Biochemical screening for Down syndrome

Maternal serum screening for Down syndrome is an established practise in many countries. In the second trimester human chorionic gonadotrophin (hCG) or free beta-hCG is the marker of first choice, with alpha-fetoprotein (AFP) as the second marker and unconjugated oestriol (uE(3)) the third. Statistical models with parameters derived by meta-analysis predict that a three marker combination will yield a 67% detection rate for a 5% false-positive rate. The model prediction have been confirmed in 21 large prospective intervention studies. A fourth marker, inhibin A, increases the detection rate by 7% for the same false-positive rate. In the first trimester, similar models predict that a combination of pregnancy associated plasma protein A, free beta-hCG, AFP and uE(3) will yield a 70% detection rate. This is increased to 88% if ultrasound nuchal translucency is used as an additional marker. Screening can also be extended to Edwards' syndrome, yielding high detection rates with little increase in the false-positive rate. Abnormal marker levels are also associated with a variety of adverse outcomes of pregnancy. High quality information and decision aids are needed to minimise anxiety among screenees.

Screening for fragile X syndrome in women of reproductive age

We conducted a prospective intervention study of screening for fragile X syndrome in the general population. Antenatal and preconceptional screening were carried out in 9459 women aged between 19 and 44 with no known family history of fragile X syndrome. 80% were tested antenatally. 134 carriers were detected (a frequency of 1 in 70); 130 had a premutation (PM) and 4 had a full mutation (FM). Prenatal diagnosis was carried out in 108 concurrent or subsequent pregnancies among carriers involving 111 fetuses. Nine had an FM, a rate of 1 in 12; two of the affected embryos received the FM directly from the mother and in seven it was the result of expansion from a PM. In all cases with an FM the pregnancy was terminated. In PM carriers there was evidence of a selection against the mutated chromosome with a segregation ratio of 0.40. Owing to the high rate of premutated chromosomes in our population we conclude that screening for fragile X syndrome among women of reproductive age should be more widely available.

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.

Screening for trisomies 21 and 18 with maternal serum placental isoferritin p43 component

A component of placental isoferritin, p43, is an immuno-regulatory protein associated with suppression of the immune system. Maternal serum p43 levels increase throughout pregnancy and low serum levels have been associated with various pathological pregnancies, particularly those with a defect of placentation. We measured maternal serum p43 retrospectively in banked samples from 42 Down syndrome, 20 Edwards' syndrome and 281 unaffected pregnancies to assess its screening potential in both the first and second trimesters. The median maternal serum p43 level in Down syndrome was 1.58 times higher than that in the unaffected pregnancies (p=0.01, two-tail). The median level was slightly, but not significantly, reduced in Edwards' syndrome. Statistical modelling, including parameters for alpha-fetoprotein, free beta-human chorionic gonadotrophin, and unconjugated oestriol suggested that it might have a role in Down syndrome screening when combined with two or three of these markers. Larger scale studies are now needed.

Serum inhibin A levels in pregnant women with systemic lupus erythematosus or antiphospholipid syndrome

Maternal serum inhibin A levels are increased on average in pregnancies affected by Down syndrome (DS). However, some reports have found increased serum levels in women with pre-eclamptic toxaemia as well. In the current study, maternal serum inhibin A was retrospectively measured in a series of 32 serum samples from pregnant women previously diagnosed as having either systemic lupus erythematosus (SLE) or primary antiphospholipid syndrome (APS). For comparison, normal medians were calculated from 57 unaffected control pregnancies together with a total of 854 samples tested at 13-19 weeks of gestation as part of the routine antenatal DS screening. All results were expressed in multiples of the gestation specific normal medians (MoM). A cubic regression formula was fitted, weighting for the number of women tested at each gestation. The median MoM value in the 16 cases of SLE and the 16 cases of primary APS is 0.60 (95% confidence interval 0.40-0.91) and 0.88 (95% confidence interval 0.66-1.17), respectively. For primary APS this was not statistically significant, whereas the SLE patients had a highly statistically significant reduction of serum inhibin A (p<0.002, Wilcoxon Rank sum Test, 2 tailed). Six pregnancies in the SLE group had a complicated obstetric outcome, i.e. missed abortion, placental abruption, exacerbation of the underlying disease which necessitated delivery, and severe postpartum haemorrhage. In 85% of this subgroup, serum inhibin A levels were below the normal 10th centile. The current data suggest that serum inhibin A is decreased on average in SLE patients. Those preliminary results might have various obstetric implications such as antenatal DS screening of SLE patients, identification of pregnant women at risk of developing SLE, who have presented for routine DS screening and for monitoring SLE patients throughout their pregnancy.

