Noninvasive prenatal testing: Where we are and where we’re going

The introduction of amniocentesis in the 1960s brought to prenatal diagnosticians the ability to detect fetal chromosome abnormalities and certain structural defects (including neural tube defects). Since that time, a goal for these practitioners has been the development of effective screening algorithms to better identify women at high risk for detectable fetal abnormalities in concert with the advent of safer and more accessible diagnostic tests, with the eventual aim being the development of a noninvasive prenatal diagnostic test.

Postamniocentesis advancements have included the identification of maternal serum analytes as well as the incorporation of first-trimester ultrasonographic measurements of the fetal nuchal translucency (NT) and nasal bone, all associated with an improved ability to identify women at increased risk for fetal trisomies 21 and 18 as well as some other fetal abnormalities. In addition, targeted ultrasound has greatly improved the ability to detect fetal structural and growth abnormalities in women of all risk levels, although it remains a highly subjective process with considerable inter/intraoperator and equipment variability.

Related article: NIPT is expanding rapidly--but don't throw out that CVS kit just yet! (Update on Obstetrics; Jaimey M. Pauli, MD, and John T. Repke, MD; January 2014)

Noninvasive prenatal screening has the advantages of being noninvasive and carrying no increased risk for fetal loss compared with chorionic villus sampling (CVS) and amniocentesis, which are associated with a small increased risk for pregnancy loss (1/500 to 1/1,500 over baseline risk for loss). However, noninvasive screening is limited compared with diagnostic procedures because it provides only a risk adjustment rather than a definitive diagnostic outcome and is mostly limited to assessment for fetal trisomies 18 and 21.

Targeted ultrasound can identify structural abnormalities associated with other chromosomal, genetic, and genomic abnormalities, but again depends on operator experience, equipment used, maternal habitus, and fetal position. Accordingly, considerable interest has remained in developing a more effective approach for detecting fetal aneuploidy and other fetal abnormalities, including assays that eventually could serve to provide noninvasive prenatal diagnosis.

RECENT ADVANCES BRING US CLOSER TO OUR ULTIMATE GOAL
The recent introduction of circulating cell-free nucleic acids (ccfna) technologies for prenatal screening for common fetal aneuploidies, better known as noninvasive prenatal testing, or NIPT, has presented a far more effective prenatal screening protocol for certain groups of women compared with the aforementioned screening algorithms that rely on measurements of the fetal NT in the late first trimester and maternal serum measurements of analytes in the first and second trimesters.

Currently, four NIPT screening products are available commercially in the United States: MaterniT21 Plus (Sequenom, San Diego, California); Verifi (Illumina, San Diego, California); Harmony Prenatal Test (Ariosa Diagnostics, San Jose, California); and Panorama Prenatal Test (Natera, San Carlos, California). While the technologies and algorithms used by each of the companies differ, they all rely on the premise that 5% to 10% of ccfna in maternal blood are fetal in nature.¹ Calculating the ratios of the expected amount of each chromosome-specific nucleic acid to that actually measured in the sample, a prediction of a normal or abnormal complement for that specific chromosome is then made. None of the commercially available tests specifically identify fetal DNA or differentiate fetal from maternal DNA.

Current validation studies have thus far limited the offering of NIPT to women at increased risk for fetal aneuploidy, including those:^2–6

• of advanced maternal age
• with a positive conventional screening test
• with abnormal ultrasound results suggestive of aneuploidy, or
• who have had a prior pregnancy with a chromosome aneuploidy found in the NIPT panel.

Studies of all available technologies tested on women at increased risk for fetal aneuploidy have thus far shown considerably higher sensitivities and specificities and detection rates for fetal trisomies 21, 18, and 13 than conventional screening algorithms, although detection rates for trisomy 13 are slightly lower than those observed for trisomies 21 and 18.

WE STILL HAVE MANY HURDLES TO LEAP
However, the groups of women at high risk for fetal aneuploidy just outlined represent only a small segment of the community of pregnant women. A multicenter study involving 1,914 women published February 2014 in the New England Journal of Medicine⁷ showed considerably and significantly lower false-positive rates and higher positive predictive values for the detection of trisomies 21 and 18 by NIPT compared with conventional fetal aneuploidy screening. This study incorporated women at low risk for fetal aneuploidies in the study cohort, although women at high risk (based on the stated range of maternal age) also were included in the cohort. Unfortunately, no information was provided in the report about the percentage of low-risk women among the study participants.