Contingency Screening Research Articles

O59 - Psychological benefits and harms of Non-Invasive Prenatal Screening (NIPS): Women’s perspectives

Non-invasive prenatal screening (NIPS) is being increasingly used by expectant parents. Much provision of this test in Australia is occurring in clinical settings where specialised genetic counselling is unavailable. Potential psychosocial consequences of NIPS remain largely unexplored. This study aimed to understand potential psychosocial consequences from prenatal genetic screening tests. We conducted a thematic analysis of semi-structured qualitative interviews with 16 women who received an increased-chance prenatal screening (NIPS, combined first trimester screening, or morphology scan) result. All participants had NIPS as a first line screening test (n=12) or contingent screening test after receiving an increased chance results from another screening test (n=4). Women held positive opinions of NIPS and discussed the need for it to be covered by Medicare. However, some received inadequate pre-test counselling which potentially contributed to negative psychosocial consequences, including increased self-reported anxiety and distress. Several women were unaware of the conditions that NIPS screened for, particularly SCA and microdeletions and were subsequently surprised when they received increased-chance results. False positive results caused some women to feel that bonding with their baby had been significantly disrupted in the period between receiving an increased chance screening result and receiving their contingent NIPS and/or diagnostic testing results. Women discussed several strategies that would have made their experience receiving an increased-chance screening result easier, including better information provision about the conditions tested for, and the possible consequences of an increased-chance screening result, particularly for conditions beyond the three common trisomies. Our results indicate that some expectant parents are not fully informed of the scope and implications of NIPS prior to testing. This may prevent them from making fully considered choices that reflect their personal values and aspirations for parenthood. Better pre-test counselling may facilitate informed decision-making and improve prospective parents’ experiences.

Non-invasive prenatal screening tests – update 2022

Abstract Since 2012, non-invasive prenatal testing (NIPT) using cell-free DNA from maternal plasma is applied all over the world as highly efficient first-line or contingent screening approach for trisomy 13, 18 and 21. With further technical development the screening has expanded to other genetic conditions such as sex chromosome anomalies (SCAs), rare autosomal trisomies (RATs), microdeletions/microduplications, structural chromosomal aberrations and monogenic diseases. Meanwhile, commercial providers are offering a number of different tests, with variable performance, the application of which needs to be carefully evaluated to apply to the true needs of clinical practice. In our review we present the different NIPT methodologies and discuss the main strengths and limitations in the context of providing a responsible pregnancy management.

Hierarchical Clustering-Based Framework for Interconnected Power System Contingency Analysis

This paper investigates a conceptual, theoretical framework for power system contingency analysis based on agglomerative hierarchical clustering. The security and integrity of modern power system networks have received considerable critical attention, and contingency analysis plays a vital role in assessing the adverse effects of losing a single element or more on the integrity of the power system network. However, the number of possible scenarios that should be investigated would be enormous, even for a small network. On the other hand, artificial intelligence (AI) techniques are well known for their remarkable ability to deal with massive data. Rapid developments in AI have led to a renewed interest in its applications in many power system studies over the last decades. Hence, this paper addresses the application of the hierarchical clustering algorithm supported by principal component analysis (PCA) for power system contingency screening and ranking. The study investigates the hierarchy clustering under different clustering numbers and similarity measures. The performance of the developed framework has been evaluated using the IEEE 24-bus test system. The simulation results show the effectiveness of the proposed framework for contingency analysis.

Reduction in Preterm Preeclampsia after Contingent First-Trimester Screening and Aspirin Prophylaxis in a Routine Care Setting.

