Urinary screening tests for fetal Down syndrome: II. Hyperglycosylated hCG

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.

Direct and rapid mass spectral fingerprinting of maternal urine for the detection of Down syndrome pregnancy

Background

The established methods of antenatal screening for Down syndrome are based on immunoassay for a panel of maternal serum biomarkers together with ultrasound measures. Recently, genetic analysis of maternal plasma cell free (cf) DNA has begun to be used but has a number of limitations including excessive turn-around time and cost. We aimed to develop an alternative method based on urinalysis that is simple, affordable and accurate.

Method

101 maternal urine samples sampled at 12–17 weeks gestation were taken from an archival collection of 2567 spot urines collected from women attending a prenatal screening clinic. 18 pregnancies in this set subsequently proved to be Down pregnancies. Samples were either neat urine or diluted between 10 to 1000 fold in dH₂O and subjected to matrix assisted laser desorption ionization (MALDI), time of flight (ToF) mass spectrometry (MS). Data profiles were examined in the region 6,000 to 14,000 m/z. Spectral data was normalised and quantitative characteristics of the profile were compared between Down and controls.

Results

In Down cases there were additional spectral profile peaks at 11,000-12,000 m/z and a corresponding reduction in intensity at 6,000-8,000 m/z. The ratio of the normalised values at these two ranges completely separated the 8 Down syndrome from the 39 controls at 12–14 weeks. Discrimination was poorer at 15–17 weeks where 3 of the 10 Down syndrome cases had values within the normal range.

Conclusions

Direct MALDI ToF mass spectral profiling of maternal urinary has the potential for an affordable, simple, accurate and rapid alternative to current Down syndrome screening protocols.

Hyperglycosylated hCG

Hyperglycosylated hCG (hCG-H) is a glycosylation variant of the hormone hCG. Here we review all that is known about this independently functioning molecule. As discussed, it is a very different molecule to the hormone hCG. First, hCG-H is produced by cytotrophoblast cells while regular hCG is made in syncytiotrophoblast cell. Second, it is an autocrine acting directly on the cells which produce it, while regular hCG is an endocrine acting on maternal corpus luteal cells. Third, hCG-H has minimal biological activity in promoting progesterone production compared to regular hCG. Fourth, hCG-H functions unlike regular hCG as an invasion promoter, whether invasion as in choriocarcinoma and testicular germ cell malignancies, or as in implantation of pregnancy. These functions seemingly occur through action on cytotrophoblast cell TGFbeta receptors. Fifth, hCG-H is an essential component for successful human implantation to prevent early pregnancy loss and spontaneous abortion. Sixth, hCG-H is critical for promoting the midtrimester hemochorial implantation, and for preventing preeclampsia. Seventh, measurements of hCG-H have advantages over measurements of regular hCG or total hCG, in detecting pregnancy, pregnancy outcome (failing or term pregnancy), predicting preeclampsia in pregnancy, or as a tumor marker for gestational trophoblastic diseases.

Hyperglycosylated HCG expression in pregnancy: cellular origin and clinical applications