Calculating correct Down's syndrome risks

Women having both first trimester nuchal translucency and second trimester serum screening tests are likely to receive two different Down's syndrome risks. Neither will be correct, and we describe how to calculate a valid combined risk. This uses the reported serum-based risk and a likelihood ratio derived from the nuchal translucency report. Tables, figures and examples are provided to aid the calculation of the likelihood ratio from either the nuchal translucency in multiples of the normal median, the nuchal translucency and crown-rump length in millimetres, or the reported prior and nuchal translucency-based risks.

Maternal serum inhibin A can predict pre-eclampsia

OBJECTIVE

To compare median levels of maternal serum inhibin A in the second trimester blood samples of women who subsequently develop pre-eclampsia and those who do not.

DESIGN

Retrospective analysis of 13 18 week samples from a bank of serum stored at -40 degrees C, originally taken for Down's syndrome screening.

SETTING

Antenatal clinics in a teaching hospital.

SAMPLE

Twenty-eight pregnancies with pre-eclampsia and 701 controls. Samples were taken, on average, 22 weeks before the diagnosis.

MAIN OUTCOME MEASURE

Median inhibin A level.

RESULTS

The median inhibin A level in the cases was 2.01 multiples of the gestation-specific median in the controls, a statistically significant elevation (P < 0.001). Twenty-three (82%) had levels above the normal median; 19 (68%), 15 (54%), and 11 (39%) exceeded the normal 75th, 90th and 95th centiles, respectively.

CONCLUSIONS

In pre-eclampsia the maternal serum inhibin A level can be increased months before the onset of symptoms. This provides an opportunity to study the early natural history of the disease and possibly to conduct treatment trials.

Variation in biochemical screening for Down's syndrome in the United Kingdom

A questionnaire survey was undertaken of all 73 laboratories performing Down's syndrome screening in 1995. An estimated 352,000 tests were performed representing 47% of maternities. Three-quarters of these tests have ultrasound dating information at the time of testing. The majority of laboratories (70%) commenced screening at 15 weeks of gestation or later, and there was considerable variation in the upper limit of screening (17 to 24 weeks). Eighty-six percent of laboratories screened all women regardless of age. The reported Down's syndrome risk was based on term in 85% of laboratories. There was an inconsistent approach to determining and reporting high risk for trisomy 18 (Edwards' syndrome): 5% reported risks on report forms and 42% notified the clinicians if the risk was considered to be raised.

