Evaluation of maternal serum immunoreactive inhibin as a first trimester marker of Down's syndrome

First trimester ultrasound tests alone or in combination with first trimester serum tests for Down's syndrome screening

BACKGROUND

Down's syndrome occurs when a person has three, rather than two copies of chromosome 21; or the specific area of chromosome 21 implicated in causing Down's syndrome. It is the commonest congenital cause of mental disability and also leads to numerous metabolic and structural problems. It can be life-threatening, or lead to considerable ill health, although some individuals have only mild problems and can lead relatively normal lives. Having a baby with Down's syndrome is likely to have a significant impact on family life.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 and false negative screening tests (i.e. invasive diagnostic testing, and the possibility that a miscarried fetus may be chromosomally normal). 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 trimester ultrasound markers alone, and in combination with first trimester serum tests for the detection of Down's syndrome.

SEARCH METHODS

We carried out extensive literature searches including 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), and The Database of Abstracts of Reviews of Effects (the Cochrane Library 2011, Issue 7). We checked reference lists and published review articles for additional potentially relevant studies.

SELECTION CRITERIA

Studies evaluating tests of first trimester ultrasound screening, alone or in combination with first trimester serum tests (up to 14 weeks' 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

We included 126 studies (152 publications) involving 1,604,040 fetuses (including 8454 Down's syndrome cases). Studies were generally good quality, although differential verification was common with invasive testing of only high-risk pregnancies. Sixty test combinations were evaluated formed from combinations of 11 different ultrasound markers (nuchal translucency (NT), nasal bone, ductus venosus Doppler, maxillary bone length, fetal heart rate, aberrant right subclavian artery, frontomaxillary facial angle, presence of mitral gap, tricuspid regurgitation, tricuspid blood flow and iliac angle 90 degrees); 12 serum tests (inhibin A, alpha-fetoprotein (AFP), free beta human chorionic gonadotrophin (ßhCG), total hCG, pregnancy-associated plasma protein A (PAPP-A), unconjugated oestriol (uE3), disintegrin and metalloprotease 12 (ADAM 12), placental growth factor (PlGF), placental growth hormone (PGH), invasive trophoblast antigen (ITA) (synonymous with hyperglycosylated hCG), growth hormone binding protein (GHBP) and placental protein 13 (PP13)); and maternal age. The most frequently evaluated serum markers in combination with ultrasound markers were PAPP-A and free ßhCG.Comparisons of the 10 most frequently evaluated test strategies showed that a combined NT, PAPP-A, free ßhCG and maternal age test strategy significantly outperformed ultrasound markers alone (with or without maternal age) except nasal bone, detecting about nine out of every 10 Down's syndrome pregnancies at a 5% false positive rate (FPR). In both direct and indirect comparisons, the combined NT, PAPP-A, free ßhCG and maternal age test strategy showed superior diagnostic accuracy to an NT and maternal age test strategy (P < 0.0001). Based on the indirect comparison of all available studies for the two tests, the sensitivity (95% confidence interval) estimated at a 5% FPR for the combined NT, PAPP-A, free ßhCG and maternal age test strategy (69 studies; 1,173,853 fetuses including 6010 with Down's syndrome) was 87% (86 to 89) and for the NT and maternal age test strategy (50 studies; 530,874 fetuses including 2701 Down's syndrome pregnancies) was 71% (66 to 75). Combinations of NT with other ultrasound markers, PAPP-A and free ßhCG were evaluated in one or two studies and showed sensitivities of more than 90% and specificities of more than 95%.High-risk populations (defined before screening was done, mainly due to advanced maternal age of 35 years or more, or previous pregnancies affected with Down's syndrome) showed lower detection rates compared to routine screening populations at a 5% FPR. 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 test sensitivity. Conversely, for the NT, PAPP-A, free ßhCG and maternal age test strategy, detection rates and false positive rates increased with maternal age in the five studies that provided data separately for the subset of women aged 35 years or more.