Employing a monoclonal antibody (B152) specific for a carbohydrate epitope found on a choriocarcinoma derived hCG, it was discovered that a similar hCG isoform is expressed during early pregnancy. This form differs from later pregnancy hCG in carbohydrate moieties. Profiling of these two hCG isoforms throughout pregnancy utilized two IRMA's: B152-B207 ("hyperglycosylated hCG"-specific assay) and B109-B108 (an IRMA for standard intact hCG isoforms in the WHO hCG reference preparation). The WHO hCG standard was used in both assays. Values were presented as a ratio of hCG isoform concentrations (B152/B109 ratio). In early pregnancy urine concentrations of B152 hCG were significantly higher in normal pregnancy (NP) compared to early pregnancy loss (EPL). Matched serum-urine samples from the first and third trimesters revealed that the B152 hCG form is predominant in both serum and urine in the first trimester compared with the third trimester. The proportion of the B152 hCG (HhCG) form is higher in urine than in matched serum. There was a significant difference in the B152/B109 ratio between days 5 and 20 from time of embryo transfer in normally developing pregnancy versus EPL in the urine of IVF patients. In spontaneous abortion (SA) the level of B109 hCG remained higher in NP compared with SA. However, the B152/B109 ratio declined with gestational age faster in SA than in NP suggesting perhaps a different loss mechanism in SA versus EPL. The cellular origin of the different hCG glycoforms was identified by assay of cell media from cytotrophoblasts (CTBs) and syncytiotrophoblasts (STBs). Isolated CTBs expressed predominantly HhCG. The level of expression was the highest in the first trimester. STBs were the source of the less glycosylated B109 hCG isoform. Analysis of hCG glycoforms during early pregnancy can distinguish pregnancies that will fail from those that will proceed normally. Since the B152 assay does not effectively discriminate between intact HhCG and free beta HhCG (HhCGbeta), a new HhCGbeta assay was developed. This assay recognizes the HhCGbeta which is produced by CTBs. We hypothesize that the measurement of HhCGbeta may have a potential use in screening for Down syndrome and perhaps other pregnancy disorders and certain types of cancer.

Hyperglycosylated hCG: a variant with separate biological functions to regular hCG

Hyperglycosylated hCG (hCG-H) is an over-glycosylated variant of hCG. While regular hCG is produced by differentiated syncytotrophoblast cells, hCG-H is independently secreted by stem cytotrophoblast cells. hCG-H has an independent function to regular hCG. It is the direct promoter of trophoblast invasion and malignancy. Invasion as in implantation of pregnancy and malignancy as in choriocarcinoma. Neither will occur in the absence of hCG-H. hCG-H measurements have multiple functions. Primarily or exclusively hCG-H is produced at the time of implantation of pregnancy and in the 2 weeks that follows. As such, a good pregnancy test should measure regular hCG and hCG-H equally. This is not commonly the case. Most tests poorly detect hCG-H. New pregnancy tests are needed, including those that measure only hCG-H. Considering that hCG-H is critical for implantation, hCG-H is also invaluable for determining pregnancy outcome and detecting failures. It makes a considerable more accurate test for detecting pregnancy failures and ectopic pregnancies than regular hCG. Down syndrome pregnancies are marked by poor trophoblast differentiation. As such, they are very well marked by using a combination of hCG-H measurements and other screening tests. hCG-H is also an absolute tumor marker for malignant or invasive gestational trophoblastic disease, it can discriminate active and inactive (quiescent) disease, and the need for chemotherapy.

A single serum test for measuring early pregnancy outcome with high predictive value

OBJECTIVES

Current testing to determine a failing pregnancy requires two separate clinic visits to measure the hCG doubling rate. Diagnosing a failing pregnancy is often done in emergency departments where simplified and accelerated testing methods are needed. Here, we investigated hyperglycosylated hCG (hCG-H) for predicting pregnancy failure.

DESIGN AND METHODS

We studied two independent sets of patient samples collected in the early weeks of gestation. One set was urine samples, and the other was serum samples. In all cases, hCG and hCG-H were measured using automated chemiluminescence immunoassays. Concentrations of hCG and hCG-H were plotted on a scattergram, and levels in failing pregnancies were compared to those in continuing pregnancies.

RESULTS

Data indicated that a threshold level of hCG-H (13 microg/L) in both serum and urine samples defined the concentration below where pregnancies were likely to fail. This cut-off corresponded to 73% detection of failures at a 2.9% false positive rate using serum and 75% detection at a 15% false positive rate using urine. Using an hCG cut-off that corresponded to the same false positive rates, hCG detected only 42% of failures using serum and 43% of failures using urine.

CONCLUSIONS

Our data indicate that hCG-H provides a much more accurate single test than hCG for assessing pregnancy outcome. Compatible with the use of serum or urine samples, a single hCG-H test might provide simpler, faster, and more accurate results for predicting the progress of a pregnancy than standard hCG testing.