Screening for fragile X syndrome

BACKGROUND AND AIM OF REVIEW. In 1991, the gene responsible for fragile X syndrome, a common cause of learning disability, was discovered. As a result, diagnosis of the disorder has improved and its molecular genetics are now understood. This report seems to provide the information needed to decide whether to use DNA testing to screen for the disorder. HOW THE RESEARCH WAS CONDUCTED. A literature search of electronic reference databases of published and 'grey' literature was undertaken together with hand searching of the most recent publications. RESEARCH FINDINGS. NATURAL HISTORY. Physical characteristics of fragile X syndrome include facial atypia, joint laxity and, in boys, macro-orchidism. Most affected males have moderate-to-severe learning disabilities with IQs under 50 whereas most females have borderline IQs of 70-85. Behavioural problems are similar to those seen with autism and attention-deficit disorders. Although fragile X syndrome is not curable there are a number of medical, educational, psychological and social interventions that can improve the symptoms. About 6% of those with learning disabilities tested in institutions have fragile X syndrome. Population prevalence figures are 1 in 4000 in males and 1 in 8000 in females. GENETICS. The disorder is caused by a mutation in a gene on the X chromosome which includes a trinucleotide repeat sequence. The mutation is characterized by hyper-expansion of the repeat sequence leading to down-regulation of the gene. In males an allele with repeat size in excess of 200, termed a full mutation (FM), is always associated with the affected phenotype, whereas in females only half are affected. Individuals with alleles having repeat size in the range 55-199 are unaffected but in females the sequence is heritably unstable so that it is at high risk of expansion to an FM in her offspring. This allele is known as a pre-mutation (PM) to contrast it with the FM found in the affected individual. No spontaneous expansions directly from a normal allele to an FM have been observed. SCREENING STRATEGIES. The principal aims of screenng for fragile X syndrome is to reduce the birth prevalence of the disorder, by prenatal diagnosis and selective termination of pregnancy, or by reducing the number of pregnancies in women who have the FM or PM alleles. Possible screening strategies are: routine antenatal testing of apparently low risk pregnancies, preconceptual testing of young women, and systematic testing in affected families ('cascade' screening). A secondary aim is to bring forward the diagnosis of affected individuals so that they might benefit from early treatment. Active paediatric screening and neonatal screening could achieve this but there is no direct evidence of any great benefit from early diagnosis. SCREENING TESTS. Cytogenetic methods are unsuitable for screening purposes. Southern blotting of genomic DNA can be used but is inaccurate in measuring the size of small PMs, there is a long laboratory turnaround time, and it is relatively expensive. The best protocol is to amplify the DNA using polymerase chain reaction on all samples, and when there is a possible failure to amplify, a Southern blot.(ABSTRACT TRUNCATED)

Down's syndrome screening marker levels following assisted reproduction

Maternal serum alpha-fetoprotein (AFP), human chorionic gonadotrophin (hCG), and unconjugated oestriol (uE3) levels were examined in 1632 women who had ovulation induction and 327 who had in vitro fertilization. There was a highly statistically significant increase in hCG and reduction in uE3 among those with ovulation induction. The median levels were respectively 1.09 and 0.92 multiples of the normal gestation-specific median (MOM) based on a total of 34582 women. Ovulation induction appeared to have no material effect on the median AFP level but this masked a significant increase when treatment was with Clomiphene (1.05 MOM) and a significant decrease when Pergonal was used (0.93 MOM). There was a highly statistically significant reduction in uE3 among women having in vitro fertilization with a median level of 0.92 MOM. Those fertilized with a donor egg (21) had significantly higher AFP and uE3 levels than when their own egg was used. Our results were confounded by differences in gravidity, but formally allowing for this factor did not materially change the findings. None of the observed effects is great enough to warrant routine adjustment of marker levels to allow for them. Moreover, women with positive Down's syndrome screening results can be reassured that this is unlikely to be due to them having had assisted reproduction.

Antenatal screening for cystic fibrosis

OBJECTIVE

To assess the practicality of implementing antenatal screening for cystic fibrosis in Yorkshire.

DESIGN

Prospective study in which all pregnant women were offered testing for the delta F508 mutation which accounts for about 85% of carriers in Yorkshire. The reproductive partners of those found to be cystic fibrosis carriers were then tested and any carrier referred for genetic counselling.

SETTING

Antenatal clinics in two hospitals and eight general practices.

POPULATION

Six thousand and seventy-one pregnant women.

RESULTS

A total of 3773 women (62%) accepted the screening offer. This was a lower uptake rate than in other published UK studies: Aberdeen (85-91%), Manchester (85%), Edinburgh (76-84%) and Oxford (67%). Nonetheless there were large and statistically significant differences in the uptake rate between centres within the study: 78% and 60% for the two hospitals and 67% for the general practices. One hundred and thirty women (3.4%) were found to be carriers and three carrier couples were identified. The median time interval for the laboratory to produce a result was five days and the cost was pounds 16 on average.

