[Combined first trimester screening and cell-free fetal DNA - “next generation screening”]

Principles of first trimester screening in the age of non-invasive prenatal diagnosis: screening for chromosomal abnormalities

PURPOSE

First trimester risk assessment for chromosomal abnormalities plays a major role in the contemporary pregnancy care. It has evolved significantly since its introduction in the 1990s, when it essentially consisted of just the nuchal translucency measurement. Today, it involves the measurement of several biophysical and biochemical markers and it is often combined with a cell-free DNA (cfDNA) analysis as a secondary test.

METHODS

A search of the Medline and Embase databases was done looking for articles about first trimester aneuploidy screening. We performed a detailed review of the literature to evaluate the screening tests currently available and their respective test performance.

RESULTS

Combined screening for trisomy 21 based on maternal age, fetal NT, and the serum markers free beta-hCG and PAPP-A results in a detection rate of about 90% for a false positive of 3-5%. With the addition of further ultrasound markers, the false positive rate can be roughly halved. Screening based on cfDNA identifies about 99% of the affected fetuses for a false positive rate of 0.1%. However, there is a test failure rate of about 2%. The ideal combination between combined and cfDNA screening is still under discussion. Currently, a contingent screening policy seems most favorable where combined screening is offered for everyone and cfDNA analysis only for those with a borderline risk result after combined screening.

CONCLUSION

Significant advances in screening for trisomy 21 have been made over the past 2 decades. Contemporary screening policies can detect for more than 95% of affected fetuses for false positive rate of less than 3%.

Contingent non-invasive prenatal testing: an opportunity to improve non-genetic aspects of fetal aneuploidy screening

BACKGROUND

Several countries today struggle with suboptimal performances in many aspects of the fetal aneuploidy screening process and consider introducing non-invasive prenatal screening (NIPT) as a solution. In this study, costs and benefits of different scenarios for contingent NIPT screening in Belgium are evaluated with respect to partial redistribution of the national screening budget into quality improving measures for those screening activities that will be maintained when full NIPT screening is implemented.

METHODS

Data from the Belgian National Institute for Health and Disability Insurance and the Study Centre for Perinatal Epidemiology were used in modeled calculations of medical and economic impact of NIPT after prior conventional screening (1) at thresholds 1:300 and 1:600, and (2) at current and improved screening sensitivity.

RESULTS

Contingent NIPT screening under current screening conditions would maintain today's 7.9(0)/000 live birth prevalence of Down syndrome (LBPD) at an 11% reduction of overall short-term costs. Lowering the screening threshold to 1:600 or increasing sensitivity by 10% would reduce LBPD to 7(0)/000 at a maximum 3% increase of overall short-term costs.

CONCLUSION

Today, in Belgium and in many other countries, full NIPT screening is considered too expensive for immediate introduction into the national fetal aneuploidy screening program. Contingent NIPT screening is both clinically and economically beneficial. A temporary contingent NIPT protocol allows for reinvesting economic savings into optimization of those screening aspects, which are to be maintained in parallel to full NIPT screening.

Single Nucleotide Polymorphism-Based Analysis of Cell-Free Fetal DNA in 3000 Cases from Germany and Austria

BACKGROUND & PATIENT

Data from 3 008 patients, who underwent single-nucleotide-polymorphism (SNP)-based noninvasive prenatal testing (NIPT) are presented.

METHOD

The PanoramaTM test (Natera, San Carlos, CA) was used to analyze cell-free fetal DNA from maternal blood for trisomies 21, 18, and 13, triploidy and sex-chromosome aneuploidies.

RESULT

In 2 942 (97.8%) cases, a result was obtained. The average fetal fraction was 10.2%. A high-risk result for fetal aneuploidy was made for 65 (2.2%) cases. In 59 (90.8%) of these cases, invasive testing confirmed the aneuploidy. There were 6 false-positive cases. In the false-positive group, the fetal fraction was significantly lower. The overall positive predictive value was 90.8%. No false-negative cases were reported but many patients in this study have not delivered yet. Therefore, exact data cannot be given for potential false-negative cases.

CONCLUSION

SNP-based NIPT is a reliable screening method for evaluating the risk of aneuploidies of chromosomes 21, 18 and 13. By using NIPT, the number of invasive procedures may be reduced significantly compared to maternal age and first-trimester screening.

Screening Performance and Costs of Different Strategies in Prenatal Screening for Trisomy 21

Objective: Cell-free fetal DNA (cffDNA) testing has opened new options in prenatal screening for trisomy 21. Due to the higher costs of cffDNA testing there is an ongoing debate on how to combine different screening strategies. Methods: For this study, a model-based approach was used to evaluate all births in Germany in 2012 together with the percentage of euploid and trisomic pregnancies. Detection rates (DR), false positive rates (FPR), the costs of different screening strategies for trisomy 21 and combinations of these strategies were compared. The number of fetuses with trisomy 21 at 12 + 0 weeks of gestation was estimated based on maternal age distribution. We examined the screening performance of a screening strategy based on maternal age, first trimester screening (FTS) and cffDNA testing as well as the combinations "maternal age and cffDNA" and "FTS and cffDNA". Results: In 2012 673 544 children were born. Median maternal age at delivery was 30.2 years (25th-75th quartile: 27.0-34.0). Based on maternal age distribution the expected number of fetuses with trisomy 21 at 12 weeks' gestation was 1788. Our study population therefore consisted of 675 332 pregnancies. Screening based only on maternal age or FTS or cffDNA resulted in detection rates of 63.3 %, 92.2 % and 99.0 % and false positive rates of 21.8 %, 8.0 % and 0.1 %, respectively. When maternal age was combined with cffDNA, cffDNA testing was only offered to women over a certain age; if a cut-off of 30 years was used, this resulted in a DR of 85.2 % and a FPR of 1.7 %. If primary screening consisted of FTS with cffDNA testing only done when the risk was between 1 : 10 and 1 : 1000, the detection rate was 96.7 % and the false positive rate was 1.2 %. Conclusion: In this model-based study we showed that prenatal screening for trisomy 21 can be improved even more by combining FTS and cffDNA. Further studies are necessary to examine whether these results can be reproduced in reality.

Nichtinvasive Pränataldiagnostik

Das Ersttrimesterscreening zur Risikobestimmung für die Trisomien 21, 18 und 13 hat sich in den letzten 15 Jahren in Deutschland etabliert. Die optimale Durchführung setzt die Einhaltung bestimmter Messkriterien beim Ultraschall und bei der biochemischen Analyse voraus sowie die Benutzung evaluierter Risikoberechnungsprogramme wie dem Berechnungsprogramm PRC der Fetal Medicine Foundation Deutschland (FMF-D). Durch die neue Version des Berechnungsprogramms PRC konnten die Trisomie-21-, -18- und -13-Detektionsraten erhöht werden bei gleichzeitiger Senkung der Falsch-positiv-Raten, was einen großen Fortschritt verglichen mit der mütterlichen Altersindikation darstellt.

Durch die Analyse der zellfreien fetalen DNA aus dem mütterlichen Plasma können seit 2 Jahren aber wesentlich bessere Screeningvorhersagen getroffen werden. Über 99 % aller Trisomie-21-Schwangerschaften können mit dieser Methodik als Risikogruppe beschrieben werden. Die Falsch-positiv-Rate liegt unter 1 %. Durch diese Methode ist ein Paradigmenwechsel in der Pränataldiagnostik zu erwarten.