Biochemical screening for congenital defects

This article discusses biochemical screening for congenital defects. Biochemical screening remains the standard for estimating the risk of aneuploidy in pregnancy. The combination of first and second trimester markers promises to further improve diagnostic accuracy for anomaly detection.

Population genetic screening programmes: principles, techniques, practices, and policies

This paper examines the professional and scientific views on the principles, techniques, practices, and policies that impact on the population genetic screening programmes in Europe. This paper focuses on the issues surrounding potential screening programmes, which require further discussion before their introduction. It aims to increase, among the health-care professions and health policy-makers, awareness of the potential screening programmes as an issue of increasing concern to public health. The methods comprised primarily the review of the existing professional guidelines, regulatory frameworks and other documents related to population genetic screening programmes in Europe. Then, the questions that need debate, in regard to different types of genetic screening before and after birth, were examined. Screening for conditions such as cystic fibrosis, Duchenne muscular dystrophy, familial hypercholesterolemia, fragile X syndrome, hemochromatosis, and cancer susceptibility was discussed. Special issues related to genetic screening were also examined, such as informed consent, family aspects, commercialization, the players on the scene and monitoring genetic screening programmes. Afterwards, these questions were debated by 51 experts from 15 European countries during an international workshop organized by the European Society of Human Genetics Public and Professional Policy Committee in Amsterdam, The Netherlands, 19-20, November, 1999. Arguments for and against starting screening programmes have been put forward. It has been questioned whether genetic screening differs from other types of screening and testing in terms of ethical issues. The general impression on the future of genetic screening is that one wants to 'proceed with caution', with more active impetus from the side of patients' organizations and more reluctance from the policy-makers. The latter try to obviate the potential problems about the abortion and eugenics issues that might be perceived as a greater problem than it is in reality. However, it seems important to maintain a balance between a 'professional duty of care' and 'personal autonomy'.

Invasive Trophoblast Antigen (Hyperglycosylated Human Chorionic Gonadotropin) in Second-Trimester Maternal Urine as a Marker for Down Syndrome: Preliminary Results of an Observational Study on Fresh Samples

Background: Down syndrome screening is commonly performed in the US using maternal age and three or four second-trimester maternal serum markers that can identify up to 75% of affected pregnancies by offering diagnostic studies to 5% of women. Invasive trophoblast antigen [ITA; hyperglycosylated human chorionic gonadotropin (hCG)] is a promising marker that can be measured in urine or serum in the first or second trimester. We report preliminary results for urinary ITA in an ongoing observational study.

Methods: Women undergoing second-trimester amniocentesis for reasons not associated with biochemical testing provided consent and a urine (and possibly serum) sample that was tested within a few days. Demographic and pregnancy-related information was collected, along with karyotype. Screening performance was modeled for ITA alone and in combination with serum markers

Results: Twelve recruitment centers collected urine from 2055 women with singleton pregnancies between 15 and 20 weeks of gestation (2023 unaffected, 28 Down syndrome, and 4 pregnancies with other chromosome abnormalities). After correction for gestational age, urine concentration, and maternal race and weight, the ITA measurements were higher in women with a Down syndrome pregnancy (median ITA, 4.33 multiples of the median). At a 75% detection rate, the false-positive rate could be reduced by substituting ITA for hCG measurements (from 5.6% to 2.6% for the triple test) or by adding ITA measurements to existing combinations (from 3.3% to 2.0% for the quadruple test).

Conclusions: Our data provide preliminary confirmation of the potential usefulness of urinary ITA measurements in detecting Down syndrome in a setting that simulates routine usage.

Detection of maternal serum hCG glycoform variants in the second trimester of pregnancies affected by Down syndrome using a lectin immunoassay

AIM

To assess whether glycoform variants of human chorionic gonadotrophin (hCG) are present in altered concentrations in the maternal serum in pregnancies affected by Down syndrome.