Objectives: Several multivariate algorithms for preeclampsia (PE) screening in the first trimester have been developed over the past few years. These models include maternal factors, mean arterial pressure (MAP), uterine artery Doppler (UtA-PI), and biochemical markers (pregnancy-associated plasma protein-A (PAPP-A) or placental growth factor (PlGF)). Treatment with low-dose aspirin (LDA) has shown a reduction in the incidence of preterm PE in women with a high-risk assessment in the first trimester. An important barrier to the implementation of first-trimester screening is the cost of performing tests for biochemical markers in the whole population. Theoretical contingent strategies suggest that two-stage screening models could also achieve high detection rates for preterm PE with lower costs. However, no data derived from routine care settings are currently available. This study was conducted to validate and assess the performance of a first-trimester contingent screening process using PlGF for PE, with prophylactic LDA, for decreasing the incidence of preterm PE. Methods: This was a two-phase study. In phase one, a contingent screening model for PE was developed using a multivariate validated model and a historical cohort participating in a non-interventional PE screening study (n = 525). First-stage risk assessment included maternal factors, MAP, UtA-PI, and PAPP-A. Several cut-off levels were tested to determine the best screening performance, and three groups were then defined (high-, medium-, and low-risk groups). PlGF was determined in the medium-risk group to calculate the final risk. Phase two included a validation cohort of 847 singleton pregnancies prospectively undergoing first-trimester PE screening using this approach. Women at high risk of PE received prophylactic treatment with 150 mg of LDA. The clinical impact of the model was evaluated by comparing the incidence of early-onset (<34 weeks) and preterm (<37 weeks) PE between groups. Results: Cut-off levels for the contingent screening model were chosen in the first and second stages of screening to achieve a performance with sensitivities of 100% and 80% for early-onset and preterm PE detection, respectively, with a 15% false positive rate. In the development phase, 21.5% (n = 113) of the women had a medium risk of PE and required second-stage screening. In the prospective validation phase, 15.3% (n = 130) of the women required second-stage screening for PlGF, yielding an overall screen-positive rate of 14.9% (n = 126). The incidence of preterm PE was reduced by 68.4% (1.9% vs. 0.6%, p = 0.031) after one year of screening implementation. Conclusions: Implementation of contingent screening for PE using PlGF in a routine care setting led to a significant reduction (68.4%) in preterm PE, suggesting that contingent screening can achieve similar results to protocols using PlGF in the whole population. This could have financial benefits, with a similar reduction in the rate of preterm PE.

An update of diagnostic efficacy of ultrasound and magnetic resonance imaging in the diagnosis of clinically significant placenta accreta spectrum disorders.

Purpose of reviewSystematic screening and diagnosis of placenta accreta spectrum disorder (PAS) either by ultrasound or magnetic resonance imaging (MRI) would allow referral of high-risk women to specialized multidisciplinary teams. We aimed to report recent findings regarding the diagnostic accuracy of ultrasound and magnetic resonance imaging in the diagnosis of PAS.Recent findingsRecent evidence from the literature shows that both ultrasound and MRI are good tests to identify PAS in high-risk populations. Ultrasound can also be used safely to guide management decisions, concentrating greater resources in patients with the higher risk of clinically significant PAS requiring complex peripartum management. Moreover, there are increasing data showing that routine contingent screening for PAS disorders based on the finding of a placenta implanted low in the uterine cavity and previous uterine surgery is effective in a public healthcare setting. A contingent screening strategy for PAS is feasible if placental location is routinely assessed during routine scans, and may even start from the first trimester of pregnancy.SummaryUltrasound is an effective tool to screen pregnancies at high risk of PAS. In such pregnancies, ultrasound and MRI are effective imaging modalities for guiding management.

Line outage distribution factors of a linearized AC model with reactive power and voltage magnitude for resilience-constrained economic dispatch

Research on preventive generation dispatch schemes for resilient power system operation is often based on the DC power flow model, ignoring the influence of reactive power and voltage magnitude. This paper presents an in-depth study of a linearized AC power flow (LAC) model with reactive power and voltage magnitude to derive sensitivity factors, including shift factors and line outage distribution factors, for pre-contingency and post-contingency power flow calculations for N-k contingencies. Based on the derived sensitivity factors, a resilience-constrained economic dispatch (LAC-RCED) strategy is developed, which considers the security constraints of N-1 contingency for all lines and N-2 contingency for the affected lines, as well as optimization objectives to improve the power flow distribution in the transmission system. To deal with the computational difficulties associated with the N-k contingency constraints, an iterative contingency filtering algorithm based on the derived line outage distribution factor is proposed for contingency screening and creating security constraints for violated contingency scenarios. In the case study, the accuracy of the power flow solution obtained from the derived sensitivity factors is investigated by comparison with the AC model. The proposed LAC-RCED model and the iterative contingency filtering algorithm are tested on the IEEE 30-bus and 118-bus systems.

