Genetic sonography after first-trimester Down syndrome screening

OBJECTIVE

Approximately 90% of Down syndrome cases are detected during first-trimester screening. We aimed to determine the potential effectiveness of second-trimester genetic sonography as a sequential screen for Down syndrome.

METHODS

In this simulation study, published statistical parameters for first-trimester free beta-human chorionic gonadotropin, pregnancy-associated plasma protein-A and nuchal translucency thickness, and second-trimester ultrasound markers (nuchal fold, hyperechoic bowel, short humerus, short femur, echogenic intracardiac focus, pyelectasis and major abnormality) were used to model the effectiveness of second-trimester genetic sonography combined with first-trimester screening.

RESULTS

First-trimester combined screening alone resulted in a detection rate of 88.5% with a 4.2% false-positive rate. A follow-up genetic ultrasound examination in which only one sonographic marker was found and previous results were not taken into account would detect an additional 8% of Down syndrome cases for an additional false-positive rate of 13.2%. Using individual marker likelihood ratios to modify the first-trimester risk for screen-negative patients, genetic sonography detected an additional 6.1% of Down syndrome cases for an additional 1.2% false-positive rate, giving a total detection rate of 94.6% and a total false-positive rate of 5.4%. In a contingent protocol, in which genetic sonography would be performed only for patients with a first-trimester risk of between 1/300 and 1/2500, the detection rate was 4.8% and the false-positive rate was 0.7%, giving a total detection rate of 93.3% and a total false-positive rate of 4.9%.

CONCLUSION

Second-trimester genetic sonography, if used properly, can be an effective sequential screen following first-trimester Down syndrome screening. Further studies on the role of the genetic sonogram as a follow-up to first-trimester combined screening are warranted.

First-trimester Down syndrome screening using dried blood biochemistry and nuchal translucency

OBJECTIVE

To assess the effectiveness of free beta-hCG, pregnancy-associated plasma protein A, and nuchal translucency in a prospective first-trimester prenatal screening study for Down syndrome and trisomy 18.

METHODS

Risks were calculated for Down syndrome and trisomy 18 based on maternal age and biochemistry only (n = 10,251), nuchal translucency only (n = 5809), and the combination of nuchal translucency and biochemistry (n = 5809).

RESULTS

The study population included 50 Down syndrome and 20 trisomy 18 cases. Nuchal translucency measurement was done on 33 Down syndrome and 13 trisomy 18 cases. Down syndrome screening using combined biochemistry and ultrasound resulted in a false-positive rate of 4.5% (95% confidence interval [CI] 3.9%, 5.2%) and detection rate of 87.5% (95% CI 47%, 100%) in patients under age 35 years. In older patients, the false-positive rate was 14.3% (95% CI 12.7%, 15. 8%) and detection rate was 92% (95% CI 74%, 99%). For trisomy 18 screening, the false-positive rate was 0.4% (95% CI 0.24%, 0.69%) and detection rate was 100% (95% CI 40%, 100%) in younger patients, whereas in older patients the false-positive rate was 1.4% (95% CI 0. 9%, 2.0%) and detection rate was 100% (95% CI 66%, 100%). Using modeling, at a fixed 5% false-positive rate, the Down syndrome detection rate was 91%. Conversely, at a fixed 70% Down syndrome detection rate, the false-positive rate was 1.4%.

CONCLUSION

First-trimester screening for Down syndrome and trisomy 18 is effective and offers substantial benefits to clinicians and patients.

Second-trimester pregnancy associated plasma protein-A levels are reduced in Cornelia de Lange syndrome pregnancies

Maternal serum samples were collected from 19 pregnancies which resulted in the birth of a child with the classical Cornelia de Lange syndrome phenotype ascertained by careful clinical review. Using specific immunoassays, the serum levels of pregnancy associated plasma protein-A, free-beta human chorionic gonadotrophin and inhibin A were investigated. Pregnancy associated plasma protein-A was detectable in all cases but the levels were significantly reduced in second-trimester maternal serum from 18 affected pregnancies. Expressed as multiples of the median (MOM), the results ranged from 0.03 MOM to 0.71 MOM with an overall median value of 0.21 MOM (Mann-Whitney p<0.001). From these data it is possible to estimate a probability that any given level of this serum marker is associated with an affected pregnancy. One further sample taken in the first trimester from an affected pregnancy at 11 weeks' gestation had a normal pregnancy associated plasma protein-A level (1.22 MOM). Less markedly reduced levels were found for free beta human chorionic gonadotrophin and inhibin A. We conclude that second-trimester maternal serum pregnancy associated plasma protein-A measurements may be of value as an adjunct to ultrasonography in the prenatal diagnosis of Cornelia de Lange syndrome. A table of likelihood ratios is presented.

