Humerus and femur length in fetuses with Down syndrome

Methods for the joint meta-analysis of multiple tests

Existing methods for meta-analysis of diagnostic test accuracy focus primarily on a single index test. We propose models for the joint meta-analysis of studies comparing multiple index tests on the same participants in paired designs. These models respect the grouping of data by studies, account for the within-study correlation between the tests' true-positive rates (TPRs) and between their false-positive rates (FPRs) (induced because tests are applied to the same participants), and allow for between-study correlations between TPRs and FPRs (such as those induced by threshold effects). We estimate models in the Bayesian setting. We demonstrate using a meta-analysis of screening for Down syndrome with two tests: shortened humerus (arm bone), and shortened femur (thigh bone). Separate and joint meta-analyses yielded similar TPR and FPR estimates. For example, the summary TPR for a shortened humerus was 35.3% (95% credible interval (CrI): 26.9, 41.8%) versus 37.9% (27.7, 50.3%) with joint versus separate meta-analysis. Joint meta-analysis is more efficient when calculating comparative accuracy: the difference in the summary TPRs was 0.0% (-8.9, 9.5%; TPR higher for shortened humerus) with joint versus 2.6% (-14.7, 19.8%) with separate meta-analyses. Simulation and empirical analyses are needed to refine the role of the proposed methodology.

Improved prenatal aneuploidy screening using the novel advanced first-trimester screening algorithm: a multicenter study of 10,017 pregnancies

PURPOSE

: It has been postulated that the maternal age component should be completely excluded from first-trimester screening (FTS) for fetal aneuploidies. In this study, we tested a new algorithm known as advanced first-trimester screening (AFS), which disregards maternal age.

METHOD

: In a multicenter study, FTS findings were retrieved from 10,017 pregnancies. FTS risk assessment was performed using the Nicolaides method, and the AFS score was calculated. The results of both methods were compared.

RESULTS

: Within this population, 81 fetuses had an abnormal karyotype. The sensitivity of the 2 algorithms was 86.4%. When the AFS method was used, the positive predictive value rose from 9.6% (FTS) to 12.4% (AFS). Using AFS, the test positive rate could be decreased by 161 cases (-22.2%) (p < 0.0001), due to a reduction of false positive cases. As a result, the false positive rate of AFS was 24.5% lower than that of FTS, while the same number of aneuploidies was detected.

CONCLUSION

: AFS can markedly reduce the rate of false positive test results. If these results are confirmed by larger multicenter studies, the new AFS will represent a great improvement in fetal aneuploidy screening. (c) 2008 Wiley Periodicals, Inc. J Clin Ultrasound, 2008.

Uptake of invasive prenatal diagnostic tests in women after detection of soft markers for chromosomal abnormality on ultrasonographic evaluation

OBJECTIVE

Use of soft ultrasonographic markers during routine prenatal ultrasonography (USG) may be used for the screening of aneuploidy in the low-risk population. The aim of this study was to evaluate the acceptance of an invasive test for prenatal diagnosis and to assess the role of various factors in the decision-making regarding an invasive test when confronted with risk for aneuploidy after a soft marker is detected on routine antenatal ultrasonogram.

STUDY DESIGN

Women were referred for USG in our department by primary obstetricians for indications such as a previous child with a congenital malformation, genetic disorder, stillbirth or in women with recurrent spontaneous abortions. Some of the women were referred after prenatal detection of a soft marker on USG. They were screened for soft markers associated with chromosomal abnormality. They were counseled regarding the age-specific risk and the risk of aneuploidy after detection of a marker in comparison to the general population's risk of Down's syndrome. They were also counseled regarding the risk of a procedure-related abortion (0.5%) following an invasive procedure before their decision regarding the use of amniocentesis was made.

RESULT

Twenty women out of 50 (40%) opted for amniocentesis. Except in one case of trisomy 21 in a fetus with short femur and humerus, all others had normal karyotype. The uptake of the test was comparable between primigravida (33%), women with poor obstetric history (46%) and women with at least one normal live child (45%). There was no statistical difference in the uptake of invasive test based on gestational age as well. Uptake of amniocentesis was higher (78%) in cases with nuchal thickening as compared to other markers (35%).

