Maternal serum inhibin levels in second-trimester Down's syndrome pregnancies

Prospective study of the feasibility and effectiveness of a second-trimester quadruple test for Down syndrome in Thailand

OBJECTIVE

To evaluate the feasibility and effectiveness of a quadruple test for Down syndrome in the second trimester of pregnancy in clinical settings in Thailand.

METHODS

From October 2015 to September 2016, a prospective study was undertaken in 19 hospitals in Songkhla province, Thailand. Women with a singleton pregnancy of 14-18 weeks were enrolled and underwent the quadruple test. The risk cutoff value was set at 1:250. All women with a positive test (risk ≥1:250) were offered amniocentesis. Women were followed up until delivery.

RESULTS

Among 2375 women, 206 (8.7%) had a positive quadruple test; 98 (47.6%) of these women voluntarily underwent amniocentesis. Overall, seven pregnancies were complicated with chromosomal abnormalities (2.9 cases in 1000), including four cases of Down syndrome (1.7 in 1000) and three of other abnormalities. The detection, false-positive, and accuracy rates of the quadruple test for Down syndrome were 75.0%, 8.6%, and 91.4%, respectively.

CONCLUSION

The quadruple test was found to be a feasible and efficient method for screening for Down syndrome in the second trimester of pregnancy in a Thai clinical setting. The test should be performed for pregnant women before an invasive test for Down syndrome.

Inhibin at 90: from discovery to clinical application, a historical review

When it was initially discovered in 1923, inhibin was characterized as a hypophysiotropic hormone that acts on pituitary cells to regulate pituitary hormone secretion. Ninety years later, what we know about inhibin stretches far beyond its well-established capacity to inhibit activin signaling and suppress pituitary FSH production. Inhibin is one of the major reproductive hormones involved in the regulation of folliculogenesis and steroidogenesis. Although the physiological role of inhibin as an activin antagonist in other organ systems is not as well defined as it is in the pituitary-gonadal axis, inhibin also modulates biological processes in other organs through paracrine, autocrine, and/or endocrine mechanisms. Inhibin and components of its signaling pathway are expressed in many organs. Diagnostically, inhibin is used for prenatal screening of Down syndrome as part of the quadruple test and as a biochemical marker in the assessment of ovarian reserve. In this review, we provide a comprehensive summary of our current understanding of the biological role of inhibin, its relationship with activin, its signaling mechanisms, and its potential value as a diagnostic marker for reproductive function and pregnancy-associated conditions.

Second-trimester maternal serum quadruple test for Down syndrome screening: a Taiwanese population-based study

OBJECTIVE

To assess the usefulness of quadruple test screening for Down syndrome in Taiwan.

MATERIALS AND METHODS

Maternal serum concentrations of alpha-fetoprotein, human chorionic gonadotropin, unconjugated estriol, and inhibin A were measured in 21,481 pregnant women from 15 to 20 weeks of gestation.

RESULTS

Of the 21,481 women, 977 returned values greater than the high-risk cut-off value (1 in 270). Most of these women (86.2%) decided to have an invasive procedure for genetic diagnosis. Nine cases of Down syndrome and 19 cases of other chromosomal anomalies were detected prenatally. Two children with Down syndrome were diagnosed after delivery even though a low estimated risk was determined following the quadruple test. The detection rate was 81.8% (nine out of 11 cases), with a 4.4% false-positive rate. The median multiple of the median value for a-fetoprotein, human chorionic gonadotropin, unconjugated estriol and inhibin A were 0.87, 2.34, 0.77 and 2.16, respectively, in affected cases.

CONCLUSION

This is the first study of the quadruple test for Down syndrome in a Chinese population. Our findings suggested that the second-trimester quadruple test provides an effective screening tool for Down syndrome in Taiwan.

