Repeated measurement of pregnancy-associated plasma protein-A (PAPP-A) in Down syndrome screening: A validation study

Non-invasive prenatal screening: A 20-year experience in Italy

Over the past two decades, there has been a rapid evolution in prenatal screening for fetal chromosome abnormalities. Initially, testing was focused on the identification of affected pregnancies in either the first, or, the second trimester (e.g. the Combined test or the triple test). This was replaced by sequential modalities (e.g. contingent screening) that have enhanced detection while reducing the need for invasive testing. More recently, the introduction of technologies based on cell-free DNA (cfDNA) in maternal plasma and enrichment of fetal cells in maternal circulation have further refined the concept of sequential screening. In this review, we document our experience with serum and ultrasound-based contingent screening where we were able to achieve a detection rate of 96.8%, a false-positive rate of 2.8% and an odds of being affected given a positive result of 1:11. We also describe our initial experience with a novel sequential protocol that includes the analysis of fetal cells in maternal blood. Methods for enrichment for fetal cells cfDNA and cfDNA technologies offer the possibility of greater sensitivity and specificity as well as expansion in the scope of genetic disorders detectable. As costs decline, these technologies will become increasingly used as primary screening tools. In the meantime, sequential use offers a practical approach to maximizing the benefits of prenatal testing.

First and second trimester serum tests with and without first trimester ultrasound tests for Down's syndrome screening

BACKGROUND

Down's syndrome occurs when a person has three copies of chromosome 21 (or the specific area of chromosome 21 implicated in causing Down's syndrome) rather than two. It is the commonest congenital cause of mental disability. Non-invasive screening based on biochemical analysis of maternal serum or urine, or fetal ultrasound measurements, allows estimates of the risk of a pregnancy being affected and provides information to guide decisions about definitive testing.  Before agreeing to screening tests, parents need to be fully informed about the risks, benefits and possible consequences of such a test. This includes subsequent choices for further tests they may face, and the implications of both false positive (i.e. invasive diagnostic testing, and the possibility that a miscarried fetus may be chromosomally normal) and false negative screening tests (i.e. a fetus with Down's syndrome will be missed). The decisions that may be faced by expectant parents inevitably engender a high level of anxiety at all stages of the screening process, and the outcomes of screening can be associated with considerable physical and psychological morbidity. No screening test can predict the severity of problems a person with Down's syndrome will have.

OBJECTIVES

To estimate and compare the accuracy of first and second trimester serum markers with and without first trimester ultrasound markers for the detection of Down's syndrome in the antenatal period, as combinations of markers.

SEARCH METHODS

We conducted a sensitive and comprehensive literature search of MEDLINE (1980 to 25 August 2011), Embase (1980 to 25 August 2011), BIOSIS via EDINA (1985 to 25 August 2011), CINAHL via OVID (1982 to 25 August 2011), the Database of Abstracts of Reviews of Effectiveness (the Cochrane Library 25 August 2011), MEDION (25 August 2011), the Database of Systematic Reviews and Meta-Analyses in Laboratory Medicine (25 August 2011), the National Research Register (Archived 2007), and Health Services Research Projects in Progress database (25 August 2011). We did not apply a diagnostic test search filter. We did forward citation searching in ISI citation indices, Google Scholar and PubMed 'related articles'. We also searched reference lists of retrieved articles SELECTION CRITERIA: Studies evaluating tests of combining first and second trimester maternal serum markers in women up to 24 weeks of gestation for Down's syndrome, with or without first trimester ultrasound markers, compared with a reference standard, either chromosomal verification or macroscopic postnatal inspection.

DATA COLLECTION AND ANALYSIS

Data were extracted as test positive/test negative results for Down's and non-Down's pregnancies allowing estimation of detection rates (sensitivity) and false positive rates (1-specificity). We performed quality assessment according to QUADAS criteria. We used hierarchical summary ROC meta-analytical methods to analyse test performance and compare test accuracy. Analysis of studies allowing direct comparison between tests was undertaken. We investigated the impact of maternal age on test performance in subgroup analyses.

