Jeroen Pennings, PhD
Sian Taylor-Phillips and colleagues at the Warwick Medical School (UK) performed a systematic review and meta-analysis on the performance of NIPT for detection of trisomy 21, 18, and 13 . Pooled sensitivity was 99.3% for Down, 97.4% for Edwards, and 97.4% for Patau syndrome, with the pooled specificity being 99.9% for all three trisomies. Pooled sensitivity was lower in the first trimester of pregnancy, in studies in the general obstetric population, in twin pregnancies, and in cohort studies with consecutive enrolment. Positive predictive values were lower for Edwards and Patau syndrome than for Down syndrome. Overall, this meta-analysis further confirms the general consensus that NIPT has very high sensitivity and specificity, but should not be used as a final diagnosis for positive cases.
However, this consensus does not always reach pregnant women. Heather Skirton and colleagues (Plymouth University, UK) searched the Internet and identified 40 (English language based) websites advertising non-invasive prenatal testing to potential users. Not all sites provided accurate and up-to-date information, and there was inadequate information on the need for an invasive test to definitely diagnose aneuploidy. The authors conclude that guidelines are needed to ensure that companies offering prenatal testing via the Internet provide accurate and comprehensible information .
The small chance of a false negative NIPT result was the subject of a study by Diane Van Opstal at Erasmus Medical Center (Rotterdam, the Netherlands) . Because the "fetal" DNA in maternal blood originates from the cytotrophoblast of chorionic villi (CV), some false negative results will have a biological origin. Starting with 5967 (CV) samples from pregnancies with a high risk for aneuploidies, 404 cases of trisomies 13, 18 and 21 were identified, of which 14 (0.23 %) had a normal or low mosaic karyotype that might potentially be missed with NIPT. The authors point out that part of these false negatives will be discovered later on during pregnancy by ultrasound or will end in an intrauterine death. Moreover, thorough follow-up investigations in cases of normal NIPT results should ultimately lead to the real figures for NIPT sensitivity.
Elise Calonico (Stanford University School of Medicine) examined pregnant women's preferences on prenatal testing for microdeletion and microduplication conditions. Of the 124 women who returned the written questionnaire, more than half made distinctions between conditions, and would choose a mixture of invasive testing, non-invasive testing, and no testing, depending on the condition. Testing preferences depended on penetrance and features of the conditions, as well as on prior prenatal testing, ethnicity, and education level. The authors propose that a test menu or filtering process may be a more optimal method of offering prenatal testing for microdeletion and microduplication conditions than an all or none process .
1. Taylor-Phillips S, et al. Accuracy of non-invasive prenatal testing using cell-free DNA for detection of Down, Edwards and Patau syndromes: a systematic review and meta-analysis. BMJ Open. 2016 Jan 18;6(1):e010002.
2. Skirton H, et al. Non-invasive prenatal testing for aneuploidy: a systematic review of Internet advertising to potential users by commercial companies and private health providers. Prenat Diagn. 2015 Dec;35(12):1167-75.
3. Van Opstal D et al. False Negative NIPT Results: Risk Figures for Chromosomes 13, 18 and 21 Based on Chorionic Villi Results in 5967 Cases and Literature Review.PLoS One. 2016 Jan 15;11(1):e0146794.
4. Calonico E, et al. Patient preferences for prenatal testing of microdeletion and microduplication syndromes. Prenat Diagn. In press.
On the Risk of Invasive Testing
A paper by Camilla Wulff and colleagues at the Copenhagen University Hospital (Denmark) assessed the risk of fetal loss associated with chorionic villus sampling (CVS) and amniocentesis (AC) following combined first-trimester screening (cFTS) for Down syndrome . This population-based study included 147 987 women with singleton pregnancy. The risks of miscarriage and stillbirth were not higher in women exposed to CVS or AC compared with unexposed women. The authors note that whereas their findings indicate that the procedure-related risk of CVS and AC is very low, the invasive procedures were carried out mainly by fetal medicine experts. The risk of fetal loss following invasive procedures is inversely correlated to the skill and experience of the operator. NIPT will likely reduce the number of invasive procedures and this decrease will have implications not only for training, but also for maintaining skills and expertise.
This issue was more extensively discussed in a paper by Lisa Hui  (Mercy Hospital for Women, Heidelberg, Australia). The papers notes that while the procedure-attributable miscarriage risk may
be lower than previously thought, it is uncertain if this level of safety will be maintained. As the number of AC and CVS decrease owing to NIPT, so will operator and center experience. Paradoxically, as a consequence, the ‘per procedure’ risk may even rise in the future. Additionally, a reduction in invasive diagnostic procedures comes at a cost to the training of new specialists. The authors express that there is a need to consider reducing the number of centers (and the number of individuals within such centers) performing AC and CVS. However, such a change could be unpopular among patients if this results in large traveling distances to a prenatal service. This situation will of course be different in, for example, Australia compared to Denmark. For the future, the authors propose collaborations in the form of training networks, and formal differentiation of specialists according to procedural skills if non-invasive prenatal testing continues to reduce indications for invasive testing.
5. Wulff CB, et al. Risk of fetal loss associated with invasive testing following combined first-trimester screening for Down syndrome: a national cohort of 147 987 singleton pregnancies. Ultrasound Obstet Gynecol. 2016 Jan;47(1):38-44.
6. Hui L, et al. How to safeguard competency and training in invasive prenatal diagnosis: 'the elephant in the room'. Ultrasound Obstet Gynecol. 2016 Jan;47(1):8-13.
Multiple Positive Outcomes
Adverse outcomes and fetal abnormalities in women with a positive prenatal screening result for more than one disorder, were subject of an article by Rebecca Baer and colleagues at the California Department of Public Health . The study population comprised 452,901 women in their first-trimester of a singleton pregnancy. The screening consisted of serum biochemistry and ultrasound examination. Risk assessment was provided for trisomy 21, trisomy 18, neural tube defects, and Smith-Lemli-Opitz syndrome. A total of 874 women had positive screening for more than one of the conditions and a completed pregnancy outcome survey. More than 50% of these women had either a fetus with a birth defect or a poor pregnancy outcome. The authors conclude that, although it is rare to screen positive for more than one condition, such results indicate a very high risk for chromosomal abnormality, fetal demise, or structural abnormality.
Holly Snyder and colleagues at Illumina looked at NIPT results of multiple aneuploidies or autosomal monosomies . Retrospective analysis of 113,415 tests resulted in a study cohort of 138 cases (65 single autosomal monosomy, 36 single trisomy with a sex chromosome aneuploidy, 37 multiple aneuploidies). NIPT results of autosomal monosomy or multiple aneuploidies are therefore rare, but turned out to have a diversity of underlying biologic causes. Some reflect the fetal karyotype, some reflect the presence of other maternal or fetal chromosome abnormalities, and a small number (n=6) are linked to maternal disease.
7. Baer, RJ et al. Outcomes of pregnancies with more than one positive prenatal screening result in the first or second trimester. Prenat Diagn. 2015 Dec;35(12):1223-31.
8. Snyder HL, et al. Follow-up of Multiple Aneuploidies and Single Monosomies Detected by Noninvasive Prenatal Testing: Implications for Management and Counseling. Prenat Diagn. In press.