Inhibins in female and male reproductive physiology: role in gametogenesis, conception, implantation and early pregnancy

A great deal of new information has arisen in the recent years concerning inhibin physiology and clinical relevance in reproductive medicine. It is now recognized that the two inhibin isoforms, named inhibin A and inhibin B, are produced by the gonads in the course of gamete maturation and in women have a different pattern of secretion throughout the menstrual cycle. Since inhibins are also produced by placenta and fetal membranes, it has been suggested that there is an involvement in physiological adaptation of pregnancy. Evidence from several sources has underlined the clinical usefulness of the measurement of inhibin-related proteins in the diagnosis and follow-up of different fertility disturbances and early pregnancy viability. In the male, inhibin B is produced in the testis, principally by the Sertoli cells. Inhibin B expression and secretion are positively correlated with Sertoli cell function, sperm number, and spermatogenic status and are negatively correlated with FSH. This review covers the most recent advances on the role of inhibins in human reproductive function. Considerable progress in the understanding of inhibin physiology has resulted from selective measurement of the two inhibin molecular forms, named inhibin A and B. Newly recognized alterations of inhibin levels in gynaecological diseases as well as in normal and pathological pregnancy are discussed, with particular emphasis on the potential clinical usefulness of assessing inhibin levels in serum and other biological fluids.

Evaluation of a Dimeric Inhibin-A Assay for Assessing Fetal Down Syndrome: Establishment, Comparison, and Monitoring of Median Concentrations for Normal Pregnancies

Context.—Several studies report the role of dimeric inhibin-A in assessing risk for fetal Down syndrome. The majority, however, use the Serotec inhibin-A assay and not the newer Diagnostic Systems Laboratories inhibin-A enzyme-linked immunosorbent assay (ELISA).

Objectives.—To establish normal gestational age day-specific medians, to compare our results against previous studies pertaining to the inhibin-A ELISA, and to evaluate long-term assay performance.

Design.—Using the inhibin-A ELISA, 100 specimens were assayed for each completed week of gestation for weeks 15 to 20, 50 specimens for 14 weeks, and 54 specimens for 21 weeks or older. Regressed inhibin-A medians were calculated employing a second-degree polynomial fit of the arithmetic medians. Thereafter, inhibin-A ELISA lot comparisons were performed to evaluate consistency.

Results.—Regressed values of 182, 174, 175, 184, 201, and 226 pg/mL resulted for weeks 15 to 20, respectively [pg/mL inhibin-A = 4.1528(gestational age)2 − 136.49(gestational age) + 1294.9]. A comparison with 2 other studies shows our values to be lower overall by 15 ± 11.4% and 16 ± 2.6%. However, variability between kit lots was as high as 30%.

Conclusions.—The equation derived provides for the calculation of gestational age day-specific inhibin-A medians for integration into maternal serum screening programs with a subsequent decrease in false-positives expected and observed. Our medians differ considerably from those of other studies, with limited data, using the Diagnostic Systems assay. However, lot changes since the initial analysis have exhibited similar inconsistencies. Therefore, we recommend that others incorporating the assay into their screening programs carefully establish, monitor, and adjust their medians accordingly as a result of potential variations.

Screening for Down's syndrome in early and late first and second trimester using six maternal serum markers

The efficiency of six maternal serum markers for Down's syndrome (DS), alpha fetoprotein (AFP), human chorionic gonadotropin (hCG), free beta-hCG, pregnancy-associated plasma protein-A (PAPP-A), the proform of eosinophil major basic protein (ProMBP), pregnancy-specific-beta-1-glycoprotein (SP(1)), and combinations thereof, was examined. Discriminant analysis in 156 DS pregnancies and 546 controls defined three effective combinations of serum marker logMoMs (multiples of the median in control samples) in three gestational age windows, i.e. Index I (weeks 7-9) = 0.52 logMoM ProMBP + 0.28 logMoM PAPP-A - logMoM SP(1); Index II (weeks 10-12) = 1.94 logMoM free beta-hCG - logMoM SP(1), and Index III (weeks 15-19) = 0.78 logMoM free beta-hCG + 1.12 logMoM ProMBP - logMoM AFP. The estimated detection rates of indices and age for a false-positive rate (FPR) of 5% were 73% for Index I, 69% for Index II, and 60% for Index III. Including the ultrasound marker nuchal translucency, using a DS at term risk of 1 : 400 as cut-off, the detection rates of the indices increased to 86, 83, and 82% for FPRs of 4.3, 4.1, and 5.8%, respectively. The indices are promising markers for screening for DS.