MAIN RESULTS

Twenty-two studies (reported in 25 publications) involving 228,615 pregnancies (including 1067 with Down's syndrome) were included. Studies were generally high quality, although differential verification was common with invasive testing of only high risk pregnancies. Ten studies made direct comparisons between tests. Thirty-two different test combinations were evaluated formed from combinations of eight different tests and maternal age; first trimester nuchal translucency (NT) and the serum markers AFP, uE3, total hCG, free βhCG, Inhibin A, PAPP-A and ADAM 12. We looked at tests combining first and second trimester markers with or without ultrasound as complete tests, and we also examined stepwise and contingent strategies.Meta-analysis of the six most frequently evaluated test combinations showed that a test strategy involving maternal age and a combination of first trimester NT and PAPP-A, and second trimester total hCG, uE3, AFP and Inhibin A significantly outperformed other test combinations that involved only one serum marker or NT in the first trimester, detecting about nine out of every 10 Down's syndrome pregnancies at a 5% false positive rate. However, the evidence was limited in terms of the number of studies evaluating this strategy, and we therefore cannot recommend one single screening strategy.

AUTHORS' CONCLUSIONS

Tests involving first trimester ultrasound with first and second trimester serum markers in combination with maternal age are significantly better than those without ultrasound, or those evaluating first trimester ultrasound in combination with second trimester serum markers, without first trimester serum markers. We cannot make recommendations about a specific strategy on the basis of the small number of studies available.

Placental Protein 13 (PP13) - A Placental Immunoregulatory Galectin Protecting Pregnancy

Galectins are glycan-binding proteins that regulate innate and adaptive immune responses, and some confer maternal-fetal immune tolerance in eutherian mammals. A chromosome 19 cluster of galectins has emerged in anthropoid primates, species with deep placentation and long gestation. Three of the five human cluster galectins are solely expressed in the placenta, where they may confer additional immunoregulatory functions to enable deep placentation. One of these is galectin-13, also known as Placental Protein 13 (PP13). It has a "jelly-roll" fold, carbohydrate-recognition domain and sugar-binding preference resembling other mammalian galectins. PP13 is predominantly expressed by the syncytiotrophoblast and released from the placenta into the maternal circulation. Its ability to induce apoptosis of activated T cells in vitro, and to divert and kill T cells as well as macrophages in the maternal decidua in situ, suggests important immune functions. Indeed, mutations in the promoter and an exon of LGALS13 presumably leading to altered or non-functional protein expression are associated with a higher frequency of preeclampsia and other obstetrical syndromes, which involve immune dysregulation. Moreover, decreased placental expression of PP13 and its low concentrations in first trimester maternal sera are associated with elevated risk of preeclampsia. Indeed, PP13 turned to be a good early biomarker to assess maternal risk for the subsequent development of pregnancy complications caused by impaired placentation. Due to the ischemic placental stress in preterm preeclampsia, there is increased trophoblastic shedding of PP13 immunopositive microvesicles starting in the second trimester, which leads to high maternal blood PP13 concentrations. Our meta-analysis suggests that this phenomenon may enable the potential use of PP13 in directing patient management near to or at the time of delivery. Recent findings on the beneficial effects of PP13 on decreasing blood pressure due to vasodilatation in pregnant animals suggest its therapeutic potential in preeclampsia.

Second trimester serum tests for Down's Syndrome screening

BACKGROUND

Down's syndrome occurs when a person has three copies of chromosome 21 - or the specific area of chromosome 21 implicated in causing Down's syndrome - rather than two. It is the commonest congenital cause of mental retardation. Noninvasive screening based on biochemical analysis of maternal serum or urine, or fetal ultrasound measurements, allows estimates of the risk of a pregnancy being affected and provides information to guide decisions about definitive testing.  

OBJECTIVES

To estimate and compare the accuracy of second trimester serum markers for the detection of Down's syndrome.

SEARCH METHODS

We carried out a sensitive and comprehensive literature search of MEDLINE (1980 to May 2007), EMBASE (1980 to 18 May 2007), BIOSIS via EDINA (1985 to 18 May 2007), CINAHL via OVID (1982 to 18 May 2007), The Database of Abstracts of Reviews of Effectiveness (The Cochrane Library 2007, Issue 1), MEDION (May 2007), The Database of Systematic Reviews and Meta-Analyses in Laboratory Medicine (May 2007), The National Research Register (May 2007), Health Services Research Projects in Progress database (May 2007). We studied reference lists and published review articles.  