Comparison of urinary free beta (hCG) and beta-core (hCG) in prenatal screening for chromosomal abnormalities

To evaluate the potential utility of free beta (hCG) and beta-core (hCG) in a prenatal screening protocol for Down syndrome we analysed these markers in dried maternal urine specimens from 163 control, 13 Down syndrome and 5 trisomy 18 pregnancies from 8 to 25 weeks' gestation. All results are reported after normalization for urinary creatinine determined by modified Jaffe reagent assay. The correlation of urinary free beta (hCG) and urinary beta-core (hCG) was 0.61 in controls and 0.93 in Down syndrome. Median MoM values in Down syndrome were 2.42 for urinary free beta (hCG) and 2.40 for beta-core (hCG). In trisomy 18 the Median MoM was 0.35 and 0.34 for free beta (hCG) and beta-core (hCG), respectively. The degree of elevation observed in DS cases with urinary free beta (hCG) is consistent with previous reports. Studies of beta-core (hCG) in Down syndrome have yielded discrepant results. In this study, beta-core (hCG) in Down syndrome is lower than values observed in early reports but consistent with more recent reports.

First-trimester screening for fetal aneuploidy: biochemistry and nuchal translucency

Maternal dried whole-blood specimens were collected prospectively from 2010 singleton pregnancies between 9 + 0 and 13 + 4 weeks that included 18 chromosomally abnormal pregnancies (11 Down's syndrome, four trisomy 18, two trisomy 13 and one triploidy). A subset of 744 pregnancies underwent ultrasound nuchal translucency measurement and included seven Down's syndrome, four trisomy 18, two trisomy 13 and one triploidy. Patients were evaluated for risk of Down's syndrome and trisomy 18 based on biochemistry (free beta-human chorionic gonadotropin and pregnancy-associated plasma protein A), nuchal translucency and the combination of both. In prospective biochemical screening, false-positive rates for Down's syndrome and trisomy 18 were 5.1% (66/1297) and 1.9% (25/1297) in women < 35 years of age and 14.2% (99/695) and 1.6% (11/695) in women > or = 35 years of age, respectively. The detection efficiency of aneuploidy was 6/6 (100%) in women < 35 years and 11/12 (92%) in women > or = 35 years. Nuchal translucency measurement alone detected 57% (8/14) of cases of aneuploidy at a 5.8% (42/730) false-positive rate. Modelling with the age distribution of live births, a 5% false-positive rate resulted in Down's syndrome detection efficiency of 61% by biochemistry, 73% by nuchal translucency and 87% by combining both methods. The data in this study demonstrate that combined biochemical and ultrasound evaluation for Down's syndrome and other chromosomal abnormalities in the first trimester of pregnancy yield a detection capability that may exceed that of current second-trimester prenatal screening protocols. The potential for enhanced detection coupled to an earlier alert of fetal complications could represent a substantial advantage to both clinician and patient.

Biochemical markers of trisomy 21 and the pathophysiology of Down's syndrome pregnancies

Using biochemical and immunocytochemical methods, we have investigated endogenous levels of various markers in tissues obtained from 67 Down's syndrome pregnancies after therapeutic abortion in the second trimester and in corresponding tissues from unaffected abortuses. Alpha-fetoprotein (AFP), intact and free beta human chorionic gonadotrophin (hCG), pregnancy-specific beta-1 glycoprotein (SP-1), placental alkaline phosphatase (PALP), pregnancy-associated plasma protein A (PAPP-A), and gamma glutamyl transferase (GGT) were investigated in placental tissue; AFP and GGT in fetal liver; and GGT in fetal intestine. The results indicate that maternal serum levels of placental products reflect those found in the placenta: intact hCG, free beta hCG, and SP-1 levels were elevated in Down's syndrome pregnancies, while PAPP-A and PALP levels were little changed. This suggests that membrane passage of these markers is not affected but there is altered synthesis of hCG and SP-1. AFP levels were strikingly elevated in placental homogenates and unchanged in liver homogenates from Down's syndrome pregnancies, while the levels in maternal serum were reduced, pointing to a possible transport defect specific to AFP. GGT levels were high in placenta and liver from Down's syndrome pregnancies but low in fetal intestine.

Screening for Down's syndrome: changes in marker levels and detection rates between first and second trimesters

OBJECTIVE

To monitor changes with gestation in levels of alpha-fetoprotein (AFP), free beta human chorionic gonadotrophin (F beta hCG) and pregnancy associated plasma protein-A (PAPP-A) in Down's syndrome pregnancies and to compare risks estimated in the first trimester with those obtained by routine screening in the second trimester for the same pregnancies.

DESIGN

In each of 47 Down's syndrome pregnancies two maternal serum samples were obtained, one in the first trimester and one in the second trimester. Comparison of marker levels with 10,600 first trimester controls and a smaller sample of second trimester controls allowed case identification criteria based on optimum marker combinations to be developed and compared directly between trimesters.

SETTING

Biochemical genetics laboratory.

RESULTS

F beta hCG was an effective marker of Down's syndrome in both the first and second trimesters. PAPP-A levels were significantly reduced in trisomy 21 pregnancies in the first trimester only. Using a population model, these two markers in combination with maternal age gave an overall detection rate of 55% for a 5% false positive rate in the first trimester. For the paired first and second trimester samples, three of six cases classified as low risk by routine second trimester screening were classified as high risk by the first trimester screening protocol of F beta hCG/PAPP-A/maternal age. However, fifteen cases identified as high risk by routine second trimester screening were classified as low risk in the first trimester, a net loss in detection of 12 cases by first trimester screening.