AUTHORS' CONCLUSIONS

Test strategies that combine ultrasound markers with serum markers, especially PAPP-A and free ßhCG, and maternal age were significantly better than those involving only ultrasound markers (with or without maternal age) except nasal bone. They detect about nine out of 10 Down's affected pregnancies for a fixed 5% FPR. Although the absence of nasal bone appeared to have a high diagnostic accuracy, only five out of 10 affected Down's pregnancies were detected at a 1% FPR.

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.

A prospective clinical trial to compare the performance of dried blood spots prenatal screening for Down's syndrome with conventional non-invasive testing technology

To evaluate, side by side, the efficiency of dried blood spots (DBSs) against serum screening for Down's syndrome, and then, to construct a two-tier strategy by topping up the fetal cell-free DNA (cfDNA) secondary screening over the high-risk women marked by the primary blood testing to build a practical screening tactic to identify fetal Down's syndrome. One thousand eight hundred and thirty-seven low-risk Chinese women, with singleton pregnancy, were enrolled for the study. Alpha-fetoprotein and free beta human chorionic gonadotropin were measured for the serum as well as for the parallel DBS samples. Partial high-risk pregnant women identified by primary blood testing (n = 38) were also subject to the secondary cfDNA screening. Diagnostic amniocentesis was utilized to confirm the screening results. The true positive rate for Down's syndrome detection was 100% for both blood screening methods; however, the false-positive rate was 3.0% for DBS and 4.0% for serum screening, respectively. DBS correlated well with serum screening on Down's syndrome detection. Three out of 38 primary high-risk women displayed chromosomal abnormalities by cfDNA analysis, which were confirmed by amniocentesis. Either the true detection rate or the false-positive rate for Down's syndrome between DBS and the serum test is comparable. In addition, blood primary screening aligned with secondary cfDNA analysis, a "before and after" two-tier screening strategy, can massively decrease the false-positive rate, which, then, dramatically reduces the demand for invasive diagnostic operation. Impact statement Children born with Down's syndrome display a wide range of mental and physical disability. Currently, there is no effective treatment to ease the burden and anxiety of the Down's syndrome family and the surrounding society. This study is to evaluate the efficiency of dried blood spots against serum screening for Down's syndrome and to construct a two-tier strategy by topping up the fetal cell-free DNA (cfDNA) secondary screening over the high-risk women marked by the primary blood testing to build a practical screening tactic to identify fetal Down's syndrome. Results demonstrate that fetal cfDNA can significantly reduce false-positive rate close to none while distinguishing all true positives. Thus, we recommend that fetal cfDNA analysis to be utilized as a secondary screening tool atop of the primary blood protein screening to further minimize the capacity of undesirable invasive diagnostic operations.

Urine 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 and also leads to numerous metabolic and structural problems. It can be life-threatening, or lead to considerable ill health, although some individuals have only mild problems and can lead relatively normal lives. Having a baby with Down's syndrome is likely to have a significant impact on family life. The risk of a Down's syndrome affected pregnancy increases with advancing maternal age.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. 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 and false negative screening tests (i.e. invasive diagnostic testing, and the possibility that a miscarried fetus may be chromosomally normal). 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 urine markers for the detection of Down's syndrome.

SEARCH METHODS

We carried out 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 2011, Issue 7), MEDION (25 August 2011), The Database of Systematic Reviews and Meta-Analyses in Laboratory Medicine (25 August 2011), The National Research Register (archived 2007), Health Services Research Projects in Progress database (25 August 2011). We studied reference lists and published review articles.

SELECTION CRITERIA

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

DATA COLLECTION AND ANALYSIS

We extracted data as test positive or 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 (Quality Assessment of Diagnostic Accuracy Studies) criteria. We used hierarchical summary ROC (receiver operating characteristic) meta-analytical methods to analyse test performance and compare test accuracy. We performed analysis of studies allowing direct comparison between tests. We investigated the impact of maternal age on test performance in subgroup analyses.