METHODS

In a series of 50 cases of pregnancies complicated by Down syndrome and 278 unaffected pregnancies, we have examined maternal serum levels of hCG glycoforms (GlyhCG) in samples collected in the second trimester (14 to 21 weeks) using a sialic acid binding lectin immunoassay. We have compared these levels with those of other second trimester serum markers (Free beta-hCG, alpha fetaprotein (AFP) and Total hCG) and modelled detection rates and false positive rates of various biochemical markers in conjunction with maternal age using a maternal age standardized population.

RESULTS

Maternal serum GlyhCG in cases of Down syndrome was significantly elevated (Median MoM 1.81) with 15 of 50 (30%) cases above the 95th centile for unaffected pregnancies. Free beta-hCG was also elevated (Median MoM 2.16) with 18 of 50 (36%) cases above the 95th centile. AFP levels were reduced (Median MoM 0.75) with 9 of 50 (18%) cases below the 5th centile. Total hCG levels whilst elevated (Median MoM 1.88) had only 15 of 50 (30%) cases above the 95th centile. Maternal serum GlyhCG levels showed significant correlation with total hCG and free beta-hCG (r = 0.6880 and 0.6922) in the Down group but not with AFP (r = 0.1237). When GlyhCG was combined together with AFP and maternal age, at a 5% false positive rate, the modelled detection rate was 53%, some 13% lower than when free beta-hCG was used and some 7% lower than when total hCG was used.

CONCLUSION

Maternal serum GlyhCG, as measured by the sialic acid-binding lectin immunoassay is unlikely to be of additional value when screening for Down syndrome in the second trimester.

Maternal serum hyperglycosylated human chorionic gonadotrophin (HhCG) in the first trimester of pregnancies affected by Down syndrome, using a sialic acid-specific lectin immunoassay

In a series of 54 cases of pregnancies complicated by Down syndrome and 224 unaffected pregnancies we examined maternal serum levels of hyperglycosylated human chorionic gonadotrophin (HhCG) in samples collected in the first trimester (11-13 weeks) using a sialic acid-specific lectin immunoassay. We compared these levels with those of other potential first trimester serum markers [free beta-hCG, pregnancy-associated plasma protein A (PAPP-A) and total hCG (ThCG)] and modeled detection rates and false-positive rates of various biochemical markers in conjunction with fetal nuchal translucency (NT) and maternal age using an maternal age standardized population. Maternal serum HhCG in cases of Down syndrome were significantly elevated (median MoM 1.97) with 24/54 (44%) of cases above the 95th centile for unaffected pregnancies. Free beta-hCG was also elevated (median MoM 2.09) with 33% of cases above the 95th centile. PAPP-A levels were reduced (median MoM 0.47) with 38% below the 5th centile. ThCG levels, whilst elevated (median MoM 1.34), had only 20% of cases above the 95th centile. Maternal serum HhCG levels were not correlated with fetal NT but showed significant correlation with ThCG and free beta-hCG and with PAPP-A in the Down syndrome group (r=0.536). Maternal serum HhCG levels in cases with Down syndrome had a significant correlation with gestational age, increasing as the gestation increased. When HhCG was combined together with fetal NT, PAPP-A and maternal age, at a 5% false-positive rate the modeled detection rate was 83%, some 6% lower than when free beta-hCG was used and some 4% better than when ThCG was used. Maternal serum HhCG is unlikely to be of additional value when screening for Down syndrome in the first trimester.

The comprehensive midtrimester test: high-sensitivity Down syndrome test

OBJECTIVE

The purpose of this study was to develop a highly sensitive algorithm for midtrimester Down syndrome detection.

STUDY DESIGN

Urine (hyperglycosylated human chorionic gonadotropin, beta-core fragment of human chorionic gonadotropin), serum (alpha-fetoprotein, human chorionic gonadotropin and unconjugated estriol [uE(3)]), and ultrasound biometry (nuchal thickness, humerus length, the presence of gross ultrasonographic anomalies), and maternal age were measured at genetic amniocentesis. Stepwise logistic regression analysis was used to identify the most significant markers. A multivariate Gaussian algorithm plus age was used to derive patient-specific Down syndrome risk. Sensitivity and false-positive rates at different risk thresholds and the area under the receiver-operating characteristic curve were determined. A probability value of <.05 was significant.