CONCLUSION

The data suggest that first trimester detection rates for Down's syndrome using a combination of F beta hCG and PAPP-A may vary with gestation and will be lower than those currently obtained by routine second trimester screening with AFP/hCG.

Free beta hCG screening of hydropic and non-hydropic Turner syndrome pregnancies

Fourteen cases of Turner syndrome (45,X), two cases of mosaic Turner syndrome (45,X/47,XXX and 45,X/ 46,XX), and one case of Turner syndrome involving an isochromosome X [46,X,i(X)(q10)] were ascertained by prenatal maternal serum alpha-fetoprotein (MSAFP) and free beta human chorionic gonadotropin (hCG) screening or by ultrasound. Patient-specific risks for Down syndrome were calculated and used as the criteria to determine offering further testing. Eleven of the 17 cases had hydrops and presented with an increased Down syndrome risk based on MSAFP and free beta hCG screening. The median MOM level was 0.98 and 4.04 for MSAFP and free beta hCG, respectively. Three cases had hydrops but screened negative. The two cases of mosaic Turner syndrome were non-hydropic and screened positive. The 46,X,i(X)(q10) case was non-hydropic but had elevated MSAFP and free beta hCG levels. These data suggest that Turner syndrome pregnancies do not appear to screen positive due to hydrops alone, but screening may also be influenced by the inherent genetic imbalance in the fetus and placenta. Because the MSAFP levels in our series were within the normative range in all except one case with an elevated MSAFP, free beta hCG alone was the most effective screening marker for Turner syndrome pregnancies.

Urine free beta hCG and beta core in pregnancies affected by Down's syndrome

Urine beta core was shown in recent studies to be markedly elevated in pregnancies affected by Down's syndrome in the late second trimester. Free beta human chorionic gonadotropin (hCG) has also been shown to be the most discriminatory maternal serum marker of Down's syndrome. Since free beta hCG is rapidly cleared from the maternal circulation, we have carried out a study to evaluate whether free beta hCG is elevated in the urine of pregnancies affected by Down's syndrome and to investigate whether urine beta core or urine free beta hCG may be used as possible screening markers. Urine samples from 29 cases of Down's syndrome, three cases of trisomy 18, and 400 control pregnancies were analysed for the two prospective markers. Results were corrected for urine concentration by expressing marker concentrations at a fixed creatinine concentration and then expressing the results as multiples of the median for unaffected pregnancies of the same gestation. The median value of beta core in the Down's syndrome pregnancies was 2.35 compared with 2.47 for free beta hCG. Free beta hCG distributions were closely similar to those in maternal serum. Using free beta hCG, we predict Down's syndrome detection rates of 58 per cent at a 5 per cent false-positive rate. Using beta core, however, this rate fell to 41 per cent. Measurement of free beta hCG in urine may present a feasible route for screening pregnant populations, particularly where community-based obstetric care is the norm and/or if early first-trimester screening becomes a reality.

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 maternal dried blood screening with alpha-fetoprotein and free beta-human chorionic gonadotropin for open neural tube defect and Down syndrome

OBJECTIVE

Our purpose was to evaluate second-trimester prenatal screening for open neural tube defects and Down syndrome by use of dried blood specimen collection and transport.

STUDY DESIGN

A prospective study of 7497 dried blood specimens from patients <35 years old was performed. Specimens were assayed for maternal blood alpha-fetoprotein and free beta-human chorionic gonadotropin. Patient-specific risks for both disorders were calculated and used to determine whether further evaluation was indicated. The study included an evaluation of the median and SD of analyte multiple of the median levels.

RESULTS

The initial positive rate for open neural tube defect was 4.4% adjusted to 2.7% after ultrasonographic revision and collection of a second sample. The initial positive rate for Down syndrome was 3.6% adjusted to 2.8% after ultrasonographic revision. All seven cases of open neural tube defect were detected within the increased risk group. Six of 8 (75%) cases of Down syndrome were detected. The median alpha-fetoprotein multiple of the median was 3.5 in open neural tube defect cases and 0.6 in Down syndrome cases. The median free beta-human chorionic gonadotropin multiple of the median was 2.4 in Down syndrome cases. The SD (log e) of alpha- fetoprotein and free beta-human chorionic gonadotropin in 5868 unaffected white patients was 0.4022 and 0.5635, respectively.

CONCLUSION

Second-trimester dried blood screening for open neural tube defects and Down syndrome can achieve screening efficiency comparable to serum-based protocols with distinct advantages over the conventional method of blood collection.

First-trimester Down syndrome screening: free beta-human chorionic gonadotropin and pregnancy-associated plasma protein A

OBJECTIVE

Our purpose was to determine the feasibility of a first-trimester Down syndrome screening protocol including free beta-human chorionic gonadotropin and pregnancy-associated plasma protein A.

