Jacob A. Canick

Maternal plasma contains circulating cell‐free DNA fragments originating from both the mother and the placenta. The proportion derived from the placenta is known as the fetal fraction. When measured between 10 and 20 gestational weeks, the average fetal fraction in the maternal plasma is 10% to 15% but can range from under 3% to over 30%. Screening performance using next‐generation sequencing of circulating cell‐free DNA is better with increasing fetal fraction and, generally, samples whose values are less than 3% or 4% are unsuitable. Three examples of the clinical impact of fetal fraction are discussed. First, the distribution of test results for Down syndrome pregnancies improves as fetal fraction increases, and this can be exploited in reporting patient results. Second, the strongest factor associated with fetal fraction is maternal weight; the false negative rate and rate of low fetal fractions are highest for women with high maternal weights. Third, in a mosaic, the degree of mosaicism will impact the performance of the test because it will reduce the effective fetal fraction. By understanding these aspects of the role of fetal fraction in maternal plasma DNA testing for aneuploidy, we can better appreciate the power and the limitations of this impressive new methodology. © 2013 John Wiley & Sons, Ltd.

Objective: Studies on prenatal testing for Down syndrome (trisomy 21), trisomy 18, and trisomy 13 by massively parallel shotgun sequencing (MPSS) of circulating cell free DNA have been, for the most part, limited to singleton pregnancies. If MPSS testing is offered clinically, it is important to know if these trisomies will also be identified in multiple pregnancies.Method: Among a cohort of 4664 high‐risk pregnancies, maternal plasma samples were tested from 25 twin pregnancies (17 euploid, five discordant and two concordant for Down syndrome; one discordant for trisomy 13) and two euploid triplet pregnancies [Correction made here after initial online publication.]. Results were corrected for GC content bias. For each target chromosome (21, 18, and 13), z‐scores of 3 or higher were considered consistent with trisomy.Results: Seven twin pregnancies with Down syndrome, one with trisomy 13, and all 17 twin euploid pregnancies were correctly classified [detection rate 100%, 95% confidence interval (CI) 59%–100%, false positive rate 0%, 95% CI 0%–19.5%], as were the two triplet euploid pregnancies.Conclusion: Although study size is limited, the underlying biology combined with the present data provide evidence that MPSS testing can be reliably used as a secondary screening test for Down syndrome in women with high‐risk twin gestations. © 2012 John Wiley & Sons, Ltd.

The performance of prenatal screening tests for the identification of trisomy 21 (Down syndrome) has markedly improved since the 1970s and early 1980s when maternal age was the sole mode of screening the general pregnant population. With the discovery of second trimester serum markers in the 1980s and 1990s and implementation of double, triple, and quad marker testing; the discovery of first trimester serum and ultrasound markers in the 1990s and implementation of the combined test; and the development of the integrated test and sequential screening strategies over the past decade, the performance of screening has improved to a detection rate of 90%–95% at a false positive rate of 2%–5%. In this review, I will describe the advances in prenatal screening for trisomy 21, present current screening strategies, and discuss guidelines published by professional societies and regulatory bodies, with a focus on current prenatal screening practice in the USA.

We initiated a voluntary, self-funded interlaboratory comparison program in the fall of 2005 because no proficiency testing program was available to laboratories in North America offering first-trimester, combined serum and ultrasound, Down syndrome screening. Objectives.-To evaluate the first 4 years of the interlaboratory comparison program against stated goals, to identify areas of concern, and to create new initiatives as indicated. Five serum samples are distributed 3 times a year to be tested for pregnancy-associated plasma protein A, human chorionic gonadotropin or its β subunit, and dimeric inhibin-A; participants convert these results into multiples of the median. Patient histories include nuchal translucency information that enables the calculation of the risk of Down syndrome. Also included are educational components linked to interlaboratory comparison program results. Assessment of integrated (first- and second-trimester markers) risks is accomplished by having participants combine interlaboratory comparison program results with their results from a second-trimester proficiency testing program administered by the College of American Pathologists. The precision profile for pregnancy-associated plasma protein A shows decreasing coefficients of variation with increasing pregnancy-associated plasma protein A concentrations and multiples of the median (25% to 11% and 30% to 15%, respectively). In contrast, coefficients of variation are a relatively constant 12% throughout the entire range of human chorionic gonadotropin results. On a logarithmic scale, the median coefficient of variation of the risk of Down syndrome is 9%. Participants in the interlaboratory comparison program reliably measure analytes, compute multiples of the median, and calculate consistent Down syndrome risks. Assays for the measurement of pregnancy-associated plasma protein A are not standardized and are less precise than those for human chorionic gonadotropin. Participants calculate reliable median equations given sonographer-specific sets of paired crown-rump length and nuchal translucency measurements.