RESULTS

There were 568 study cases with 17 Down syndrome cases (3.0%). The mean (+/-SD) maternal and gestational ages for the study group were 36.9 (+/-3.5) years and 16.2 (+/-1.4) weeks, respectively. The significant markers were nuchal thickness (P =.0001), hyperglycosylated human chorionic gonadotropin(P <.001), and beta-core fragment (P <.002). Neither maternal age nor gross sonographic anomaly contributed significantly to Down syndrome detection. The comprehensive midtrimester test was extremely efficient for Down syndrome detection in advanced maternal age only cases with a sensitivity of 92.3% at a 0.8% false-positive rate. In women <35 years old, all the Down syndrome cases were detected at 2.2% false positive rate. For the overall population, the sensitivity was 93.7% at 5% false-positive rate.

CONCLUSION

In a preliminary study, the comprehensive midtrimester test appeared highly sensitive in different age groups. Gross anomaly detection was not required for high performance, which makes the comprehensive midtrimester test potentially suitable for low-risk screening and as an alternative to amniocentesis in women who wish to avoid the procedure. This was a small study; thus, the clinical value of this test can only be established in large trials.

Urinary hyperglycosylated hCG in first trimester screening for chromosomal abnormalities

Hyperglycosylated human chorionic gonadotrophin (H-hCG), also known as Invasive Trophoblast Antigen or ITA, is a unique metabolic variant of hCG with more complex oligosaccharide side chains. Concentrations are independent of regular hCG. Urine H-hCG has recently proved to be a highly sensitive marker for Down syndrome screening in the second trimester of pregnancy. We evaluated H-hCG as a potential marker in the first trimester of pregnancy. Maternal urine samples were collected from 10(+0) to 11(+6) weeks of gestation prior to genetic analysis and stored in frozen form. Samples from eight cases of Down syndrome, two cases of trisomy 13, one case of trisomy 18, and 55 control pregnancies were hand-carried frozen to the USA and tested blindly. Samples were tested in a specific H-hCG immunoassay and values were normalized to creatinine concentration. Values were plotted against gestational age, and multiples of control pregnancy median (MoM) calculated. The median level of the MoMs of the eight Down syndrome cases was 3.6 MoM. Five of the eight Down syndrome cases exceeded the 90th centile of the 55 unaffected cases. The MoMs of the trisomy 13 and 18 pregnancies were 0.2, 0.2 and 0.3. All three cases were under the 10th centile of unaffected pregnancies. The results of this study indicate that H-hCG testing may be useful in screening for Down syndrome in the first trimester of pregnancy. Further studies are needed to assess the potential screening values of urine H-hCG and the combination of this test with free beta-subunit, PAPP-A and other markers for Down syndrome in the first trimester of pregnancy.

Determination of the Glycoforms of Human Chorionic Gonadotropin β-Core Fragment by Matrix-assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry

Background: Metabolism of human chorionic gonadotropin (hCG) in the serum and kidney yields the terminal urinary product hCG β-core fragment (hCGβcf), comprising two disulfide-linked peptides (β6-β40 and β55-β92) of which one (β6-β40) retains truncated N-linked sugars. Hyperglycosylated hCGβcf may indicate choriocarcinoma or Down syndrome, but the glycosylation profile of hCGβcf has not been thoroughly evaluated.

Methods: hCGβcf, purified from pregnancy urine, was reduced by “on-target” dithiothreitol (DTT) reduction and analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The mass ([M+H]+) of the primary sequence of the glycosylated peptide β6-β40 was subtracted from the m/z values of the discrete peaks observed to give the masses of the carbohydrate moieties. Carbohydrate structure was predicted by sequentially subtracting the masses of the monosaccharide residues corresponding to N-linked carbohydrates of the hCG β-subunit reported in the literature.