STUDY DESIGN

First-trimester maternal blood samples from 22 Down syndrome and 483 control cases were assayed for free beta-human chorionic gonadotropin and pregnancy-associated plasma protein A by enzyme-linked immunosorbent assay procedures. False-positive and detection rates were determined on the basis of Down syndrome risks calculated from the levels of biochemical markers and maternal age. Because 11 of the 22 Down syndrome cases were from older pregnancies (> or = 35 years old), rates were recalculated with the United States age distribution of live births to get a more representative estimate of false positives and detection efficiency.

RESULTS

The median free beta-human chorionic gonadotropin and pregnancy-associated plasma protein A levels in cases of Down syndrome was 2.09 (95% confidence interval 1.69 to 2.62) and 0.405 multiples of the median (95% confidence interval 0.28 to 0.67), respectively. At a 5.0% false-positive rate, 15 (68.2%) Down syndrome cases were detected. By the use of the age distribution of live births, 63% of cases could be expected to be detected at a 5.0% false-positive rate.

CONCLUSION

First-trimester free beta-human chorionic gonadotropin and pregnancy-associated plasma protein A screening for Down syndrome can achieve detection rates as high as those associated with alpha-fetoprotein and human chorionic gonadotropin or alpha-fetoprotein, human chorionic gonadotropin, and unconjugated estriol screening in the second trimester. Prospective studies are needed to further assess first-trimester screening.

Enhanced twin pregnancy detection within an open neural tube defect and Down syndrome screening protocol using free-beta hCG and AFP

We have applied our multimarker approach of maternal serum alpha-fetoprotein (AFP) and free-beta human chorionic gonadotropin (hCG) for Down syndrome screening to multiple gestations to assess its efficacy for improved detection of twin and triplet pregnancies. This study matched 225 cases of twin pregnancy and 39 cases of triplet pregnancy each with ten singleton pregnancies based on gestational week, race, time to receive sample, time of year of sample, and geographical area. The ratios of the MOM for each group at the tenth, 50th, and 90th percentiles were compared by the Wilcoxon test. Risks for twins were calculated using Bayes' rule, the age-related incidence of twins, and the levels of AFP and free-beta hCG. The tenth, 50th and 90th percentiles of free-beta hCG MOMs in twin and triplet cases were 0.85, 1.99, and 4.51, and 1.38, 2.78, and 4.07, respectively. For AFP, the MOMs at these percentiles were 1.26, 1.91, and 2.99, and 2.02, 2.68, and 5.30, respectively. The twin and triplet distributions for each marker were statistically significantly different from the singleton distributions (P < 0.0001) and from each other (P = 0.0012). At a twin risk cut-off of 1 in 50, 77.4 per cent of all twin gestations can be detected in a second-trimester AFP and free-beta hCG screening protocol with 5.1 per cent of singleton pregnancies falsely identified as at risk for twins. Our dual marker protocol for mid-trimester pregnancy screening combining AFP and free-beta hCG can identify over 77 per cent of twin pregnancies in women less than 35 years of age. This benefit may contribute to an improved outcome of pregnancy by early detection of multiple gestation.

Analysis of maternal serum alpha-fetoprotein and free beta human chorionic gonadotrophin in the first trimester: implications for Down's syndrome screening

The aim of this study was to determine the maternal population, pregnancy, serum alpha-fetoprotein (AFP) and free beta subunit of human chorionic gonadotrophin (F beta hCG) parameters in a large series of women attending prenatal clinics before 15 weeks' gestation and to assess the practical problems of population screening for Down's syndrome in the first trimester using these markers. Serum samples were collected from 8600 women attending prenatal clinic booking visits. Maternal serum AFP and F beta hCG medians were calculated for each day of gestation (49-104 days), using both dates and ultrasound estimates of gestation. The effects of maternal weight, twin pregnancies, and threatened abortion on AFP and F beta hCG levels were analysed. The median age of the population was 27.1 years and the median weight 62.1 kg. Twenty-six per cent of samples were collected before 70 days and 50 per cent before 78 days' gestation. Eighty-nine per cent of all samples had gestational estimates by dates, 60 per cent by ultrasound and 52 per cent by both dates and ultrasound. The AFP median was 5 kU/l at 49 days, 5.9 kU/l at 70 days, and 17.9 kU/l at 100 days. The peak median F beta hCG level was 66.4 ng/ml at 64 days, falling to 20.6 ng/ml at 100 days' gestation. Both AFP and F beta hCG levels showed log Gaussian distributions but the standard deviation for AFP was 20 per cent greater than that found in the second trimester. AFP and F beta hCG levels showed an inverse relationship with maternal weight and were increased in twin pregnancies (1.68 and 1.97 multiples of the median, respectively). AFP and F beta hCG can be readily measured in a large screening population in the first trimester. Down's syndrome screening protocols based on these markers could be refined by the use of gestations in individual days but AFP is likely to be a less effective marker and detection rates are likely to be lower than in the second trimester. To realize the potential of first-trimester screening, more women should be encouraged to attend the prenatal clinic in early pregnancy and ultrasound dating should be carried out for all pregnancies at this stage.

