Non‐invasive prenatal testing for everybody or contingent screening?

OBJECTIVE
To compare the advantages and disadvantages of two main implementation strategies for non-invasive prenatal testing (NIPT) for the detection of trisomies 21, 13 and 18 in public healthcare settings. These concern NIPT as a first-tier screening test for all pregnant women, or NIPT contingent on an increased risk first-trimester combined test (FCT) result.


METHOD
Comparison based on (un)published data and literature.


RESULTS
Despite the lower prevalence of trisomies in the low-risk population, data show that the positive predictive value (PPV) and detection-miscarriage ratio of first-tier NIPT are comparable to NIPT used as contingency test. Advantages of NIPT as a first-tier test compared to FCT with contingent NIPT are that there is no time-window for testing, sensitivity is higher, fewer women need follow-up testing (PPV of NIPT is higher than PPV of FCT), less anxiety for pregnant women and partners. When given the choice, pregnant women prefer first-tier NIPT. Although contingency testing still seems to be the cheaper option, the differences in costs are becoming increasingly smaller and might soon disappear.


CONCLUSION
We argue that NIPT should preferably be offered as a first-tier test for the detection of fetal aneuploidies for all pregnant women, provided that women are supported to make informed decisions. This article is protected by copyright. All rights reserved.

performed each year. 1 The fast dissemination of NIPT was driven by demand of pregnant women and healthcare professionals, technological developments and commercial incentives.
For long time, fetal aneuploidy screening was primarily done by first-trimester combined testing (FCT), which is a screening test based on a combination of serum measurements (i.e., pregnancyassociated plasma protein A and free β-human chorionic gonadotropin [hCG]), a nuchal translucency (NT) ultrasound measurement and maternal age. The result is provided as a risk estimate and different cutoffs (e.g., 1 in 200, 1 in 150 or 1 in 1000) can be chosen to indicate an increased risk result. 2 Previously, an increased FCT risk result was followed by an invasive confirmatory diagnostic test either chorionic villus sampling (CVS) or amniocentesis (AC). Both procedures are associated with a slightly increased risk of miscarriage. 3 NIPT uses cell-free DNA to screen maternal blood for fetal chromosomal aneuploidies. NIPT results can be either "high-risk", "low-risk" or "no-result". Due to the placental origin of cell-free DNA, genetic differences between the placenta and fetus can cause rare discordant NIPT results. Therefore, in order to obtain a definite diagnosis after a high-risk NIPT result, the result needs to be confirmed by invasive testing. 4 Two main implementation strategies are used within public healthcare settings for the introduction of NIPT within national prenatal screening programs. The most common approach is that of contingent (second-tier) NIPT, offered only to high-risk pregnant women based on a previous increased-risk FCT result. Besides this, NIPT can be offered to women with increased risk based on maternal age or medical history (e.g., a previous child with a trisomy). 5 Offering women with an increased FCT result NIPT as a contingent test greatly reduced the number of invasive tests needed as the majority of these women will receive a low-risk NIPT result, 6,7 and follow-up testing is not recommended.
Results described in this paper have been presented at the 2022 ISPD conference in Montreal, Canada. ISPD 2022 Montreal-Erik Sistermans.
NIPT can also be offered as a first-tier test to all pregnant women replacing conventional screening (FCT) for fetal aneuploidies.
A 2020 paper describing the implementation of NIPT showed that 14 out of 30 European countries included in the study had adopted a national policy for NIPT. 5 The majority of these European countries offer NIPT as a contingent test, 5 with the exception of Belgium 8 and the Netherlands, 9 where NIPT is offered as a first-tier test within their respective national prenatal screening programs. In several countries, there is a commercial offer of NIPT as a first-tier test. The American College of Obstetrics and Gynecologists recommends that NIPT is offered to all pregnant women regardless of their personal risk. 10 Here, we will compare the advantages and disadvantages of the two main implementation approaches for NIPT for the detection of trisomies 21, 18 and 13: NIPT as contingent screening test and firsttier NIPT for all pregnant women. This is done by looking at the following characteristics: test performance, timing, societal and ethical aspects, and financial aspects.