Molecular weight forms of inhibin A, inhibin B and pro-alphaC in maternal serum, amniotic fluid and placental extracts of normal and Down syndrome pregnancies

BACKGROUND

Inhibin A, an established prenatal marker of Down syndrome (DS), exists in the maternal circulation in a number of isoforms. The present study explored whether specific inhibin A isoforms may be selectively increased in DS, offering the prospect of improved marker performance.

METHODS

Second trimester maternal serum, placental extracts and amniotic fluid (AF) pools from both normal and DS pregnancies were fractionated by a combined immunoaffinity (IA) chromatography, preparative polyacrylamide gel electrophoresis (Prep-PAGE) and electroelution procedure. Inhibins A, B and pro-alphaC were determined in the eluted fractions by specific enzyme-linked immunosorbent assays (ELISAs) and the profiles of immunoactivity (IA) characterized in terms of molecular weight (MW) and percentage recovery.

RESULTS

The MW patterns of inhibin A and pro-alphaC in maternal serum and AF were similar between DS and control pregnancies, both showing peaks between 25-40 k and approximately 65 k. AF contained, in addition, a higher proportion of <30 k inhibins A and B, and <25 k pro-alphaC forms. There were large differences in the inhibin forms present in DS placentae, with more 70 k and less 30-40 k inhibin A than in controls.

CONCLUSIONS

The present data suggest that the processing, cleavage or secretion of inhibin MW forms by the DS placenta differs from normal. However, these differences are not reflected in maternal serum and so improvements in serum screening will not be afforded by measuring specific inhibin A isoforms.

Raised maternal serum placenta growth factor concentration during the second trimester is associated with Down syndrome

OBJECTIVES

To compare early second-trimester maternal serum placenta growth factor concentrations in Down syndrome pregnancies and those in normal pregnancies.

METHODS

A case-control study was performed to evaluate the maternal serum placenta growth factor concentrations in 36 Down syndrome and 320 normal pregnancies with matched gestational age during the second trimester. For the detection of serum concentrations of placenta growth factor, a quantitative sandwich enzyme immunoassay technique (R & D Systems Inc., Minneapolis, Minnesota, USA) was performed.

RESULTS

Using a multiple linear regression model, maternal serum placenta growth factor level was associated with gestational age (p<0.001) and the existence of Down syndrome pregnancy (p<0.001). After converting maternal serum placenta growth factor concentrations of each analyte to multiples of the appropriate gestational median (MoM), placenta growth factor MoM (p<0.001) was revealed to be an independent variable for Down syndrome pregnancies after adjusting for the effects of maternal age (p<0.001), free beta-hCG (p<0.001) and AFP (p=0.014) by multivariate logistic regression analysis.

CONCLUSIONS

Maternal serum placenta growth factor concentration was elevated in Down syndrome pregnancies during the early second trimester. Placenta growth factor might be a novel marker for maternal serum Down syndrome screening.