Security-constrained AC–DC hybrid distribution system expansion planning with high penetration of renewable energy

With the increasing penetration of renewables into the AC–DC hybrid distribution system, it is necessary to develop a security-constrained planning approach to address the uncertainty brought by renewables. Motivated by the urgent need to accommodate high penetration of renewable energy resources in weak grids, this paper presents a novel planning approach for AC–DC hybrid distribution systems considering the N-1 security criterion. Network reconfiguration is incorporated as a method to improve the flexibility of the system under normal and post-contingency conditions. To alleviate the burden of computing all possible N-1 contingencies, a novel solving strategy with a nested loop structure is proposed. In the inner loop, a novel contingency screening method is designed for the fast identification of high-risk contingencies under a given planning solution. In the outer loop, the obtained high-risk contingency set is adopted to refine and filter the security constraints, so that the planning solution can be improved iteratively. The superiority and effectiveness of the proposed planning model and solving strategy are tested on a real-world distribution network in Zhejiang Province, China. Numerical results verify that the proposed planning approach is able to deal with the installation of large-scale renewable energy while ensuring the security of the system.

A new method for operating point free power system transient stability situational awareness based on single machine equivalent approach

Transient stability assessment is an essential element for situational awareness and decision making process of a power system operator in maintaining the secure operation. Continuous efforts have been made for devising methods to assess the transient stability. However, the existing methods have dependency on pre-fault operating condition. This article aims to report a new method, towards deriving contingency wise operating point free transient stability assessment. The method is based on single machine equivalent (SIME) approach and has two phases (a) off line tuning (b) online application. In offline tuning phase each plausible contingency is characterized by an operating point free parameter called mean critical return angle over the specified operating region. In online application phase, transient stability for a given plausible contingency is predicted using SIME approach. Two new severity indices namely stable severity index and unstable severity index are proposed and evaluated for assessing the degree of stability and instability. Further, these indices are used in contingency screening. The proposed method is tested on 3-machine 9-bus system and, 10-machine 39-bus system. Results shows that the proposed method is simple and can aid the existing methods in providing reliable transient stability situational awareness to the system operator.

Second-Trimester Circulating Maternal Placenta Growth Factor as a Contingent Screening Tool for Placental Mediated Complications in Women with Abnormal First-Trimester Serum Analytes

First-trimester analytes are thought to reflect in part on placental function, and therefore, are associated with an increased risk of placental-mediated complications. At present, no specific management protocol is available. We aimed to assess the role of placental growth factor (PlGF) in patients with unexplained abnormal first-trimester markers.

Implementation of non-invasive prenatal testing within a national UK antenatal screening programme: Impact on women's choices.

To evaluate the implementation of non-invasive prenatal testing (NIPT) on pregnant women's choices in a national NHS antenatal screening programme for Down's syndrome, Edwards' syndrome and Patau's syndrome. An observational study of all pregnant women with a singleton pregnancy and higher chance (≤1:150) combined or quadruple screening result from 30 April 2018 to 25 September 2020 in Wales, UK. Pregnant women's journey through the pathway was determined including uptake of NIPT, performance of NIPT in a non-research setting and invasive procedures performed. Of the 1273 women with a higher chance initial screening, 1073 (84%) chose NIPT contingent test, 174 (14%) no further testing and 26 (2%) invasive procedure. There were 1001 (93%) low chance NIPT results; 11 (1%) failed results and 61 (6%) high chance results. Average annual incidence of 27 invasive procedures undertaken compared to 229 pre-NIPT implementation, a nearly ninefold reduction. Down's syndrome annual live birth rate remained unchanged across the implementation period. This study demonstrates that NIPT contingent screening was highly acceptable to women with a resulting reduction in invasive procedures performed. The high uptake of NIPT in NHS antenatal screening pathway conditions should inform planning for other national screening programmes.