MAIN RESULTS

We included 19 studies involving 18,013 pregnancies (including 527 with Down's syndrome). Studies were generally of high quality, although differential verification was common with invasive testing of only high-risk pregnancies. Twenty-four test combinations were evaluated formed from combinations of the following seven different markers with and without maternal age: AFP (alpha-fetoprotein), ITA (invasive trophoblast antigen), ß-core fragment, free ßhCG (beta human chorionic gonadotrophin), total hCG, oestriol, gonadotropin peptide and various marker ratios. The strategies evaluated included three double tests and seven single tests in combination with maternal age, and one triple test, two double tests and 11 single tests without maternal age. Twelve of the 19 studies only evaluated the performance of a single test strategy while the remaining seven evaluated at least two test strategies. Two marker combinations were evaluated in more than four studies; second trimester ß-core fragment (six studies), and second trimester ß-core fragment with maternal age (five studies).In direct test comparisons, for a 5% false positive rate (FPR), the diagnostic accuracy of the double marker second trimester ß-core fragment and oestriol with maternal age test combination was significantly better (ratio of diagnostic odds ratio (RDOR): 2.2 (95% confidence interval (CI) 1.1 to 4.5), P = 0.02) (summary sensitivity of 73% (CI 57 to 85) at a cut-point of 5% FPR) than that of the single marker test strategy of second trimester ß-core fragment and maternal age (summary sensitivity of 56% (CI 45 to 66) at a cut-point of 5% FPR), but was not significantly better (RDOR: 1.5 (0.8 to 2.8), P = 0.21) than that of the second trimester ß-core fragment to oestriol ratio and maternal age test strategy (summary sensitivity of 71% (CI 51 to 86) at a cut-point of 5% FPR).

AUTHORS' CONCLUSIONS

Tests involving second trimester ß-core fragment and oestriol with maternal age are significantly more sensitive than the single marker second trimester ß-core fragment and maternal age, however, there were few studies. There is a paucity of evidence available to support the use of urine testing for Down's syndrome screening in clinical practice where alternatives are available.

First trimester serum tests for Down's syndrome screening

BACKGROUND

Down's syndrome occurs when a person has three, rather than two copies of chromosome 21; or the specific area of chromosome 21 implicated in causing Down's syndrome. It is the commonest congenital cause of mental disability and also leads to numerous metabolic and structural problems. It can be life-threatening, or lead to considerable ill health, although some individuals have only mild problems and can lead relatively normal lives. Having a baby with Down's syndrome is likely to have a significant impact on family life.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. However, no test can predict the severity of problems a person with Down's syndrome will have.

OBJECTIVES

The aim of this review was to estimate and compare the accuracy of first trimester serum markers for the detection of Down's syndrome in the antenatal period, both as individual markers and as combinations of markers. Accuracy is described by the proportion of fetuses with Down's syndrome detected by screening before birth (sensitivity or detection rate) and the proportion of women with a low risk (normal) screening test result who subsequently had a baby unaffected by Down's syndrome (specificity).

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), Health Services Research Projects in Progress database (25 August 2011). We did forward citation searching ISI citation indices, Google Scholar and PubMed 'related articles'. We did not apply a diagnostic test search filter. We also searched reference lists and published review articles.  

SELECTION CRITERIA

We included studies in which all women from a given population had one or more index test(s) compared to a reference standard (either chromosomal verification or macroscopic postnatal inspection). Both consecutive series and diagnostic case-control study designs were included. Randomised trials where individuals were randomised to different screening strategies and all verified using a reference standard were also eligible for inclusion. Studies in which test strategies were compared head-to-head either in the same women, or between randomised groups were identified for inclusion in separate comparisons of test strategies. We excluded studies if they included less than five Down's syndrome cases, or more than 20% of participants were not followed up.