Changes in inhibins and activin secretion in healthy and pathological pregnancies

Inhibin-related proteins are involved in the control of the feto-maternal communication required to maintain pregnancy. Human placenta, decidua, and fetal membranes are the major sites of production and secretion of activin A, inhibin A and inhibin B in maternal serum, amniotic fluid, and cord blood. The availability of suitable assays developed in the last years has enabled the measurement of inhibins and activin A in their dimeric forms, in order to investigate their role in physiological conditions of pregnancy. The studies conducted on inhibin-related proteins and human pregnancy suggested the possibility of an involvement of inhibin A and activin A in the pathogenesis of gestational diseases. In fact, several lines of evidence underline the potential role and the clinical usefulness of inhibin-related proteins measurement in the diagnosis, prevention, prognosis and follow-up of different gestational pathologies such as early pregnancy viability, Down's syndrome, fetal demise, pre-eclampsia, pregnancy-induced hypertension, preterm delivery and intrauterine growth restriction. The measurement of inhibin A and activin A into the biological fluids of pregnancy will offer in the future, further possibilities in the early diagnosis, prediction, and monitoring diseases of pregnancy.

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.

Maternal serum levels of dimeric inhibin A in pregnancies affected by trisomy 21 in the first trimester

Dimeric inhibin A was measured in maternal serum samples from 45 pregnancies affected by trisomy 21 and 493 samples from unaffected pregnancies at 10-14 weeks of gestation. Inhibin A levels in affected pregnancies were compared with levels of free beta-hCG and PAPP-A in the same series. In the trisomy 21 group, the median multiple of the median (MoM) inhibin A was not significantly elevated (1.28 vs 1.00) with only 15.5% being above the 95th centile. In contrast, the median MoM free beta-hCG was significantly increased (2.05 vs 1.00) with 36% above the 95th centile and PAPP-A was significantly reduced (0.49 vs 1.00) with 42% below the 5th centile. Inhibin A levels in the trisomy 21 group were significantly correlated with gestational age such that median levels rose from 1.04 at 11 weeks to 1.30 at 12 weeks and 1.67 at 13 weeks. These findings suggest that first trimester biochemical screening for trisomy 21, which is currently optimised using maternal serum free beta-hCG and PAPP-A and fetal nuchal translucency, will not benefit from the inclusion of inhibin A.

Triple marker screening for trisomy 21, trisomy 18 and open neural tube defects in singleton pregnancies of native Japanese pregnant women

OBJECTIVE

To report the results of prenatal triple marker screening on a population of Japanese pregnant women.

METHODS

From April 1994 through March 1999, a total of 32,925 native Japanese women with singleton pregnancies requested a triple marker-screening test. Multiples of the median values for 3 markers and individual risks for each patient were calculated following adjustment for the Japanese weight correction factor. The risk cut-off values used for Down syndrome (T21), open spina bifida (OSB) and trisomy 18 (T18) were 1: 295, 1: 290, and 1: 100, respectively. Follow-up information was collected postpartum and statistically analyzed.

RESULTS

Detection rates (DR) of T21 for women less than 35 years, over 35 years and overall were 58, 94, and 83%, respectively. DR of T18 for women less than 35 years, over 35 years and overall were 75, 79, and 79%, respectively. DR of open neural tube defects (ONTD) was 100%.

CONCLUSIONS

The first cumulative data of an intervention program and prospective follow-up studies in Japan have proven to be similar to other published reports. Individual risk values were calculated for each pregnancy for T21, T18 and ONTD. This screening program is more effective than age-dependent screening for detecting T21, T18 and ONTD pregnancies.

Dimeric inhibin A as a fourth marker for Down's syndrome maternal serum screening in native Japanese women

OBJECTIVE

This study was conducted to assess the usefulness of dimeric inhibin A as a fourth marker for Down's syndrome screening in addition to AFP, hCG and uE3 markers for native Japanese women.

METHODS

Serum specimens from 367 native Japanese women in the second trimester were assayed for dimeric inhibin A levels. Day specific dimeric inhibin A medians were established for gestational ages 15.0-21.9. Weekly median values for the native Japanese were compared with those of a U.S. population. Selected Japanese specimens from 15 diagnosed Down's syndrome and 3 trisomy 18 cases were also assayed for dimeric inhibin A.