CONCLUSIONS

Antenatal screening for cystic fibrosis does not pose any special practical difficulties. It would be feasible to introduce it into routine practice in Yorkshire.

Effect of gravidity on maternal serum markers for Down's syndrome

The effect of gravidity on maternal serum alpha-fetoprotein (AFP), unconjugated oestriol (uE3), and human chorionic gonadotrophin (hCG) levels was investigated in 27,592 women being screened for Down's syndrome. There was no difference in the median AFP level in primigravid and multigravid women, but the median hCG level in multigravid women was 5.9 per cent lower than in those tested in their first pregnancy (P < 0.0001) and the median uE3 level was 3.9 per cent lower (P < 0.0001). Among multigravid women, there was no material difference in hCG levels according to the number of previous pregnancies or livebirths, whereas uE3 levels declined steadily with increasing numbers. Both markers declined with advancing maternal age: for hCG this was an independent effect, but for uE3 it was due to the correlation between age and gravidity. Allowing for these effects will not greatly alter the Down's syndrome screening detection and false-positive rates.

Improved parameters for risk estimation in Down's syndrome screening

A new method is described for calculating maternal serum marker distribution parameters which will improve risk estimation when screening for Down's syndrome. The approach is to calculate parameters using data from the local screened population and data obtained by meta-analysis from all published studies. The local data are used to derive the variance and covariance in unaffected pregnancies. The meta-analysis is used for the mean level in Down's syndrome pregnancies together with the differences in variance and covariance between affected and unaffected pregnancies. Forty-four published studies were analysed. The mean level for Down's syndrome in multiples of the normal median was 0.73 for alpha-fetoprotein (AFP) in total of 1140 pregnancies, 0.73 for unconjugated oestriol (uE3) in 613, 2.02 for human chorionic gonadotropin (hCG) in 850, and 2.30 for free beta-hCG in 477. For all four markers, the variance in Down's syndrome was higher than in unaffected pregnancies; for AFP and uE3, the covariances were also higher in Down's syndrome, but for the other markers they were lower. The method was illustrated using data from 6387 pregnancies screened in Leeds.

Taking account of vaginal bleeding in screening for Down's syndrome

OBJECTIVE

To derive a method for revising the risk of Down's syndrome in maternal serum marker screening when there is vaginal bleeding. The effect on screening performance of routinely allowing for the presence or absence of bleeding in all women is also assessed.

DESIGN

Overview of published studies on the rate of reported vaginal bleeding in pregnancies with Down's syndrome, on the rate according to maternal age and on the association of bleeding with alpha-fetoprotein (AFP) level. The publications are supplemented with data on unconjugated oestriol (uE3), human chorionic gonadotrophin (hCG) and AFP levels in a consecutive series of screened women.

SETTING

Routine Down's syndrome screening tests carried out on women having antenatal care at the St James's University Hospital, Leeds.

SUBJECTS

Eight hundred and nine screened women.

RESULTS

In five studies the rate of vaginal bleeding in Down's syndrome pregnancies was 1.7 times that in unaffected pregnancies on average. In three studies, the vaginal bleeding rate increased proportionally by 2.2% on average for each year of maternal age. Three studies and our own data were consistent with a 10% increase in the mean AFP level associated with vaginal bleeding, but it did not appear to materially alter uE3 and hCG levels or the standard deviations and correlation coefficients for any of the three analytes. An individual woman's risk was calculated by multiplying her age-specific odds of Down's syndrome by two likelihood ratios, one relating to the vaginal bleeding itself and one from the marker levels. Routine allowance for the presence or absence of vaginal bleeding was estimated to increase the detection rate by less than 1%.

CONCLUSION

Our method is of clinical value in revising the risk when there is concern that vaginal bleeding might be responsible for a negative maternal serum Down's syndrome screening result. A policy of routinely incorporating information on vaginal bleeding in risk estimation for all women would have too small an effect on overall screening performance to recommend it.