Contingent cfDNA Screening Implementation: Increasing Diagnostic Accuracy and Reducing Invasive Testing – 6 Years’ Results in a Single Center

Objectives: Universal screening for trisomy using cell-free DNA (cfDNA) has proven to be more effective than combined test, but it is not cost efficient currently. Contingent cfDNA screening on the results of the first-trimester combined test can improve the detection rate of the combined test and reduce the number of invasive tests at a lower cost than universal screening. In 2018, a contingent screening program was implemented in the community of Castilla y Leon (Spain). This study aims to compare the results achieved in Salamanca University Hospital during the first 3 years of contingent screening (2018–2020) with those of the previous 3 years (2015–2017) to assess the changes in the trisomy detection rate and the number of invasive tests. Methods: A total of 9,903 singleton pregnancies without malformations nor nuchal translucency >p99 were included. 5,165 patients underwent combined screening and 4,738 had contingent screening based on the combined test risk. In the combined test group, women were offered an invasive test if the risk was ≥1:270, while risks under 1:270 were considered low risks, and no further testing was offered. In the contingent screening group, invasive testing was offered if the risk was ≥1:100 (≥1:50 from 2020 onwards), while cfDNA was offered if the combined test risk was between 1:100 and 1:1,000 (1:50–1:1,000 from 2020 onwards). When risk was <1:1,000, no further testing was offered. Aneuploidies detected by cfDNA were confirmed by invasive diagnostic testing. Results: There were 33 cases of trisomy 21 (T21) throughout the 6 years of study. Four cases had low/intermediate risks and were spotted by cfDNA. Risk >1:1,000 threshold for contingent test detected 100% T21. There was a false-positive result for trisomy 13. There were no false-negative results. “No-call” cfDNA results were minimized by repeating blood collection 2 weeks later, as fetal fraction (FF) was doubled. Invasive testing had a drop rate of 84% after contingent screening implementation. Discussion: The implementation of population-based contingent screening significantly reduces the number of invasive tests without lowering diagnostic accuracy. To achieve the maximum efficiency of the program, it is important to know the best cut-offs according to the population where the program is to be implemented. The number of uninformative results due to low FF can be reduced by repeating the test 2 weeks after the initial extraction: this increases the FF to twice the initial one, achieving informative results and avoiding unnecessary invasive tests.

Reachability-Based False Data Injection Attacks and Defence Mechanisms for Cyberpower System

With the push for higher efficiency and reliability, an increasing number of intelligent electronic devices (IEDs) and associated information and communication technology (ICT) are integrated into the Internet of Things (IoT)-enabled smart grid. These advanced technologies and IEDs also bring potential vulnerabilities to the intelligent cyber–physical smart grid. State estimation, as a primary step of system monitoring and situational awareness, is a potential target for attackers. A number of other smart grid applications, such as voltage stability assessment and contingency screening, utilize state estimation results as input data. False data injection (FDI) is a specific way to attack state estimation by manipulating input data. Existing research mainly focuses on the mathematical analysis of FDI attacks; however, in these methods, discussions of reachability requirements to compromise measurements considering cyberinfrastructure are limited. Reachability is defined as a measure that estimates the number of hosts to compromise for the possible FDI. Most of the existing FDI attack methods require the simultaneous manipulation on multiple measurement devices in different substations, in order to bypass the bad data detection, which may be impractical. In this paper, a new type of reachability-based FDI attack considering the cybernetwork with a practical attack is proposed and validated on two IEEE test systems. The corresponding defence mechanisms are (a) decentralized state estimation (DSE), (b) DSE with additional backup computational nodes, (c) communication network rerouting, and (d) intrusion detection system, and they were developed and presented with validation for two IEEE test systems with superior performance for an IoT-enabled intelligent smart grid system.