Results: Mass spectra of hCGβcf revealed a broad triple peak at m/z 8700–11300. After reduction, the triple peak was replaced by a discrete set of peaks between m/z 4156 and 6354. A peak at m/z 4156.8 corresponded to the nonglycosylated peptide (β55-β92). The remaining nine peaks indicated that urinary hCGβcf comprises a set of glycoforms smaller and larger than the trimannosyl core.

Conclusions: hCGβcf comprises a wider set of glycoforms than reported previously. Peaks of highest mass indicate evidence of hyperglycosylated carbohydrate moieties. The data support previous reports that hCGβcf oligosaccharides lack sialic acid and galactose residues. No indication was found of a β6-β40 peptide that was entirely devoid of carbohydrate.

Comparison of urinary hyperglycosylated human chorionic gonadotropin concentration with the serum triple screen for Down syndrome detection in high-risk pregnancies

OBJECTIVE

Both modest screening performance and declining patient and physician acceptance have stimulated interest in alternative markers to the triple screen for the detection of Down syndrome. Our purpose was to compare the concentration of a single urinary analyte, hyperglycosylated human chorionic gonadotropin, with the serum triple screen (alpha-fetoprotein, human chorionic gonadotropin, and unconjugated estriol concentrations combined with age) for second-trimester Down syndrome detection.

STUDY DESIGN

Urine and blood were obtained from pregnant women in the second trimester undergoing genetic amniocentesis. Urinary hyperglycosylated human chorionic gonadotropin concentration and serum triple-screen values were measured. Individuals undergoing amniocentesis because of abnormal triple-screen results were excluded. Individual Down syndrome risks on the basis of urinary hyperglycosylated human chorionic gonadotropin concentration plus maternal age and on the basis of the triple-screen results were calculated. For each algorithm the sensitivity and false-positive rate for Down syndrome detection at different risk thresholds were determined. From these values receiver operating characteristic curves were constructed, and the area under the curve was determined for each algorithm. Finally, the performance of a new combination in which urinary hyperglycosylated human chorionic gonadotropin concentration replaced serum human chorionic gonadotropin concentration in the triple screen was ascertained.

RESULTS

We studied 24 pregnancies complicated by Down syndrome and 500 unaffected pregnancies between 14 and 22 weeks' gestation in a mostly white (93.5%) population undergoing amniocentesis primarily because of advanced maternal age. The sensitivity and false-positive rate for urinary hyperglycosylated human chorionic gonadotropin concentration were 75. 0% and 5.6%, respectively, whereas those for the triple screen were 75.0% and 33.2%, respectively. Urinary hyperglycosylated human chorionic gonadotropin concentration was superior to the triple screen (area under the curve, 0.9337 vs 0.7887; P =.02). The substitution of urinary hyperglycosylated human chorionic gonadotropin concentration for serum human chorionic gonadotropin concentration in the triple screen resulted in a 91.7% sensitivity at a 10.0% false-positive rate, versus a 54.2% sensitivity for the traditional triple screen at the same false-positive rate.

CONCLUSION

The performance of urinary hyperglycosylated human chorionic gonadotropin concentration was statistically superior to that of the serum triple screen in a high-risk population. The use of urinary hyperglycosylated human chorionic gonadotropin concentration as an alternative test or substitution of this measurement for serum human chorionic gonadotropin concentration in the triple screen would improve diagnostic accuracy and address many current concerns related to the triple screen.

Advances in antenatal screening for Down syndrome

Antenatal screening for Down Syndrome using maternal age alone is no longer an adequate standard of care. Screening in the early second trimester of pregnancy (between 15 and 20 weeks of pregnancy) using the quadruple test can identify 76% of affected pregnancies with a 5% false-positive rate. Screening in the first trimester of pregnancy (between 10 and 13 weeks of pregnancy) is possible using two biochemical markers (PAPP-A and free beta-hCG) together with an ultrasound marker nuchal translucency measurement; using these three markers together with maternal age can identify 85% of affected pregnancies, with a 5% false-positive rate. While there is debate over issues involved in choosing between first and second trimester screening, the most effective screening test for Down Syndrome is the integrated test based on the integration of the first trimester and the second trimester markers. This has a 94% detection rate for a 5% false-positive rate. If the false-positive rate were set at 1%, the detection rate would be 85%. No other screening test for Down Syndrome can detect such a high proportion of affected pregnancies with such a low false-positive rate.