Repeat maternal serum testing in multiple marker Down's syndrome screening programmes

The effect of repeat testing in maternal serum multiple marker screening for Down's syndrome was estimated using samples stored in an antenatal serum bank. Human chorionic gonadotropin (hCG) and unconjugated oestriol (uE3) levels were determined in 142 pairs of routinely collected samples which had already been tested for alpha-fetoprotein (AFP). For each marker, about two-thirds of the pairs of values were within 20 per cent of each other and most were within 40 per cent. A multivariate Gaussian model was used to estimate the detection and false-positive rates for different repeat testing policies. A policy of repeat testing those with a high risk of a Down's syndrome term pregnancy given age and marker levels would reduce the false-positive rate but there would also be a reduction in the detection rate. For example, using all three markers and a 1 in 250 cut-off risk, the estimated false-positive rate would fall from 5.3 to 3.8 per cent but the detection rate would decrease from 58 to 55 per cent. A policy of repeating those with either high or borderline risks would produce a modest improvement in screening efficiency. Repeating the 11 per cent with a risk exceeding 1 in 500 yields an estimated false-positive rate of 5.0 per cent and a detection rate of 60 per cent. A policy of selective repeat testing is not recommended as it would not substantially improve screening efficiency. Nonetheless, if a repeat test has been performed, the parameters given in this paper will enable an unbiased estimate of the Down's syndrome risk to be calculated for individual women.

Maternal age-specific risks for trisomies at 9-14 weeks' gestation

This study provides data on the incidence of fetal trisomies 21, 18, and 13 at 9-14 weeks' gestation in women aged 35-45 years and estimates of maternal age-specific risks in women aged 20-45 years. Our data from 5814 singleton pregnancies undergoing first-trimester karyotyping for the sole indication of maternal age > or = 35 years were combined with those from two previous reports and the incidence of the trisomies was calculated from a total of 15,793 pregnancies. Comparison of incidences at 9-14 weeks' gestation with published data at 15-20 weeks' gestation and in livebirths demonstrated that at birth the maternal age-specific incidence of trisomy 21 is 33 per cent lower than at 15-20 weeks' gestation and 54 per cent lower than at 9-14 weeks' gestation. Furthermore, the relative frequency of trisomies 18 and 13 decreases from 30 per cent at 9-14 weeks to 22 per cent at 15-20 weeks and 14 per cent at birth.

Cystic fibrosis screening strategies

Cystic fibrosis is the most common single gene disorder in Western populations. With the discovery of the principal genetic mutations responsible for the disease, screening has become feasible. The challenge is now to devise an effective screening strategy. Various approaches are possible, and each has its own merits, but on balance antenatal screening would appear to be the most practical.

Expanding multiple marker screening for Down's syndrome to include Edward's syndrome

Information on maternal age and maternal serum alpha-fetoprotein, unconjugated oestriol (uE3), and human chorionic gonadotrophin (hCG) levels was used to investigate retrospectively the effect of estimating Edward's syndrome risk in women having multi-marker screening for Down's syndrome. The screened population comprised 15 pregnancies affected by Edward's syndrome, 15 with Down's syndrome and 5472 unaffected pregnancies. The use of all three markers to estimate Edward's syndrome risk would have led to the detection of 10-12 (67-80 per cent) cases with a false-positive rate of 0.3-0.6 per cent depending on the risk cut-off. A further case would have been detected as a result of screening for Down's syndrome alone. Similar results were obtained when the Edward's syndrome risk was based on uE3 and hCG only. These data suggest that extending Down's syndrome screening to include Edward's syndrome risk will yield a high detection rate with only a small increase in the false-positive rate.

Serum PAPP-A levels are depressed in women with fetal Down syndrome in early pregnancy

The value of maternal serum pregnancy-associated plasma protein (PAPP)-A in screening for Down syndrome in early pregnancy was assessed using stored samples. Seventeen cases of Down syndrome and 66 unaffected control pregnancies were studied. The median PAPP-A level in the cases was 0.42 multiples of the expected value in controls (p < 0.0001). Eleven cases (65 per cent) had levels less than half the expected value compared with only six controls (9 per cent). A commercial assay kit is now needed so that prospective screening with this marker can begin.