CONCLUSION

Ultrasonographic detection of soft markers is associated with a high frequency of uptake for invasive prenatal testing. Increased nuchal thickening is associated with a higher acceptance of amniocentesis. Maternal age, gestational age or previous obstetric history were not associated with the decision to undergo amniocentesis.

The role of the second trimester genetic sonogram in screening for fetal Down syndrome

The Genetic Sonogram is an ultrasound examination done on second trimester fetuses that not only evaluates the fetus for structural malformations, but also searches for the sonographic markers of fetal Down syndrome. The main markers that comprise the genetic sonogram include the nuchal fold, short femur and humerus, pyelectasis, hyperechoic bowel, echogenic intracardiac focus, and any major abnormality. The absence of any marker on a second trimester scan conveys a 60-80% reduction in prior risk of Down syndrome based on advanced maternal age or serum screen risk. The presence of sonographic markers, either singly or in combination, will raise the baseline risk of Down syndrome using likelihood ratios calculated for each individual marker. Using this approach, approximately 75% of fetuses with Down syndrome can be identified by modifying the patient's baseline risk according to the results of the ultrasound. The second trimester scan will likely continue to play an important role in the future in the detection of aneuploidy.

[Role of "subtle" ultrasonographic signs during antenatal screening for trisomy 21 during the second trimester of pregnancy: meta-analysis and CPDPN protocol of the Grenoble University Hospital]

OBJECTIVE

A meta-analysis about subtle ultrasonographic signs in second trimester of pregnancy.

MATERIALS AND METHODS

196 articles dealing with the subject--from 1985 to July 2002--were studied. Data on the 11 reported signs were collected from 92 theoretically and/or statistically valid studies. Then, the studies were selected according to several criteria: isolated characteristic, defined thresholds, calculable sensitivity and specificity. After checking for homogeneity, a likelihood ratio was calculated for some of the signs.

RESULTS

This meta-analysis of the second trimester ultrasonographic signs of Down's syndrome enabled us to estimate the likelihood ratio (LHR) of six signs. At 22 weeks'gestation (WG) these signs are: pyelectasis equal to or greater than 5 mm; nuchal fold thickness equal to or greater than 6 mm; persistence of choroid plexus cysts; shortness of the femur and humerus below the tenth percentile; hyperechogenic bowe; and nasal bone length less than 2.5 mm.

CONCLUSION

These validated ultrasonographic signs are independent of nuchal translucency thickness at 12 WG and of maternal serum biochemistry. This allows to calculate a combinate risk for nuchal translucency, maternal serum biochemistry and second trimester ultrasonographic signs when they are validated.

Second trimester ultrasound screening for chromosomal abnormalities

The use of prenatal ultrasound has proven efficacious for the prenatal diagnosis of chromosomal abnormalities. The first sonographic sign of Down syndrome, the thickened nuchal fold, was first described in 1985. Since that time, multiple sonographically-identified markers have been described as associated with Down syndrome. The genetic sonogram, involving a detailed search for sonographic signs of aneuploidy, can be used to both identify fetuses at high risk for aneuploidy and, when normal, can be used to decrease the risk for aneuploidy for a pregnancy when no sonographic markers are identified. Combining the genetic sonogram with maternal serum screening may be the best method of assessing aneuploidy risk for women who desire such an assessment in the second trimester. Trisomy 18, Trisomy 13, and triploidy are typically associated with sonographically identified abnormalities and have a high prenatal detection rate. The use of the described sonographic signs in low-risk women requires further investigation, however, patients at increased risk for aneuploidy due to advanced maternal age or abnormal serum screening can benefit from a genetic sonogram screening for sonographic signs of aneuploidy to adjust their baseline risk of an affected fetus.

Second trimester markers of aneuploidy

The risks of aneuploidy associated with identification of a sonographic marker in the low risk population is controversial. Prior risk estimates have been derived usually from high risk populations. Screening programmes in the first trimester, second trimester and combined first and second trimester will undoubtedly alter the second trimester scan as a screening tool for aneuploidy. This chapter reviews the current sonographic markers and the difficulties in their application to the general population.