First trimester biochemical screening for trisomy 21: the role of free beta hCG, alpha fetoprotein and pregnancy associated plasma protein A

The potential efficacy of screening for trisomy 21 in the first trimester, using maternal serum markers alpha fetoprotein, free beta human chorionic gonadotropin, unconjugated oestriol and pregnancy associated plasma protein A, was studied in an unselected population of women between the seventh and fourteenth week of gestation. Using a combination of alpha fetoprotein and free beta human chorionic gonadotropin, 53% of affected pregnancies could be identified at a false positive rate of 5%. Unconjugated oestriol and pregnancy associated plasma protein A levels were lower in cases of trisomy 21, but their inclusion with other markers did not significantly improve detection rate. Monitoring the same pregnancies also in the second trimester showed that screening in the first trimester identified the same cases as in the second. We conclude that first trimester screening using free beta human chorionic gonadotropin and alpha fetoprotein, is a viable possibility and will lead to detection rates in excess of 50%. Prospective studies are needed to confirm these observations.

Maternal serum free beta hCG screening: results of studies including 480 cases of Down syndrome

The median maternal serum free beta human chorionic gonadotropin (hCG) multiple of the median (MOM) of 480 Down syndrome cases in the second trimester was 2.64, significantly greater than the reported median MOM of intact hCG (p < 0.0001). In 234 of these cases from retrospective and prospective studies, the effectiveness of maternal serum free beta hCG was evaluated in combination with alpha-fetoprotein (AFP) and maternal age in second-trimester Down syndrome screening. Down syndrome detection in the gestational age range of 14-16 weeks was 82 per cent. In all gestational weeks (14-22), a 77.7 per cent Down syndrome detection rate was achieved. In prospective screening of 44,272 patients under the age of 35 years, 69 per cent of Down syndrome cases were detected (73 per cent in gestational weeks 14-16). The false-positive rate for the prospective study was 3.8 per cent. The use of free beta hCG combined with maternal serum AFP and maternal age-related risk for Down syndrome in a screening population (i.e., women under 35 years) yields an improved detection efficiency over other protocols.

Prenatal screening for trisomy 18 with free beta human chorionic gonadotrophin as a marker

OBJECTIVE

To determine the relation between maternal serum alpha fetoprotein and free beta human chorionic gonadotrophin concentrations in pregnancies complicated by trisomy 18 and establish whether prenatal biochemical screening for this condition could be developed in a way similar to that proposed for trisomy 21.

DESIGN

Serum alpha fetoprotein and free beta human chorionic gonadotrophin concentrations in women with singleton pregnancies affected by cytogenetically confirmed trisomy 18, uncomplicated by neural tube defect or ventral wall defect, were identified from prospective trisomy 21 screening programmes. Additionally, stored maternal serum from similar pregnancies was analysed retrospectively. Analyte concentrations from singleton unaffected pregnancies were identified from a prospective screening programme as controls. Statistical parameters of the affected and unaffected populations were compiled.

SETTING

Biochemical screening laboratories in Britain and the United States.

SUBJECTS

52 women with singleton pregnancies complicated by trisomy 18; control population of 6661 women with unaffected singleton pregnancies.

MAIN OUTCOME MEASURES

Median values of each analyte and their distribution in the affected and unaffected populations; detection rate of trisomy 18 and the false positive rate.

RESULTS

Maternal serum alpha fetoprotein and free beta human chorionic gonadotrophin concentrations were significantly lower in pregnancies complicated by trisomy 18 (median values 0.71 and 0.37 respectively). By using a multivariate risk algorithm incorporating maternal age risk of trisomy 18 and the concentration of the two biochemical markers it was predicted that 50% of trisomy 18 cases (unaffected by neural tube defect or ventral wall defect) could be detected with a 1% false positive rate.

CONCLUSION

Second trimester biochemical screening for trisomy 18 could be a valuable addition to trisomy 21 screening programmes.

First-trimester biochemical screening for fetal chromosome abnormalities and neural tube defects

Alpha-fetoprotein (AFP), unconjugated oestriol (UE3), intact human chorionic gonadotrophin (intHCG), and the free beta subunit of chorionic gonadotrophin (F beta HCG) were investigated in a series of 21 chromosomally abnormal and 14 open neural tube defect pregnancies ascertained from a series of 14,000 prospectively collected maternal serum samples at 6-14 weeks' gestation. In 16 cases of Down's syndrome, significant reductions were found for AFP (0.65 multiples of the normal median) and UE3 (0.67 MOM). IntHCG levels were unaltered (0.97 MOM) but a significant increase was found for F beta HCG (1.96 MOM). Significant correlations were found for AFP and UE3 in the controls and for intHCG and F beta HCG in both the control and the Down's syndrome pregnancies. In a group of five trisomy 18 pregnancies, median MOMs were for AFP 0.71, for UE3 0.34, for intHCG 0.27, and for F beta HCG 0.15. None of 13 pregnancies with open neural tube defects at 8-13 weeks gestation had elevated maternal serum AFP levels, whereas matched second-trimester samples from the same pregnancies at 16-18 weeks gestation all had significantly elevated AFP levels. Thus, biochemical screening for chromosome abnormalities may be practicable in the first trimester using free beta human chorionic gonadotrophin in combination with AFP and maternal age. However, a separate screening protocol using AFP at 15-18 weeks gestation would still be required for effective detection of neural tube defects.

