the differences were not significant. CONCLUSION: PPV of an abnormal cfDNA test for SCAs is low, particularly for monosomy X. The confirmation rate depends on the type of confirmatory test.

False-Positive Rate in First-Trimester Screening Based on Ultrasound and Cell-Free DNA versus First-Trimester Combined Screening with Additional Ultrasound Markers.

OBJECTIVE: To determine whether screening for trisomy 21 based on first-trimester combined screening (FTCS) with assessment of nasal bone (NB), tricuspid flow (TCF), and ductus venosus flow (DVF) results in similar false-positive rates compared to ultrasound and cell-free DNA (cfDNA) screening. METHODS: This is a subanalysis of a prospective randomized controlled trial which was performed between October 2015 and December 2016. Pregnant women with a normal first-trimester ultrasound examination at 11 to 13 weeks' gestation were randomized into two groups: (1) FTCS with assessment of the NB, TCF, and DVF (extended FTCS [eFTCS]), and (2) ultrasound + cfDNA screening. The false-positive rate in screening for trisomy 21 was defined as the primary outcome parameter. RESULTS: The study population consisted of 688 women in each study arm. In the eFTCS group, the median delta fetal nuchal translucency thickness (NT) was 0.0 mm, free beta-hCG and PAPP-A were 0.96 and 1.11 MoM, and NB, TCF, and DVF PIV were abnormal in 0.9, 0.6, and 7.0% cases. In the ultrasound + cfDNA group, the median delta NT was 0.0 mm. In 10 pregnancies the cfDNA analysis was uninformative and the risk of trisomy 21 was based on eFTCS. There were no false-positive cases in the ultrasound + cfDNA group, whereas the false-positive rates were between 0.9 and 2.2% with eFTCS. CONCLUSION: Screening for trisomy 21 based on ultrasound + cfDNA has a lower false-positive rate than screening based on eFTCS.