Femur length and trisomy 21: impact of gestational age on screening efficiency

OBJECTIVE

This study assesses two methods used to define relatively short femur in screening for trisomy 21 and examines changes in performance of screening with gestational age.

DESIGN

Retrospective analysis of data on menstrual age, femur length (FL) and biparietal diameter (BPD) in 49 trisomy 21 pregnancies and 6069 normal controls. Reference ranges were derived for BPD/FL versus menstrual age and for FL versus BPD. Two methods of defining short femur (BPD/FL and observed-to-expected FL ratio) were examined for false-positive rates and detection rates for trisomy 21 at different gestational ages.

RESULTS

In the control group the BPD/FL ratio and its standard deviation decreased with menstrual age. Trisomy 21 was associated with a significantly higher BPD/FL ratio (P < 0.001) and the deviation increased significantly with menstrual age (P < 0.05). Eleven percent of 28 fetuses examined at 15-17 weeks had a BPD/FL above the 95th centile compared with 24% of 21 fetuses examined at 18-20 weeks (P = 0.40). The median observed-to-expected FL ratio in the control group was 1.0 throughout the gestational age range but the standard deviation decreased significantly with menstrual age (P < 0.01). Trisomy 21 was associated with a significantly reduced observed-to-expected FL ratio (P < 0.001) and the deviation increased significantly with menstrual age (P < 0.05). A fixed cut-off of 0.91 for observed-to-expected FL ratio provided a false-positive rate of 12% at 15-17 weeks compared with 6% at 18-20 weeks of gestation (P < 0.001) with detection rates of 29 and 38%, respectively (P = 0.73).

CONCLUSION

Irrespective of the definition used to define the condition, relatively short femur is a poor marker for trisomy 21 particularly when the assessment takes place before 18 weeks of gestation.

Ultrasound screening for fetal chromosome anomalies

Ultrasound evidence for aneuploidy may be found in almost every organ of the fetus and can be used to modify the risk of aneuploidy. The diagnosis of these minor anomalies on second-trimester ultrasonography will increase the risk of an abnormal karyotype whereas the absence of these findings may reduce this danger. The most specific and most ominous isolated markers for fetal aneuploidy are nuchal findings (edema or cysts), indicating the need to obtain a fetal karyotype in all cases irrespective of maternal age or results of biochemical serum screening. Hyperechoic fetal bowel is apparently also a strong indicator of fetal aneuploidy. Other isolated sonographic markers may increase the risk of an abnormal karyotype three- to ninefold. Most sonographic markers for aneuploidy specify an increased risk for Down syndrome, but choroid plexus cysts are apparently more specific for trisomy 18. Along with other screening methods, ultrasound screening for fetal aneuploidy should be used routinely to identify additional pregnancies at need for evaluation of fetal karyotype.

Antenatal screening for Down's syndrome

BACKGROUND

In 1968 the first antenatal diagnosis of Down's syndrome was made and screening on the basis of selecting women of advanced maternal age for amniocentesis was gradually introduced into medical practice. In 1983 it was shown that low maternal serum alpha fetoprotein (AFP) was associated with Down's syndrome. Later, raised maternal serum human chorionic gonadotrophin (hCG), and low unconjugated oestriol (uE3) were found to be markers of Down's syndrome. In 1988 the three biochemical markers were used together with maternal age as a method of screening, and this has been widely adopted. PRINCIPLES OF ANTENATAL SCREENING FOR DOWN'S SYNDROME: Methods of screening need to be fully evaluated before being introduced into routine clinical practice. This included choosing markers for which there is sufficient scientific evidence of efficacy, quantifying performance in terms of detection and false positive rates, and establishing methods of monitoring performance. Screening needs to be provided as an integrated service, coordinating and managing the separate aspects of the screening process. SERUM MARKERS AT 15-22 WEEKS OF PREGNANCY: A large number of serum markers have been found to be associated with Down's syndrome between 15 and 22 weeks of pregnancy. The principal markers are AFP, hCG or its individual subunits (free alpha- and free beta-hCG), uE3, and inhibin A. Screening performance varies according to the choice of markers used and whether ultrasound is used to estimate gestational age (table 1). When an ultrasound scan is used to estimate gestational age the detection rate for a 5% false positive rate is estimated to be 59% using the double test (AFP and hCG), 69% using the triple test (AFP, hCG, uE3), and 76% using the quadruple test (AFP, hCG, uE3, inhibin A), all in combination with maternal age. Other factors that can usefully be taken into account in screening are maternal weight, the presence of insulin dependent diabetes mellitus, multiple pregnancy, ethnic origin, previous Down's syndrome pregnancy, and whether the test is the first one in a pregnancy or a repeat. Factors such as parity and smoking are associated with one or more of the serum markers, but the effect is too small to justify adjusting for these factors in interpreting a screening test.