| TEST PERFORMANCE
In their recently published guideline, the American College for Medical Geneticists strongly recommends the use of first-tier NIPT over traditional screening methods due to its superior performance. 11 For FCT, the sensitivity and specificity depend on the chosen increased risk cutoff (e.g., 1 in 200) and the maternal age distribution in the population. 12 The sensitivity or detection rate for FCT is about 80%-90%. 10,13 Compared to FCT, NIPT has a higher sensitivity (>99% for Down syndrome), which means that fewer women will be confronted with a false-negative result after screening. In addition, NIPT has fewer false positives (0.1%) compared to FCT (5%). 14 This means that NIPT leads to fewer (unnecessary) diagnostic tests compared to FCT. Biological mechanisms, such as placental mosaicism, low fetal fraction, fetal mosaicism, maternal chromosomal aberrations or vanishing twin, may cause false-positive, false-negative or inconclusive NIPT results. 4 Therefore, a high-risk NIPT result should be confirmed with an invasive diagnostic test.
As the prevalence of a condition in the population influences the positive predictive value (PPV), 15 at the start of the implementation of first-tier NIPT in the Netherlands, it was expected that the PPV would be much lower compared to contingent NIPT. 16 However, as shown in Table 1, first-tier NIPT performs better or comparable to contingent NIPT, despite the lower population frequency of the common trisomies in the general-risk population. 9 These results are confirmed by data from Belgium. 17 Compared to FCT, both contingent and first-tier NIPT have much higher PPVs for the detection of trisomies 21, 18 and 13. 18 However, it is important to note that in the contingent program, the detection rate of trisomies 21, 18 and 13 and the number of false-negative cases are determined by the firsttier test that is used. In the case of contingent testing, this is FCT, the detection rate will therefore be lower and the number of false-negative cases will be higher compared to first-tier NIPT. 19 A way to increase the detection rate would be to lower the high-risk threshold (cutoff) for FCT (e.g., from 1 in 200 to 1 in 1000). However, this also increases the number of false positives. 20 It should be noted that the test performance of FCT can vary between programs, regions and countries, as the sonographic NT measurement is dependent on individual skills and quality control. Some argue that an important advantage of contingent testing is that FCT includes a first-trimester ultrasound for NT measurement, which can detect other conditions than trisomies 21, 18 and 13. 21 While NIPT is used for the detection of fetal aneuploidy (trisomies 21, 18 and 13), it is important for pregnant couples to be aware that these are not the only conditions a fetus can have and that can be detected in the first trimester. Severe structural defects can be detected using ultrasound scan. During pre-test counseling, it should be made clear that these are two different types of screening methods, screening for different conditions, to avoid false reassurance with NIPT regarding fetal conditions as a whole.
The detection-miscarriage ratio represents the number of fetuses that are detected with the disorder that is being screened for at the cost of one (CVS/AC) invasive procedure-related miscarriage. Table 2 shows the detection-miscarriage ratio for FCT (1:200), contingent NIPT after FCT (1:200) and first-tier NIPT. A higher ratio is more favorable. With first-tier NIPT, one case of procedure-related miscarriage will occur for every 480 fetuses with Down syndrome that is detected, which is almost similar to contingent NIPT.

| TIMING
Compared to contingent NIPT screening based on increased FCT results, first-tier NIPT has several timing benefits: it can be done early in pregnancy (starting from 9 weeks gestation) and throughout the entire pregnancy, 22 whereas FCT has a time window and can only be performed between 11 and 14 weeks gestation. Furthermore, more women will receive a final result after a single test with first-T A B L E 1 Test performance for FCT, contingent NIPT and first-tier NIPT for the common trisomies in the Netherlands (TRIDENT studies)

| SOCIETAL AND ETHICAL ASPECTS
The introduction of NIPT sparked heavy ethical and societal debates. 23 Concerns were raised that the many advantages of NIPT might lead to routinization, or uncritical use of prenatal screening, resulting in a large increase in prenatal screening uptake, and women making unconsidered choices. 24 The aim of a prenatal screening for fetal aneuploidy is to enhance reproductive autonomy for pregnant couples. Therefore, the success of a prenatal screening program is not measured in high uptake but instead in high levels of informed decision-making. 23 In the Netherlands, similar levels of informed choices were found in women offered FCT, 25 contingent NIPT 26 and first-tier NIPT. 27 To achieve high levels of informed decision-making, high-quality and non-directive pre-test counseling is required.
When given the choice between FCT and NIPT as a first-tier test, the large majority of women prefer NIPT. 17,28 Before the introduction of first-tier NIPT in the Netherlands in April 2017, an estimated 3%-5% of pregnant women traveled abroad to neighboring countries to obtain NIPT there, so-called "prenatal tourism". 9 After the introduction of first-tier NIPT, uptake increased to some extent from 34% in 2016 to 46% in 2018. However, the majority of women still declined to have NIPT. 28 This indicates that women experienced freedom to decline prenatal screening. A survey in 2017 among Dutch prenatal counselors (mostly midwives) showed that before its introduction, 75% held a positive attitude toward first-tier NIPT; this percentage increased to 86% after NIPT implementation and participating in a blended learning education program for counselors. 29 In the Netherlands, introducing first-tier NIPT caused no changes in the livebirth prevalence of Down syndrome. A register study by de concluded that the decreasing livebirth prevalence of Down syndrome was not affected by the introduction of first-tier NIPT. 30 The authors argued that the gradual decrease in the livebirth prevalence of Down syndrome most likely resulted from a broader development of more prenatal testing that had started before the introduction of NIPT.

| FINANCIAL ASPECTS
A systematic review of economic evaluations comparing the costeffectiveness of first-tier or contingent NIPT with usual screening found conflicting results for contingent NIPT. 31 Factors that influenced the results were test-uptake, FCT cutoff, maternal age and perspective of the analysis (i.e., what costs and effects are considered). For first-tier NIPT, studies were consistent and concluded that this is a more effective but also more expensive strategy. 31 However, due to declining sequencing costs and improved logistics by centralization, first-tier NIPT is likely to become cost-effective soon.
Already, the differences between contingent NIPT and first-tier NIPT The detection-miscarriage ratio represents the number of fetuses that are detected with the disorder that is being screened for, at the cost of one invasive procedure-related miscarriage. In the Netherlands (TRIDENT studies), a miscarriage risk of 2 in 1000 was used based on advice of the Health Council of the Netherlands. 16 b From: Health Council of the Netherlands. 16

| CONCLUSION
First-tier NIPT has several clear benefits over contingency testing: no time window for testing, higher detection rate, and fewer false positives; more women will receive a final result after a single test and less women need invasive follow-up testing. When given the choice between FCT and NIPT, most pregnant women prefer firsttier NIPT. Contingency NIPT testing is still cheaper compared to first-tier NIPT. However, the difference with first-tier NIPT costs is getting smaller. A drawback is that first-tier NIPT does not include a first-trimester ultrasound. A combination of first-tier NIPT and an additional first-trimester ultrasound scan aimed at detecting fetal structural anomalies is likely the ideal screening approach.