Universal Cell Free DNA or Contingent Screening for Trisomy 21: Does It Make a Difference? A Comparative Study with Real Data

Objective: The aim of the objective was to compare the detection rate for trisomy 21 of universal cell free DNA (cfDNA) screening with contingent screening. Methods: Retrospective study was carried out at 3 German centers. The study included euploid and trisomy 21 pregnancies where cfDNA and first trimester (FT) screening assessment was carried out. The FT risk for trisomy 21 was computed based on combined screening and stratified into the following classes: high risk ≥1:10, intermediate risk 1:11–1,000, low risk ≤1,001. For universal cfDNA screening, the cfDNA test results were examined. For the contingent screening model, the result of the cfDNA test was taken into account in case of an intermediate FT risk. Different strategies combining maternal age, nuchal translucency, nasal bone, beta-hCG, and PAPP-A were evaluated. Screen positivity was defined as either a high risk after FT screening or a cfDNA test indicating a high-risk result. An inconclusive cfDNA test was also considered as screen positive. Results: The search of the database identified 2,255 euploid and 163 affected pregnancies. All affected fetuses were identified by universal cfDNA screening. 1.3% of the euploid fetuses were classified as screen positive due to final inconclusive cfDNA test result. The detection and false-positive rate of a contingent approach that is based on combined screening and cfDNA screening in the intermediate group would be 98.4% and 0.7%, respectively. With this approach, cfDNA screening would be necessary in only about 27% of all pregnancies. Conclusion: This study demonstrates that a contingent approach provides similar detection rates for trisomy 21 as universal cfDNA screening, by a reduction of 73% the number of cfDNA tests.

Second-trimester contingent screening for small-for-gestational-age neonate.

First, to investigate the additive value of second-trimester placental growth factor (PlGF) for the prediction of a small-for-gestational-age (SGA) neonate. Second, to examine second-trimester contingent screening strategies. This was a prospective observational study in women with singleton pregnancy undergoing routine ultrasound examination at 19-24 weeks' gestation. We used the competing-risks model for prediction of SGA. The parameters for the prior model and the likelihoods for estimated fetal weight (EFW) and uterine artery pulsatility index (UtA-PI) were those presented in previous studies. A folded-plane regression model was fitted in the dataset of this study to describe the likelihood of PlGF. We compared the prediction of screening by maternal risk factors against the prediction provided by a combination of maternal risk factors, EFW, UtA-PI and PlGF. We also examined the additive value of PlGF in a policy that uses maternal risk factors, EFW and UtA-PI. The study population included 40 241 singleton pregnancies. Overall, the prediction of SGA improved with increasing degree of prematurity, with increasing severity of smallness and in the presence of coexisting pre-eclampsia. The combination of maternal risk factors, EFW, UtA-PI and PlGF improved significantly the prediction provided by maternal risk factors alone for all the examined cut-offs of birth weight and gestational age at delivery. Screening by a combination of maternal risk factors and serum PlGF improved the prediction of SGA when compared to screening by maternal risk factors alone. However, the incremental improvement in prediction was decreased when PlGF was added to screening by a combination of maternal risk factors, EFW and UtA-PI. If first-line screening for a SGA neonate with birth weight < 10th percentile delivered at < 37 weeks' gestation was by maternal risk factors and EFW, the same detection rate of 90%, at an overall false-positive rate (FPR) of 50%, as that achieved by screening with maternal risk factors, EFW, UtA-PI and PlGF in the whole population can be achieved by reserving measurements of UtA-PI and PlGF for only 80% of the population. Similarly, in screening for a SGA neonate with birth weight < 10th percentile delivered at < 30 weeks, the same detection rate of 90%, at an overall FPR of 14%, as that achieved by screening with maternal risk factors, EFW, UtA-PI and PlGF in the whole population can be achieved by reserving measurements of UtA-PI and PlGF for only 70% of the population. The additive value of PlGF in reducing the FPR to about 10% with a simultaneous detection rate of 90% for a SGA neonate with birth weight < 3rd percentile born < 30 weeks, is gained by measuring PlGF in only 50% of the population when first-line screening is by maternal factors, EFW and UtA-PI. The combination of maternal risk factors, EFW, UtA-PI and PlGF provides effective second-trimester prediction of SGA. Serum PlGF is useful for predicting a SGA neonate with birth weight < 3rd percentile born < 30 weeks after an inclusive assessment by maternal risk factors and biophysical markers. Similar detection rates and FPRs can be achieved by application of contingent screening strategies. © 2021 International Society of Ultrasound in Obstetrics and Gynecology.