RESULTS

Dimeric inhibin A levels did not vary greatly over the gestational age range as expected. Median value comparison showed that native Japanese dimeric inhibin A medians are higher than the U.S. population medians by an average of 7.95%. Native Japanese dimeric inhibin A median values in this study are 1.77 times higher in Down's syndrome cases than in unaffected pregnancies. Trisomy 18 dimeric inhibin A levels show no significant difference from the unaffected pregnancies.

CONCLUSIONS

This report shows for the first time that dimeric inhibin A can be informative as a fourth marker for Down's syndrome screening in native Japanese women. We expect the addition of dimeric inhibin A to a triple marker protocol will increase the accuracy of predicted risk for all pregnancies screened and increase the detection rate of Down's syndrome affected pregnancies.

Second-trimester maternal serum inhibin-A screening for fetal Down syndrome in Asian women

Case control studies in the Caucasian population showed that maternal serum inhibin-A is elevated in Down syndrome pregnancies and may be a useful second-trimester marker in addition to human chorionic gonadotrophin (hCG) and alpha-fetoprotein (AFP). Data in the Asian population are lacking. We measured inhibin-A levels in the stored maternal sera of 49 Down syndrome pregnancies and 341 controls with a commercially available assay and expressed them as the multiples of the median of the gestational week. The log means and standard deviations for case and control inhibin-A MOMs were 0.209, 0.226, and 0.002 and 0.177, respectively. Median inhibin-A MOM in Down syndrome cases was elevated to 1.62 (95 per cent confidence interval, 1.29-1.82). 36 per cent of Down syndrome cases were expected to be detected at a 5 per cent false-positive rate. However, inhibin-A MOMs were strongly correlated with hCG MOMs in the cases (r=0.73, p<0.001) and the controls (r=0.56, p<0.001). This will diminish the value of adding inhibin-A to the existing hCG and AFP screening protocol.

Advance in the study of inhibin, activin and follistatin production in pregnant women

This review summarizes the new information on the studies of inhibin, activin, and follistatin production in the placenta during human pregnancy. Inhibin and activin exert suppressive and stimulatory effects, respectively, on the release of FSH in the pituitary. Follistatin is bound to inhibin and activin and indirectly modulates the FSH release. The placenta produces these three proteins. The serum levels of inhibin, activin, and follistatin are elevated in pregnant women and decrease after delivery. The trophoblast cells from term placenta secrete inhibin and activin in the primary cultures. The production and mRNA expression of inhibin and activin are regulated by several stimulatory and suppressive hormones and growth factors in placental tissues. cAMP, Ca2+, and protein kinase-C may be involved in intracellular signal transduction in trophoblasts. Activin receptors are present on placental cells. Follistatin inhibits the binding of activin to ActRII receptor. Abnormal levels of inhibin and activin in maternal serum are observed in problem pregnancies and gestational diseases. Inhibin, activin, and follistatin may play roles in the regulation of reproductive endocrinology in pregnant women and the embryo/fetal development.

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

Inhibin, activin and follistatin in the human placenta--a new family of regulatory proteins

The family of inhibin-related proteins has been investigated extensively in the last decade. It is composed of three members: inhibin, activin and follistatin. Inhibin and activin are chemically related, while follistatin acts as an activin-binding protein. Initially identified as regulators of pituitary follice stimulating hormone (FSH) secretion, inhibin, activin and follistatin have more recently been characterized as growth factors, embryo modulators and immune factors. Human placenta, amnion, chorion and maternal decidua express mRNAs for inhibin, activin and follistatin, and the presence of both immunoreactive and bioactive proteins has been demonstrated. The proteins are present in maternal and fetal circulation, and are measurable in amniotic fluid with changes related to gestational age and to the occurrence of gestational diseases. Various biological actions have been described in embryo and intrauterine tissues, which suggest a role for these proteins in the development of the gestational unit. However, several questions remain to be elucidated. The chemical forms of inhibin, activin and follistatin produced by human placenta and the mechanisms involved in the regulation of their secretion are largely unknown. The nature of the receptors for these proteins and the physiological implications of receptor activation have not yet been elucidated and this will require further investigation.