Dual analyte immunoassay in neural tube defect and Down's syndrome screening: results of a multicentre clinical trial

We report a multicentre clinical field trial of a novel dual analyte enzyme immunoassay method for the simultaneous measurement of alpha-fetoprotein (AFP) and free beta-human choriogonadotropin (hCG) in the same microtitre well. The assay was shown to have good technical performance in the hands of all trial centres, with between assay coefficients of variation better than 10% for both analyte across the whole of the assay ranges. The method compared well with single analyte measuring procedures and produced acceptable performance as judged by external quality assurance criteria. Recovery of added analyte and analyte dilution curves also showed acceptable performance. In clinical evaluation of a large set of neural tube defect cases, good clinical discrimination from unaffected cases was observed using AFP. With over 150 Down's syndrome cases, the combination of AFP and free beta hCG confirmed the high detection rates achievable using this marker combination, with detection rates in excess of 70% in early gestation. We conclude that the combination of clinically superior markers coupled with technologically innovative assay design will lead to more efficient Down's screening programmes.

First-trimester free beta (hCG) screening for Down syndrome

Maternal serum free beta (hCG) levels are elevated (median 2.20 MOM) in the first trimester of pregnancy in 38 Down syndrome cases as compared with appropriate controls. This observation may form the basis for its use as a marker in screening for Down syndrome in the first trimester. Altered levels of the free beta analyte are observed in pregnancy conditions or complications other than Down syndrome.

Stability of intact chorionic gonadotropin (hCG) in serum, liquid whole blood, and dried whole-blood filter-paper spots: impact on screening for Down syndrome by measurement of free beta-hCG subunit

The use of multiple maternal serum biochemical markers in screening for Down syndrome is gaining worldwide acceptance. We sought to study the impact of the potential instability of intact human chorionic gonadotropin (hCG) on free beta-hCG subunit, a marker that has recently been used successfully in such screening. We found that, in practice, any changes in free beta-hCG due to the instability of intact hCG do not inhibit the effectiveness of free beta-hCG as a marker for Down syndrome. This was proven by controlled laboratory experiments at various stress temperatures, freeze-thaw studies, and analysis of a large set of screening data with particular reference to time in transit for individual samples. Data from controlled dissociation studies demonstrate that any apparent increase in free beta-hCG due to the instability of intact hCG cannot be attributed simply to the dissociation of intact hCG. Finally, for large-scale mass population screening in areas of the world where transport delays, safety concerns, and high temperatures preclude the shipment of liquid whole blood, dried whole-blood spots in filter paper provide a suitable delivery system with many advantages.

Stability of intact chorionic gonadotropin (hCG) in serum, liquid whole blood, and dried whole-blood filter-paper spots: impact on screening for Down syndrome by measurement of free beta-hCG subunit

The use of multiple maternal serum biochemical markers in screening for Down syndrome is gaining worldwide acceptance. We sought to study the impact of the potential instability of intact human chorionic gonadotropin (hCG) on free beta-hCG subunit, a marker that has recently been used successfully in such screening. We found that, in practice, any changes in free beta-hCG due to the instability of intact hCG do not inhibit the effectiveness of free beta-hCG as a marker for Down syndrome. This was proven by controlled laboratory experiments at various stress temperatures, freeze-thaw studies, and analysis of a large set of screening data with particular reference to time in transit for individual samples. Data from controlled dissociation studies demonstrate that any apparent increase in free beta-hCG due to the instability of intact hCG cannot be attributed simply to the dissociation of intact hCG. Finally, for large-scale mass population screening in areas of the world where transport delays, safety concerns, and high temperatures preclude the shipment of liquid whole blood, dried whole-blood spots in filter paper provide a suitable delivery system with many advantages.

Free beta-chorionic gonadotropin: a cross-reactivity study of two immunometric assays used in prenatal maternal serum screening for Down's syndrome

We have evaluated the cross-reactivity characteristics of two distinct immunometric assays for the measurement of free beta-human chorionic gonadotropin (free beta). These maternal serum assays have been used in the initial studies which evaluated free beta as a marker in the prenatal detection of Down's syndrome. It has been suggested that free beta assays are subject to substantial potential for cross-reactivity. To confirm that free beta was the analyte responsible for enhanced detection efficiency both non-competitive and competitive cross-reactivity evaluations were undertaken. These studies demonstrated acceptably small cross-reactivity to other glycoprotein hormones or their beta components. We conclude that properly designed free beta assays will provide specific analyte measurement and improved detection efficiency in Down's syndrome screening.