Adaptive Lyapunov Function Method for Power System Transient Stability Analysis

Transient stability analysis is a crucial tool for evaluating the stability and ensuring the safe operation of power systems. Among existing methodologies for transient stability analysis, direct methods show merits in performing fast contingency screening and providing quantitative information for the degree of stability. However, the inherent conservatism of direct methods and their restriction on power system models still pose significant challenges for practical applications. This paper is devoted to further developing direct methods by proposing a novel <i>adaptive Lyapunov function method</i>, which enables simulation-free estimation of critical clearing time with drastically reduced conservatism. The novelties of the proposed method lie in three aspects. First, we propose an <i>adaptive sector condition</i> which bounds the nonlinearity of the power system model in an adjustable neighborhood of a given equilibrium. Second, we introduce an improved bounding technique for the time derivative of the Lyapunov function. Third, by exploiting the freedom of the adaptive sector condition and the <i>adjustable neighborhood</i>, the construction of Lyapunov functions along with the choice of the parameters in the sector conditions can be co-optimized to achieve the tightest possible estimation of the critical clearing time. The effectiveness of the proposed method is validated on four benchmark systems.

Cost of providing cell-free DNA screening for Down syndrome in Finland using different strategies.

A financial analysis is carried out to assess costs and benefits of providing cell-free DNA screening in Finland, using different strategies. Three cell-free DNA screening strategies are considered: Primary, all women; Secondary, those with positive Combined test; and Contingent, the 10-30% with the highest Combined test risks. Three costs are estimated: additional cost for 10,000 pregnancies compared with the Combined test; 'marginal' cost of avoiding a Down syndrome birth which occurs in a pregnancy that would have been false-negative using the Combined test; and marginal cost of preventing the iatrogenic loss of a non-Down syndrome birth which occurs in a pregnancy that would have been false-positive. Primary cell-free DNA will require additional funds of €250,000. The marginal cost per Down syndrome birth avoided is considerably less than the lifetime medical and indirect cost; the marginal cost per unaffected iatrogenic fetal loss prevented is higher than one benefit measure but lower than another. If the ultrasound component of the Combined test is retained, as would be in Finland, the additional funds required rise to €992,000. Secondary cell-free DNA is cost-saving as is a Contingent strategy with 10% selected but whilst when 20-30% costs rise they aremuch less than for the Primary strategy and are cost-beneficial. When considering the place of cell-free DNA screening it is important to make explicit the additional and marginal costs of different screening strategies and the associated benefits. Under most assumptions the balance is favorable for Contingent screening.

Multi-task Transient Contingency Screening with Temporal Graph Convolutional Network in Power Systems

Rapid transient stability assessment (TSA) is an essential requirement for power system security. In real-world applications, transient contingency screening (TCS) applies TSA approaches to address the pre-defined contingency sets under online operation conditions. TSA by time domain simulation (TDS) is time-consuming, hence we propose a high-speed temporal graph convolutional network (TGCN) that achieves TSA decisions such that a large-scale contingency set can be scanned quickly with enough precision. Based on multi-graph inputs to reflect the transient process, the TGCN utilizes the graph convolutional network (GCN) to extract topology representations and temporal convolution (TC) layers to encode temporal relations. After above graph embedding, two downstream networks are designed for stability classification and critical generator prediction, respectively. Test results on IEEE 39 Bus system demonstrate its superiority over existing models under different operation topologies, fault locations and clearing modes.