DATA COLLECTION AND ANALYSIS

We extracted data as test positive or 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 (Quality Assessment of Diagnostic Accuracy Studies) criteria. We used hierarchical summary ROC meta-analytical methods or random-effects logistic regression methods to analyse test performance and compare test accuracy as appropriate. Analyses of studies allowing direct and indirect comparisons between tests were undertaken.

MAIN RESULTS

We included 56 studies (reported in 68 publications) involving 204,759 pregnancies (including 2113 with Down's syndrome). Studies were generally of good quality, although differential verification was common with invasive testing of only high-risk pregnancies. We evaluated 78 test combinations formed from combinations of 18 different tests, with or without maternal age; ADAM12 (a disintegrin and metalloprotease), AFP (alpha-fetoprotein), inhibin, PAPP-A (pregnancy-associated plasma protein A, ITA (invasive trophoblast antigen), free βhCG (beta human chorionic gonadotrophin), PlGF (placental growth factor), SP1 (Schwangerschafts protein 1), total hCG, progesterone, uE3 (unconjugated oestriol), GHBP (growth hormone binding protein), PGH (placental growth hormone), hyperglycosylated hCG, ProMBP (proform of eosinophil major basic protein), hPL (human placental lactogen), (free αhCG, and free ßhCG to AFP ratio. Direct comparisons between two or more tests were made in 27 studies.Meta-analysis of the nine best performing or frequently evaluated test combinations showed that a test strategy involving maternal age and a double marker combination of PAPP-A and free ßhCG significantly outperformed the individual markers (with or without maternal age) detecting about seven out of every 10 Down's syndrome pregnancies at a 5% false positive rate (FPR). Limited evidence suggested that marker combinations involving PAPP-A may be more sensitive than those without PAPP-A.

AUTHORS' CONCLUSIONS

Tests involving two markers in combination with maternal age, specifically PAPP-A, free βhCG and maternal age are significantly better than those involving single markers with and without age. They detect seven out of 10 Down's affected pregnancies for a fixed 5% FPR. The addition of further markers (triple tests) has not been shown to be statistically superior; the studies included are small with limited power to detect a difference.The screening blood tests themselves have no adverse effects for the woman, over and above the risks of a routine blood test. However some women who have a 'high risk' screening test result, and are given amniocentesis or chorionic villus sampling (CVS) have a risk of miscarrying a baby unaffected by Down's. Parents will need to weigh up this risk when deciding whether or not to have an amniocentesis or CVS following a 'high risk' screening test result.

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.

First-trimester biochemical screening for Down syndrome

Of all markers evaluated for first-trimester biochemical screening for Down syndrome (DS), PAPP-A and free beta-hCG emerged as the most predictive. The combined test uses these markers in conjunction with nuchal translucency measurements, and is estimated to achieve a DS detection rate of 80% to 85% at a 5% false-positive rate. The integrated test, combining first-trimester sonographic and biochemical markers with second-trimester markers, provides a single estimate of a patients DS risk, and may yield a DS detection rate of 94% at a 5% false-positive rate. The acceptability and feasibility of this test, however, remain to be proved.

Inhibin A and pro-alpha C inhibin in Down syndrome and normal pregnancies

Using two enzyme-linked immunosorbent assays specific for inhibin A and pro-alpha C inhibin, levels of the two proteins were assessed in maternal serum from 43 Down syndrome and 300 chromosomally normal pregnancies at 15-17 weeks' gestation. Compared to the control pregnancies, both inhibin A and pro-alpha C inhibin were significantly elevated in the Down syndrome pregnancies with median levels, expressed as multiples of the normal median, of 1.53 MoM and 1.34 MoM, respectively (P < 0.001 and P = 0.046 compared to controls). Levels of inhibin A and pro-alpha C inhibin were weakly but significantly correlated in both the control and the Down syndrome sera (r = 0.25, P < 0.0001; r = 0.4, P = 0.008, respectively). These data suggest that the mechanism(s) underlying the elevated inhibin levels observed in Down syndrome may affect the regulation of both the inhibin alpha- and beta A-subunits.