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.

Second trimester levels of maternal serum inhibin A, total inhibin, alpha inhibin precursor, and activin in Down's syndrome pregnancy

OBJECTIVE

To determine the levels of various biochemical forms of the placental protein, inhibin (total inhibin, inhibin A, and alpha inhibin precursor) and activin in maternal serum samples from fetal Down's syndrome, and to determine which of these analytes most effectively identifies samples from affected pregnancies.

METHODS

Maternal serum samples were collected from 100 unaffected pregnancies and 20 cases of fetal Down's syndrome during gestational weeks 15-20 for routine triple marker screening, and were stored frozen after clinical assay. Levels of inhibin A, total inhibin, alpha inhibin precursor (pro-alphaC), and activin were compared retrospectively in the Down's syndrome cases and control samples.

RESULTS

There was no association of the inhibin or activin levels with gestational age or length of freezer storage, and therefore single median values were determined for the unaffected pregnancies for each analyte. Multiples of the unaffected median (MoM) values were calculated for all cases, showing that inhibin A (1.95 MoM) provided the best discrimination between cases and controls, followed by total inhibin (1.37 MoM). Mann-Whitney U analysis showed significant group differences in inhibin A (P = 0.0001) and total inhibin (P = 0.0005). In contrast, alpha inhibin precursor (0.81 MoM) and activin (1.16 MoM) levels in Down's syndrome cases were not significantly different from those in unaffected patients.

CONCLUSIONS

Levels of inhibin A and total inhibin, but not alpha inhibin precursor or activin, are significantly raised in maternal serum from cases of fetal Down's syndrome. These data, taken together, indicate that inhibin A levels are specifically raised in Down's syndrome pregnancy. 45% of the inhibin A levels in the Down's syndrome samples were above the 90th centile of unaffected levels, indicating that inhibin A may be as good a marker as human chorionic gonadotrophin, the most informative serum marker currently in use.

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.

Prenatal screening for Down's syndrome using inhibin-A as a serum marker

The value of measuring inhibin-A (a beta A dimer) with human chorionic gonadotrophin (total or the sub-units free a-hCG and free beta-hCG separately), alpha-fetoprotein (AFP), and unconjugated oestriol (uE3) was examined to determine the effect on the performance of serum screening for Down's syndrome between 15 and 22 weeks of pregnancy. The study was based on stored serum samples from 77 Down's syndrome singleton pregnancies and 385 unaffected singleton pregnancies, matched for maternal age, gestational age, and duration of storage of the sample, supplemented by data from 970 white women with unaffected pregnancies. Inhibin-A was elevated in the serum of women with Down's syndrome pregnancies with a median of 1.79 multiples of the median (MOM). Using the four serum markers AFP, uE3, total hCG, and inhibin-A, in addition to maternal age, 70 per cent of Down's syndrome pregnancies were detected for a 5 per cent false-positive rate compared with 59 per cent with the conventional triple test (AFP, uE3, and total hCG with maternal age). If the estimate of gestational age were based on an ultrasound scan examination, the detection rate would be 77 per cent [95 per cent confidence interval (CI) 69-85 per cent] using the four serum markers including inhibin-A, compared with 67 per cent with the triple test or 79 per cent (95 per cent CI 71-87 per cent) if marker values were adjusted for maternal weight. If the detection rate were kept at 70 per cent and the gestational age were estimated by an ultrasound scan examination, the four-marker test would reduce the false-positive rate from 6-1 per cent using the triple test to 2-9 per cent. The results were virtually the same if free beta-hCG was used instead of total hCG. The inhibin-A-based four-marker test is the most effective method of prenatal screening for Down's syndrome suitable for routine use. If the extra cost required to carry out the inhibin-A test were less than about [symbol: see text]3 per woman screened, the four-marker test including inhibin-A would be financially cost-effective.

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.

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.