Serum expression of sialyltransferase in normal and Down's syndrome-affected pregnancy

Human chorionic gonadotrophin (hCG) has been used as a serum biochemical marker of Down's syndrome (DS) pregnancies, and recent studies have reported an increase in a hyperglycosylated form of the protein associated with DS. In the present study, the activity of the combined soluble serum sialyltransferase (ST) enzymes was examined as a potential marker for the hyperglycosylated form of hCG. There were no changes in total ST activity during the course of the pregnancy in normal mothers and there was no difference in enzyme activity in the DS pregnancies when compared with controls. These results suggest that, although there may be an alteration in the activities of one or more of the glycosyltransferase enzymes in the placenta resulting in an alteration in hCG glycosylation, this is not reflected in the serum soluble enzyme activity.

The development of a sialic acid specific lectin-immunoassay for the measurement of human chorionic gonadotrophin glycoforms in serum and its application in normal and Down's syndrome pregnancies

OBJECTIVE

We have developed and validated a lectin-immunoassay for the recognition of sialic acid residues on hCG glycoforms in serum.

DESIGN

This assay employs a hCG specific capture antibody and a sialic acid specific lectin (Wheat Germ Agglutinin) labelled with horse radish peroxidase.

RESULTS

The standard curve covered hCG concentrations of 0-4000 IU/l (3rd IS for hCG, 75/537) with an analytical sensitivity of 1.0 IU/l. The within and between batch coefficient of variation was < 7% for all doses. Cross-reactivity of < 1% with TSH, LH, FSH, hCGalpha, hCGbeta and desialylated hCG confirmed assay specificity. Dilutions of serum of < 10% final concentration were parallel to the standard curve (within and between batch CV, < 6%). The assay working range was 100 - > 500 000 IU/l and the recovery of hCG from serum was in the range of 94.5% to 115.4%, with a mean value of 102.1%. The assay detected a time dependent change in hCG sialylation during normal pregnancy with the relative abundance of sialylated hCG declining after week 9 and increasing after week 15 of gestation. In addition preliminary studies showed that maternal serum hCG concentrations measured with the lectin-immunoassay were elevated in high risk Down's pregnancies (as defined by conventional screening tests between weeks 16-18 gestation, median multiple of median, 3.14; range 1.81-19.12, P < 0. 001) and low risk (1.57, 0.49-6.14, P = 0.034) compared to normal (1. 00, 0.32-3.20) pregnancies. Furthermore, the lectin immunoassay had greater discriminatory power compared to conventional immunoassay of hCG and hCGbeta between normal and both low and high risk Down's pregnancies.

CONCLUSION

This assay will allow analysis of serum samples for the investigation of sialylated variants of hCG glycoforms in various pathological and physiological situations.

Combined ultrasound biometry, serum markers and age for Down syndrome risk estimation

OBJECTIVE

To compare Down syndrome screening efficiency of the standard serum triple analyte screen to that of a four-component screen consisting of ultrasound biometry and serum markers in the second trimester.

METHODS

The Down syndrome screening efficiency of the triple screen, i.e. alpha-fetoprotein (AFP), unconjugated estriol (E3), hCG and maternal age, was compared with the four-marker algorithm, i.e. humerus length, nuchal thickness, AFP and hCG plus maternal age. A quadrivariate Gaussian algorithm was used to calculate individual Down syndrome odds. Receiver operating characteristic (ROC) curves plotting sensitivity against false-positive rate were constructed for each algorithm and the areas under the curves were compared to determine which was superior. Sensitivity and false-positive rates at different Down syndrome risk thresholds were also compared.