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

Maternal serum inhibin-A and free beta-hCG concentrations in trisomy 21 pregnancies at 10 to 14 weeks of gestation

OBJECTIVE

To determine the relation between maternal serum inhibin-A and free beta-hCG concentrations in chromosomally normal pregnancies and to compare the two biochemical markers for their sensitivity in identifying trisomy 21 pregnancies.

SAMPLE

Inhibin-A and free beta-hCG were measured in maternal serum samples from 800 chromosomally normal singleton pregnancies at 10 to 14 weeks of gestation and 76 singleton pregnancies with fetal trisomy 21.

RESULTS

In the normal group maternal serum inhibin-A was significantly associated with both maternal weight and gestational age (F = 11.2, P < 0.0001). In pregnancies with trisomy 21 the maternal serum inhibin-A and free beta-hCG concentrations were significantly increased (mean difference inhibin = 0.51 SD, F = 18, P < 0.0001 and mean difference free beta-hCG = 1.13 SD, F = 80, P < 0.0001). For a 5% false positive rate, the sensitivity of maternal serum free beta-hCG in identifying pregnancies with trisomy 21 was 28.9% compared with 12.8% for maternal serum inhibin-A. Delta inhibin-A was significantly associated with delta-free beta-hCG (r = 0.345, P < 0.01) and the deviation from the normal mean for free beta-hCG was significantly greater than the deviation for inhibin-A (t = 4.0, P < 0.0001). For a 5% false positive rate, the sensitivity achieved by combining information from delta inhibin-A and delta free beta-hCG was similar to the sensitivity of free beta-hCG alone (30.3% compared with 28.9%).

CONCLUSION

At 10 to 14 weeks of gestation fetal trisomy 21 is associated with increased maternal serum inhibin-A and free beta-hCG levels. However, the degree of elevation of inhibin-A is less than that of free beta-hCG, and there is a significant association between levels of the two proteins. The sensitivity for trisomy 21 achieved with the combination of maternal serum inhibin-A and free beta-hCG is not significantly different from that achieved with maternal serum free beta-hCG alone.

Combining inhibin A with existing second-trimester markers in maternal serum screening for Down's syndrome

To assess the value of inhibin A as an additional second-trimester maternal serum marker of Down's syndrome we studied 56 affected and 280 unaffected pregnancies matched for gestational age. The median level in the cases was 1.62 multiples of the gestation-specific median (MOM) in the controls, with 95 per cent confidence limits of 1.34-1.96. The distribution of inhibin levels in affected and unaffected pregnancies was approximately log Gaussian, with means about 1 standard deviation apart. This degree of separation was similar to that for human chorionic gonadotropin (hCG), free beta-hCG, and unconjugated oestriol (uE3), but about double that of alpha-fetoprotein (AFP) measured in the same samples. Inhibin was largely uncorrelated with AFP and uE3, whereas the log correlation coefficient with hCG was 0.29 (P = 0.19) for Down's syndrome and 0.41 (P < 0.0001) for unaffected pregnancies; with free beta-hCG, it was 0.18 (P = 0.38) and 0.38 (P < 0.0001), respectively. On the basis of these results and other published studies, we estimate that measuring inhibin A in addition to AFP and hCG or free beta-hCG (with or without uE3) will increase the detection rate for a fixed 5 per cent false-positive rate by about 7 per cent.