SELECTION CRITERIA

Studies evaluating tests of maternal serum in women at 14-24 weeks of gestation for Down's syndrome, compared with a reference standard, either chromosomal verification or macroscopic postnatal inspection.

DATA COLLECTION AND ANALYSIS

Data were extracted as test positive/test negative results for Down's and non-Down's pregnancies allowing estimation of detection rates (sensitivity) and false positive rates (1-specificity). We performed quality assessment according to QUADAS criteria. We used hierarchical summary ROC meta-analytical methods to analyse test performance and compare test accuracy. Analysis of studies allowing direct comparison between tests was undertaken. We investigated the impact of maternal age on test performance in subgroup analyses.

MAIN RESULTS

Fifty-nine studies involving 341,261 pregnancies (including 1,994 with Down's syndrome) were included. Studies were generally high quality, although differential verification was common with invasive testing of only high-risk pregnancies. Seventeen studies made direct comparisons between tests. Fifty-four test combinations were evaluated formed from combinations of 12 different tests and maternal age; alpha-fetoprotein (AFP), unconjugated oestriol (uE3), total human chorionic gonadotrophin (hCG), free beta human chorionic gonadotrophin (βhCG), free alpha human chorionic gonadotrophin (αhCG), Inhibin A, SP2, CA125, troponin, pregnancy-associated plasma protein A (PAPP-A), placental growth factor (PGF) and proform of eosinophil major basic protein (ProMBP).Meta-analysis of 12 best performing or frequently evaluated test combinations showed double and triple tests (involving AFP, uE3, total hCG, free βhCG) significantly outperform individual markers, detecting six to seven out of every 10 Down's syndrome pregnancies at a 5% false positive rate. Tests additionally involving inhibin performed best (eight out of every 10 Down's syndrome pregnancies) but were not shown to be significantly better than standard triple tests in direct comparisons. Significantly lower sensitivity occurred in women over the age of 35 years. Women who miscarried in the over 35 group were more likely to have been offered an invasive test to verify a negative screening results, whereas those under 35 were usually not offered invasive testing for a negative screening result. Pregnancy loss in women under 35 therefore leads to under ascertainment of screening results, potentially missing a proportion of affected pregnancies and affecting the accuracy of the sensitivity.

AUTHORS' CONCLUSIONS

Tests involving two or more markers in combination with maternal age are significantly more sensitive than those involving one marker. The value of combining four or more tests or including inhibin have not been proven to show statistically significant improvement. Further study is required to investigate reduced test performance in women aged over 35 and the impact of differential pregnancy loss on study findings.

Biological variation of free β chorionic gonadotropin and pregnancy-associated plasma protein A in first trimester pregnancies

BACKGROUND

Trisomy 21 risk estimation in first trimester pregnancies can be performed by a combined test based on ultrasound measurement of fetal nuchal translucency thickness and maternal plasma concentrations of free β human chorionic gonadotropin (hCGβ) and pregnancy-associated plasma protein A (PAPP-A). However, little knowledge exists regarding the biological variation of hCGβ and PAPP-A when the time interval between sampling increases.

METHODS

We estimated these variations from double measurements of hCGβ and PAPP-A in first trimester pregnancies in 167 women. Data were divided into three groups based on the number of days between sampling. The correlation coefficients and biological variation were estimated for each group.

RESULTS

The correlation coefficient between the first and second samples was 0.841 for hCGβ, and 0.706 for PAPP-A. The ranges for biological variation were 11.9%-48.5% for hCGβ and 31.6%-63.3% for PAPP-A, increasing with time between sampling. The average overall biological variation for hCGβ was 29%, and 49.7% for PAPP-A.

CONCLUSIONS

We found high biological variation for plasma concentrations of hCGβ and PAPP-A, increasing with longer time intervals between sampling. From our data that showed high correlation of hCGβ and PAPP-A in the first and second sample, we found no reason to recommend retesting. However, new studies should clarify whether PAPP-A should be collected early, and hCGβ late, in the first trimester of pregnancy.