Dual Analyte immunoassay--a new approach to neural tube defect and Down's syndrome screening

A microtiter plate based Dual Analyte enzyme-immunoassay method for the simultaneous measurement of alpha-fetoprotein (AFP) and Free-beta human chorionic gonadotrophin (hCG) was evaluated. This rapid assay, which has application in both Neural Tube Defect screening and Down's screening, shows good precision with between assay coefficients of variation between 5 and 7.5% for AFP and 3.7 to 5.8% for Free-beta(hCG). Correlation with single analyte procedures is good, with correlation coefficients being greater than 0.91 in both cases. Clinical discrimination in detecting both types of abnormalities is not compromised by this new simultaneous Dual Analyte assay. We conclude that the Dual Analyte approach, which combines analytes achieving the highest known detection efficiency, will bring about improvements in the efficiency of screening, reduce costs and improve report turnaround, all leading to better quality of patient care.

Maternal serum Down syndrome screening: free beta-protein is a more effective marker than human chorionic gonadotropin

The use of quantitative human chorionic gonadotropin measurement in obstetrics has a long and successful history. Prior studies on the utility of human chorionic gonadotropin in Down syndrome screening have utilized assays that measure the intact human chorionic gonadotropin molecule. This study targeted a distinct marker, the human chorionic gonadotropin free beta-protein, which is present in second-trimester maternal serum at much lower concentrations than is intact human chorionic gonadotropin. Our study of 29 cases of trisomy 21 and 450 control samples shows 80% detection efficiency with maternal serum alpha-fetoprotein, the free beta-protein, and maternal age in pregnancies under 17 weeks' gestation. We conclude that the combination of maternal serum alpha-fetoprotein and the human chorionic gonadotropin free beta-protein will be useful in the prenatal detection of trisomy 21.

Maternal serum alpha-fetoprotein (MSAFP) patient-specific risk reporting: its use and misuse

Fundamental to maternal serum alpha-fetoprotein screening is the clinical utility of the laboratory report. It follows that the scientific form of expression in that report is vital. Professional societies concur that patient-specific risk reporting is the preferred form. However, some intermediate steps being taken to calculate patient-specific risks are invalid because of the erroneous assumption that multiples of the median (MoMs) represent an interlaboratory common currency. The numerous methods by which MoMs may be calculated belie the foregoing assumption.

Maternal serum Down syndrome screening: unconjugated estriol is not useful

Study of 41 known Down syndrome cases and 441 matched controls did not confirm earlier reports that low unconjugated estriol levels can be used to detect fetal Down syndrome. Hence the obstetric community should exercise caution in using unconjugated estriol levels as a marker in prenatal Down syndrome screening.

Maternal serum alpha-fetoprotein screening. III. Pitfalls in evaluating black gravid women

Maternal serum alpha-fetoprotein levels are higher in black women. Misinterpretation of maternal serum alpha-fetoprotein screening results can subject black gravid women to unnecessary invasive diagnostic procedures and their calculable risks. Screening errors for black women can result from the use of normative data bases established with maternal serum samples drawn from other racial groups or the use of such data bases in conjunction with a published correction factor. Because the incidence of open neural tube defects is lower for blacks than for others, excessive false positive results for blacks (estimated to be 8817 to 28,215 cases annually) would be a pernicious misapplication of maternal serum alpha-fetoprotein screening. We address the problem outlined above and recommend independently developed, valid, normative data bases.

Maternal serum alpha-fetoprotein screening. II. Pitfalls in low-volume decentralized laboratory performance

Assays of maternal serum alpha-fetoprotein are subject to the phenomenon of assay drift, which may be defined as incorrect increase or decrease of alpha-fetoprotein values from their true values. Low maternal serum alpha-fetoprotein weekly volume (for example, fewer than 500 specimens per week) will result in a greater than 47% probability that 10% assay drift will not be recognized. Further, laboratory reports to clinicians may lead to either misdirecting 43% more pregnant women (with positive drift) into further (possibly invasive) diagnostic procedures or the offer of further diagnostic services to 32% fewer gravidas at increased risk (with negative drift) than should be so managed. We address the problem outlined above and present the reasons for establishment of regional maternal serum alpha-fetoprotein screening programs operating at sufficient volume to permit the identification and control of assay drift.

Maternal serum alpha-fetoprotein screening, maternal weight, and detection efficiency

Various methods of adjusting maternal serum alpha-fetoprotein levels based on maternal weight have been recommended in order to more appropriately assign risk in maternal serum alpha-fetoprotein screening. These adjustments may, however, result in a larger proportion of pregnancies designated as having increased risk and reduction in detection accuracy.

Critical issues in prenatal maternal serum alpha-fetoprotein screening for genetic anomalies

Prenatal screening, in this Clinical Opinion article, is defined as a population-based search for subgroups of pregnancy that by virtue of their maternal serum alpha-fetoprotein levels may be at increased risk for genetic anomalies. The identification of subgroups at risk for neural tube defects, ventral wall defects, and chromosomal trisomies presents clinicians with unprecedented information and issues, including the biomedical basis for prenatal screening, the distinction between screening and diagnosis, and the decision-making function of the patient. The interest and, perhaps, ambivalence of the clinical community to prenatal screening are understandable. Obstetricians ordering prenatal screens are challenged by medical data associated with epidemiology, pathology, and genetics, including: prevalence of disease, interpretive biochemical reporting, and the assessment and communication of patient-specific risks. Obstetricians ordering screens will be called on to provide information generally used by other specialists at other stages of care. What information, how it is biomedically determined, its form for communication to patients, and pitfalls in the process are discussed.