Second-trimester dimeric inhibin-A in Down's syndrome screening

Initial studies of immunoreactive inhibin using a commercial assay have shown levels to be increased in three second-trimester series of maternal samples from Down's syndrome-affected pregnancies. This assay detected non-specifically all forms of circulating inhibin, dimeric and free alpha subunits, whether fully or partially processed. More recently, a new specific assay for dimeric inhibin-A has shown elevated results in both a first-trimester and a second-trimester series of cases. In order to assess the value of dimeric inhibin-A as a potential marker in the second trimester, we have analysed 157 Down's syndrome cases and used 367 unaffected cases across the gestational range 14-20 weeks to establish control medians and population parameters. In our series, the median MOM in Down's cases was 1.77, significantly higher than in the controls. At a 5 per cent false-positive rate, dimeric inhibin-A alone identified 37 per cent of cases. When used in conjunction with maternal age and other marker combinations, mathematical modelling showed detection rates rising from 48 per cent (inhibin-A plus age) to 61 per cent (inhibin-A, free beta hCG, age) and 68 per cent (inhibin-A, AFP, free beta hCG, age). Our data suggest that dimeric inhibin-A may have greater potential earlier in gestation when median levels at 14-16 weeks are 1.92 compared with 1.46 at 17-23 weeks. Dimeric inhibin-A may be a valuable addition to screening protocols, particularly in early gestations.

Dimeric inhibin A as a marker for Down's syndrome in early pregnancy

BACKGROUND

In screening for Down's syndrome in the second trimester of pregnancy, the concentrations of alpha-fetoprotein, the beta subunit of human chorionic gonadotropin, and intact human chorionic gonadotropin in material serum are widely used markers. We investigated a new marker, dimeric inhibin A, and compared its predictive value with that of the established markers.

METHODS

Serum samples were obtained at 7 to 18 weeks of gestation from 58 women whose fetuses were known to be affected by Down's syndrome, 32 whose fetuses were affected by trisomy 18, and 438 whose fetuses were normal, and the samples were analyzed for each marker. Individual serum concentrations of each marker were converted to multiples of the median value at the appropriate length of gestation in the women with normal pregnancies, and rates of detection of Down's syndrome by screening for inhibin A in various combinations with the other markers were estimated by multivariate analysis.

RESULTS

In the women with fetuses affected by Down's syndrome, the serum inhibin A concentrations were 2.06 times the median value in the women with normal pregnancies (P < 0.001). This compared with 2.00 times the median for the beta subunit of human chorionic gonadotropin, 1.82 times the median for intact human chorionic gonadotropin, and 0.72 for alpha-fetoprotein. The serum concentrations of inhibin A in the women with fetuses affected by Down's syndrome did not appear to be significantly elevated above normal until the end of the first trimester and were not significantly different from normal in the women with fetuses affected by trisomy 18 (P = 0.17). The rate of detection of Down's syndrome was 53 percent and the false positive rate was 5 percent when alpha-fetoprotein, the beta subunit of human chorionic gonadotropin, the maternal age were used together as predictors. The detection rate increased to 75 percent when inhibin A was added (P = 0.002).

CONCLUSIONS

In the second trimester of pregnancy, measuring inhibin A in maternal serum, in combination with measurements of alpha-fetoprotein and beta subunit of human chorionic gonadotropin, significantly improved the rate of detection of Down's syndrome.

Evaluation of maternal serum dimeric inhibin A as a first-trimester marker of Down's syndrome

While second-trimester prenatal screening programmes for Down's syndrome have become established in prenatal care, it would be advantageous to be able to offer screening in earlier pregnancy. To this end, we have evaluated a new potential maternal serum marker, dimeric inhibin A, as a possible first-trimester marker. Dimeric inhibin A was measured in prospectively collected maternal serum from 23 cases of Down's syndrome and matched chromosomally normal controls, at 11-13 weeks' gestation. Levels of this protein were significantly elevated in the Down's pregnancies compared with the control pregnancies. The median multiple of the normal median (MOM) for the Down's samples was 2.46 (95 per cent confidence interval: 2.11-3.26, P < 0.0001 vs. controls). These results suggest that dimeric inhibin A is a useful discriminator of Down's-affected pregnancies from normal pregnancies in the first trimester and that sensitive screening in combination with maternal age and other possible markers may be practicable in the first trimester.