Factors affecting cell-free DNA fetal fraction and the consequences for test accuracy.

INTRODUCTION: Biological factors are known to influence the fetal fraction (FF) of cell-free DNA and may also influence the accuracy of non-invasive prenatal testing. MATERIAL AND METHODS: NIPT from 5267 mixed risk women across three specialist clinics in Australia were analyzed. Multivariable regression analysis was used to determine whether maternal characteristics, ultrasound, and placental biomarkers affect FF and test accuracy. RESULTS: FF ranged from 4% to 37% (mean 11.6%). Body mass index (BMI), gestation, and placental biomarkers were found to be significant factors associated with FF. For each unit increase in BMI, the logarithmically transformed FF, (lnFF), mean value decreased by 0.027. Each week increases in gestation, lnFF increased by 0.023. Each unit increase in free BhCG, PAPPA, and PlGF, the lnFF increased by 0.065, 0.050, and 0.17, respectively. There was no significant association between FF with either maternal age or nuchal translucency. The false-positive cases and one false-negative case did not have lower FF than the true-positive cases. DISCUSSION: The fetal fraction in maternal plasma cfDNA increased with gestational age, serum pregnancy-associated plasma protein A (PAPP-A), ß-hCG, and PlGF and decreased with increasing maternal BMI. There was no significant correlation between low FF and test accuracy, when FF was above 4%.

Prospective evaluation of a first trimester screening program for Down syndrome and other chromosomal abnormalities using maternal age, nuchal translucency and biochemistry in an Australian population.

BACKGROUND: A combination of maternal age and ultrasound assessment of the nuchal translucency (NT) has been used in the first trimester to screen for chromosomal abnormality. In the United Kingdom, the addition of NT screening was shown to be beneficial. AIMS: To report the sensitivity of combined first trimester biochemistry and ultrasound screening for Down syndrome in an Australian private practice specialising in obstetric ultrasound. METHODS: A prospective study in a private obstetric ultrasound practice. Over 22 months, 2121 patients were screened and data was analysed for sensitivity (detection) and false positive rates for all chromosome abnormalities. RESULTS: There were 17 chromosomal abnormalities, five of which were Down syndrome. Using maternal age alone or age and biochemistry, four of the Down syndrome cases were detected for a 29 and 19% false positive rate, respectively. Using age and NT or age, NT and biochemistry, all the Down syndrome cases were detected, for a false positive rate of 5.7 and 7.2%, respectively. The difference in detection rates for Down syndrome or other chromosomal abnormalities, using the four screening methods, did not reach statistical significance. However, the false positive rates in screening methods without ultrasound to assess the NT was significantly higher (P < 0.01). CONCLUSIONS: A combination of maternal age, NT and maternal serum biochemistry gives a high detection rate for both trisomy 21 and other chromosomal abnormalities. Down syndrome screening using either maternal age alone or age in combination with first trimester biochemistry conferred screen positive rates significantly higher than when combined with NT.

The loss rates for invasive prenatal testing in a specialised obstetric ultrasound practice.

OBJECTIVES: To establish the spontaneous miscarriage rate and compare it with the procedure related miscarriage rate for amniocentesis and chorionic villus sampling (CVS) by experienced operators. DESIGN: Retrospective audit over a two-year period of all patients having a consultation for prenatal diagnosis before 12 weeks gestation. SETTING: A specialised obstetric and gynaecological ultrasound practice. POPULATION: A total of 2366 patients, mostly over 35 years of age. METHODS: Between 1 July 1995 and 30 June 1997, all patients having a prenatal consultation decided between amniocentesis, CVS or no invasive testing. The CVS was performed either transabdominally or transcervically, depending on the position of the uterus and placenta. MAIN OUTCOME MEASURES: Delivery, termination of pregnancy for chromosomal abnormality or miscarriage. RESULTS: Over the two-year period, 2366 patients had a prenatal consultation and outcome data were available for all but 53 patients. After consultation, 346 patients decided not to have any prenatal testing and 29 (8.4%) of these had a spontaneous miscarriage. Amniocentesis was requested by 839 patients; however 10 miscarried before the scheduled procedure. After the amniocentesis, there were 13 terminations for chromosomal abnormality and three miscarriages. CVS was requested by 1128 patients; however, 23 miscarried before the scheduled procedure. Transabdominal CVS was performed in 665 patients, transcervical in 416 and in 24 cases the documentation of the method used was unclear. Eleven patients miscarried after the transabdominal CVS (1.65%) compared with nine patients miscarrying after the transcervical CVS (2.16%), which was not statistically significant (p = 0.27). CONCLUSION: In the group studied, the spontaneous miscarriage rate from nine weeks gestation is very high (8.4%). The procedure-related loss rate from amniocentesis was less than 1 in 280. Transabdominal CVS appears to have a lower fetal loss rate than transcervical CVS, but much larger numbers are needed to prove this.