An association between low maternal serum alpha-fetoprotein and fetal chromosomal abnormalities

An index case of "undetectable" maternal serum alpha-fetoprotein at 16 weeks in the first pregnancy of a 28-year-old woman was associated with birth of an infant with trisomy 18. This fortuitous finding stimulated a retrospective study of prenatally diagnosed chromosomal abnormalities. From among a series of 3,862 genetic amniocenteses, 32 cases of fetal autosomal trisomy were diagnosed for which corresponding maternal serum and amniotic fluid alpha-fetoprotein data could be retrieved. From a second laboratory, nine additional cases were added. The maternal serum alpha-fetoprotein levels expressed as multiples of the median were significantly lower in distribution for these 41 women than those from a group of normal matched control subjects (p less than 0.001). Since maternal age is shown to be a less than adequate predictor of autosomal trisomic birth, we proposed that a low level of maternal serum alpha-fetoprotein obtained through routine screening may prove to be valuable in improving the prenatal detection of these serious anomalies.

Rapid assay--possible application in the diagnosis of premature rupture of the membranes

Premature rupture of the membranes frequently presents a diagnostic dilemma to the clinician. The presence of alpha-fetoprotein (AFP) in amniotic fluid suggests a possible solution. A rapid, easy-to-use latex agglutination test for AFP was evaluated on 99 amniotic fluid samples of known AFP concentration. Body fluids (maternal serum, urine, vaginal secretions, and seminal fluid) that commonly interfere with other tests were also studied. The sensitivity for amniotic fluids from gestations less than 39 weeks was 93%, and specificity was 94%. The test is most accurate under 35 weeks, when diagnosis is critical. Equivocal results may be resolved by using radioimmunoassay. The results suggest that a rapid assay for AFP may be useful in the diagnosis of ruptured membranes. It has few interfering factors, is safe, and requires no elaborate equipment or training.

The clinical significance of low maternal serum alpha-fetoprotein

Lowered maternal serum alpha-fetoprotein was retrospectively investigated in 20,000 patients who underwent prenatal maternal serum alpha-fetoprotein screening. Two hundred thirty-two patients were initially evaluated as having maternal serum alpha-fetoprotein levels less than 0.25 multiple of the normal median. Correction factors, such as incorrect estimation of gestational age, reduced to 180 the number of patients with low alpha-fetoprotein. In this target group, patients with known pregnancy outcomes sustained a fetal loss in 38% of the cases. It appears that significantly low maternal serum alpha-fetoprotein is associated with a dramatic increase in second-trimester fetal wastage.

Second-trimester placental changes associated with elevated maternal serum alpha-fetoprotein

Measuring maternal serum alpha-fetoprotein (AFP) levels in the second trimester is an effective screening test for identifying pregnancies at increased risk for neural tube defects. In the absence of a neural tube defect there are many nonpathologic and pathologic causes for elevated AFP including underestimated gestational age, twin gestation, impending fetal death, and rare fetal malformations. In this series, intraplacental sonolucent spaces were detected in a significant percentage of second trimester pregnancies with elevated serum AFP in the absence of any other cause for the elevation. It is postulated that these cystic spaces are a conduit for the transfer of fetal blood into the maternal circulation, thus accounting for the nonpathologic AFP elevation in the maternal serum.

Prenatal serum alpha-fetoprotein screening for neural tube defects

Twenty-two cases of open neural tube defect were found in a population of 17,703 unselected pregnancies (1.2 per 1000) within the Long Island, New York, region. Voluntary screening of maternal serum alpha-fetoprotein levels identified 20 of the 22 cases (91%). Six hundred ninety-two participants demonstrated serial elevations in maternal serum alpha-fetoprotein. Of this group, which was designated at increased risk for open neural tube defect, 24% had underestimated gestational age, 13% had multiple gestations, and 53% were candidates for amniocentesis. In the amniocentesis group, the detection yield of neural tube defect was 20 per 365 (5.5%). Neither false-negative amniotic fluid evaluations nor termination of normal pregnancy due to false-positive amniotic fluid levels occurred. Perinatal outcome data, including pregnancy complications, date, mode of delivery, sex, birth weight, Apgar score, and congenital malformations of the neonate other than neural tube defect, are reported for the first 9300 consecutive participants of the 17,703 population study. These data identify a correlation between rate of perinatal loss and maternal serum alpha-fetoprotein levels.

gamma-Glutamyl transpeptidase of human amniotic fluid

gamma-Glutamyl transpeptidase (GGTP) activity in normal amniotic fluids and corresponding maternal sera obtained at various gestational periods was measured. The ontogenic pattern of enzyme activity in amniotic fluid is very similar to alpha fetoprotein (AFP). However, the levels of these two proteins behaved differently in corresponding maternal sera. Also, in amniotic fluids obtained from pregnancies with neural tube defects (NTD), only AFP concentration was abnormally high whereas GGTP activity was normal.