Application of triangle count in branch contingency screening

In this paper, a triangle count-based branch contingency screening scheme is proposed for transmission power systems. Different triangle count algorithms are compared. A parallel triangle count algorithm is implemented in the graph database for speed-up. Then, a modified performance index is proposed. A structure-related constant called Line Outage Severity Factor (LOSF) is defined to avoid redundant computation when updating the screening results under different operating points. Numerical acceleration techniques are also leveraged to update the screening results when system topology changes. Experiments of N − 1 and N − 2 analysis on three testbed systems demonstrate that about 20% of contingency cases can be ruled out with relatively low risk in missing critical contingencies, which reduces the computational burdens for succeeding stages of the contingency analysis.

Accurate, simultaneous and Real-Time screening of N-1, N-k, and N-1-1 contingencies

Generally, in contingency analysis, N-1 events are studied. However, simultaneous failure of more than one device is more dangerous and might result in cascading failures. Due to cyber-attacks and malicious motivations, attacking several components of the system is more probable. Various methods have been proposed for contingency screening, which are mainly time-consuming and cannot be applied to real systems. On the other hand, there is no integrated algorithm for screening N-1, N-k, and N-1-1 contingencies. In this paper, an indirect method has been presented for this purpose. The proposed method extracts different critical contingencies through screening contingencies in offline mode for some selected operating modes. Then, they are clustered using the Gaussian-mixture model using the expectation–maximization algorithm. Therefore, the boundary between data is specified, which helps to find the closest (the most similar) operating modes. In online mode, the critical contingencies of the closest trained data are introduced as the critical contingency set of the present operating point. To the best of our knowledge, this study employs the correlation between the loads and generators in contingency screening for the first time. The proposed method outperforms the existing methods in terms of simplicity, accuracy, and speed such that it detects contingencies with the maximum accuracy of 100% without imposing any additional computational load.

Contingent screening in stratification of pregnancy care based on risk of pre-eclampsia at 19-24 weeks' gestation.

To explore the possibility of carrying out routine screening for pre-eclampsia (PE) with delivery at < 28, < 32, < 36 weeks' gestation by maternal factors, uterine artery pulsatility index (UtA-PI) and mean arterial pressure (MAP) in all pregnancies and reserving measurements of placental growth factor (PlGF) and soluble fms-like tyrosine kinase-1 (sFlt-1) for only a subgroup of the population. This was a prospective observational study in two UK maternity hospitals involving women with singleton pregnancy attending for routine assessment at 19-24 weeks' gestation. The improvement in performance of screening for PE, at fixed risk cut-offs, by the addition of serum PlGF and sFlt-1 to screening by maternal factors, UtA-PI and MAP, was estimated. We examined a policy of contingent screening in which biochemical testing was reserved for only a subgroup of the population. The main outcome measures were the additional contribution of PlGF and sFlt-1 to the performance of screening for PE and the proportion of the population requiring measurement of PlGF and sFlt-1 for maximum performance of screening. The study population included 37 886 singleton pregnancies. At each risk cut-off, the highest detection rates for delivery with PE and the lowest screen-positive rates were achieved in screening with maternal factors, UtA-PI, MAP, PlGF and sFlt-1. The maximum performance by such screening was also achieved by contingent screening in which PlGF and sFlt-1 were measured in only about 40% of the population. The performance of screening for PE by a combination of maternal factors, UtA-PI and MAP is improved by measurement of PlGF and sFlt-1 in about 40% of the population. © 2021 International Society of Ultrasound in Obstetrics and Gynecology.