RESULTS

There were 46 cases of Down syndrome (1.9%) with 2391 normal singleton pregnancies in a referral population in which triple screen, fetal biometry and karyotype had been done. The gestational age range for the study was 14-24 completed weeks. The median maternal age for the study group was 35.0 years (14.0-46.0 years). The areas (SE) under the ROC curves were 0.75(0.04) and 0.93(0.02) for the standard triple and the four-marker screen, respectively (P < 0.001). At a 10% false-positive rate, detection was 45.7% for the triple and 80.4% for the four-marker screen.

CONCLUSIONS

A new algorithm combining humerus length and nuchal thickness measurement with serum AFP, hCG and maternal age substantially improved Down syndrome screening efficiency compared with the traditional triple screen. The model appears promising and should be evaluated in an independent data set.

Hyperglycosylated Human Chorionic Gonadotropin (Invasive Trophoblast Antigen) Immunoassay: A New Basis for Gestational Down Syndrome Screening

Background: Serum human chorionic gonadotropin (hCG) and hCG free β-subunit tests are used in combination with unconjugated estriol and α-fetoprotein in the triple screen test, and with the addition of inhibin-A in the quadruple marker test for detecting Down syndrome in the second trimester of pregnancy. These tests have a limited detection rate for Down syndrome: ∼40% for hCG or free β-subunit alone, ∼60% for the triple screen test, and ∼70% for the quadruple marker test, all at 5%, or a relatively high, false-positive rate. New tests are needed with higher detection and lower false rates. Hyperglycosylated hCG (also known as invasive trophoblast antigen or ITA) is a new test. It specifically detects a unique oligosaccharide variant of hCG associated with Down syndrome pregnancies. We evaluated this new Down syndrome-directed test in prenatal diagnosis.

Methods: Hyperglycosylated hCG was measured in urine samples from women undergoing amniocentesis for advanced maternal age concerns at 14–22 weeks of gestation, 1448 with normal karyotype and 39 with Down syndrome fetuses.

Results: The median hyperglycosylated hCG value was 9.5-fold higher in Down syndrome cases (9.5 multiples of the normal karyotype median). The single test detected 80% of Down syndrome cases at a 5% false-positive rate. Urine hyperglycosylated hCG was combined with urine β-core fragment (urine breakdown product of serum hCG free β-subunit), serum α-fetoprotein, and maternal age-related risk. This urine-serum combination detected 96% of Down syndrome cases at a 5% false-positive rate, 94% of cases at a 3% false-positive rate, and 71% of cases at a 1% false-positive rate. These detection rates exceed those of any previously reported combination of biochemical markers.

Conclusions: Hyperglycosylated hCG is a new base marker for Down syndrome screening in the second trimester of pregnancy. The measurement of hyperglycosylated hCG can fundamentally improve the performance of Down syndrome screening protocols.

Collaborative study of maternal urine beta -core human chorionic gonadotrophin screening for Down syndrome

Several studies have shown that second-trimester maternal urine beta-core human chorionic gonadotrophin (hCG) levels are raised on average in Down syndrome pregnancies. However, in all but one, testing was retrospective after extended sample storage and so we carried out a large international multicentre prospective study. 16 centres provided 6730 samples from 14-19 week pregnancies: 39 with Down syndrome, 12 with Edwards' syndrome, 42 with other aneuploidies, 52 unaffected twins and 6585 singleton unaffected pregnancies. Samples were from those having routine maternal serum screening in 6 centres and invasive prenatal diagnosis for reasons unrelated to maternal serum screening in 10 centres. Normalized levels of beta-core hCG (nmom/mmol creatinine) were expressed as multiples of the gestation-specific normal median (MoMs). The median beta-core hCG level in Down syndrome was 1.70 MoM (95 per cent confidence interval, 1.26-2.30); 14 (36 per cent) exceeded the normal 90th centile and 9 (23 per cent) the 95th centile. The median level in Edwards' syndrome was 0.23 MoM. On the basis of our results alone it is unlikely that urinary beta-core hCG will be a useful marker in Down syndrome screening practice. But the considerable variability in results between studies means that further research is needed before a reliable conclusion can be drawn.