Down syndrome screening: imagining the screening test of the future

Prenatal screening for Down syndrome (DS) is performed by risk calculation based on biochemical and biometric parameters. This way, approximately 75-85% of all DS cases can be detected. A way to improve detection rates is to search for new screening markers. Since the majority of biomarkers used in current DS screening are predominantly produced by the placenta, and the presence of an extra chromosome (as in DS) complicates placental development and function, it is plausible to assume that new potential screening markers may also originate from the placenta. Any alterations in these markers can be attributed to abnormal placental development and function. This article focuses on normal early placental development and function compared with that in DS pregnancies. Using this knowledge, we reason towards candidate biomarkers that may be useful in screening for DS.

Dependence of maternal serum [AFP]/[hCG] median ratios on age of gestation: comparison of trisomy 21 to euploid pregnancies

BACKGROUND

Current risk calculations for trisomy 21, which are based on multiples of median (MoM), do not take into account possible differences between euploid and trisomy 21 pregnancies that may develop with gestational age. In order to optimize the predictive value of screening tests, we calculated the ratio between maternal serum concentration of alpha-fetoprotein (AFP) and that of human chorionic gonadotropin (hCG) in euploid and in trisomy 21 pregnancies.

METHODS

The medians of the concentration ratios, [AFP]/[hCG] at 16-21 weeks of gestation, were plotted as a function of gestational age for 307 cases of trisomy 21 and were compared with the medians of 30 549 normal karyotype cases.

RESULTS

[AFP]/[hCG] ratio medians were independent of body weight and maternal age. There was a significant difference in the [AFP]/[hCG] ratio when comparing trisomy 21 and euploid pregnancies at each week. This difference became greater with advancing gestational age (P < 0.01).

CONCLUSION

There is a significant difference in ratios of [AFP]/[hCG] between euploid and trisomy 21 pregnancies, which may be used to improve detection rates of Down syndrome screening.

Multiple-marker screening for Down's syndrome: a method of assessing the statistical robustness of proposed tests

OBJECTIVES

Antenatal screening for Down's syndrome relies on the use of multiple markers in combination. Markers that are highly correlated can cause statistical instability. We used the maximum variance inflation factor (VIF(max)) to determine whether a screening test using multiple markers was robust to imprecision in the estimation of the marker distribution parameters.

METHODS

The VIF(max) for a specified screening test was calculated from the correlations between markers in Down's syndrome pregnancies for six tests: integrated and serum integrated tests without repeat measurements, both tests with repeat measurements across trimesters analysed in the standard way, and both tests with repeat measurements analysed as cross-trimester (CT) marker ratios. The screening performance of each test using published parameter values, in terms of the false-negative rates for a 3% false-positive rate (FN(3)), were calculated for simulated populations with medians 0.2 standard deviations (SD) higher or lower than the published values (to reflect imprecision in parameter estimation) for pregnancy-associated plasma protein A and unconjugated oestriol in affected pregnancies. For each test, the VIF(max) value was compared with the coefficient of variation of the FN(3) (FN(3) CV). An independent set of 27 Down's syndrome pregnancies was used to determine how many had meaningless low risks (<1 in 10,000) with each test.

RESULTS

Tests with VIF(max) values greater than 5 had FN(3)CV values over 50%, but those with VIF(max) values less than 5 had FN(3) CV values less than 21%. The numbers of Down's syndrome pregnancies with meaningless low risk estimates in the independent set were 18 (64%) in tests with VIF(max) values > or =5 and none for those with values <5.

CONCLUSION

VIF(max) values of 5 or more suggest instability. The tests using CT marker ratios were stable (VIF(max) < 3), but the tests using repeat measurements in the standard manner were not (VIF(max) > 5).

Aneuploidy screening in the first trimester

This article reviews the performance of first trimester screening for chromosomal anomalies using various combinations of ultrasound and maternal serum biochemical modalities. Detection rates in excess of 90% can be routinely achieved for Trisomy 21, Trisomy 13, Trisomy 18 using a combination of fetal nuchal translucency (NT) thickness and maternal serum free ss-hCG and PAPP-A at 11 + 0 to 13 + 6 weeks of gestation.

Prenatal Screening for Fetal Aneuploidy

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