URINARY MARKERS AND FETAL CELLS IN MATERNAL BLOOD

Urinary beta-core hCG has been investigated in a number of studies and shown to be raised in pregnancies with Down's syndrome. This area is currently the subject of active research and the use of urine in future screening programmes may be a practical possibility. Other urinary markers, such as total oestriol and free beta-hCG may also be of value. Fetal cells can be identified in the maternal circulation and techniques such as fluorescent in situ hybridisation can be used to identify aneuploidies, including Down's syndrome and trisomy 18. This approach may, in the future, be of value in screening or diagnosis. Currently, the techniques available do not have the performance, simplicity, or economy needed to replace existing methods.

DEMONSTRATION PROJECTS

Demonstration projects are valuable in determining the feasibility of screening and in refining the practical application of screening. They are of less value in determining the performance of different screening methods. Several demonstration projects have been conducted using the triple and double tests. In general, the uptake of screening was about 80%. The screen positive rates were about 5-6%. About 80% of women with positive screening results had an invasive diagnostic test, and of those found to have a pregnancy with Down's syndrome, about 90% chose to have a termination of pregnancy. ULTRASOUND MARKERS AT 15-22 WEEKS OF PREGNANCY: There are a number of ultrasound markers of Down's syndrome at 15-22 weeks, including nuchal fold thickness, cardiac abnormalities, duodenal atresia, femur length, humerus length, pyelectasis, and hyperechogenic bowel. (ABSTRA

Screening for Down's syndrome

Down's syndrome (DS) is the commonest cause of severe mental retardation in children. It is the result of trisomy of chromosome 21 which is usually a random event though it is commoner in older mothers. DS can be diagnosed by chorionic villus sampling (CVS) and amniocentesis followed by karyotyping. Because of the risks associated with these invasive procedures, they can only be offered to a high-risk group. At one time the sole basis for identifying this increased risk was maternal age, but within the past ten years a series of biochemical and ultrasound abnormalities have been shown in DS pregnancies. The biochemical abnormalities include changes in the levels of most fetal and placental products in the maternal circulation. The best-known of these changes are the reduced levels of alphafetoprotein (AFP) and oestriol (E3) and increased levels of human chorionic gonadotrophin (hCG). The mechanism underlying these biochemical phenomena is unknown. Screening programmes involving the measurement of hCG and AFP, with or without additional parameters such as E3, at 15-18 weeks of pregnancy can typically identify 60% or more of cases of DS with a screen-positive rate of 5%. The combined risk derived from the various biochemical parameters, together with maternal age, is calculated by one of a number of computer programmes which have been developed for this purpose. There has been considerable discussion as to the exact biochemical tests which should be used for DS screening. This had led to controversy as to whether measurement of E3 has a place, and whether or not measurement of the free beta-subunit of hCG should replace measurement of the intact molecule. A notable recent development is the suggestion that measurement of the urinary beta-core of the hCG could be a highly discriminatory marker. A number of factors can affect the results of biochemical screening for DS. These include maternal weight, gestational age, ethnic origin, smoking, and diabetes. In addition, abnormal levels of the biochemical products may be found in other chromosome abnormalities.