Assay precision of serum alpha fetoprotein in antenatal screening for neural tube defects and Down's syndrome

Down Syndrome Screening among Pregnant Women Visiting the Department of Obstetrics and Gynaecology of a Tertiary Care Centre

Introduction

The screening of Down's syndrome by the measurement of serum markers using dual and quadruple tests in the second trimester is done among obstetric patients between 13 to 22 weeks of gestation. The test readings are signified in terms of low-risk or high-risk. The aim of this study was to find out the prevalence of Downs syndrome screening among pregnant women visiting the Department of Obstetrics and Gynaecology of a tertiary care centre.

Methods

A descriptive cross-sectional study was conducted from 15 April 2022 to 15 December 2022 among patients visiting the Department of Obstetrics and Gynecology of a tertiary care centre. Ethical approval was taken from the Institutional Review Committee. Women with a singleton pregnancy who underwent dual and quadruple screening tests at 11 to 22 weeks of gestation were taken and analysed as per reports. Convenience sampling method was used. The point estimate was calculated at a 95% Confidence Interval.

Results

Among 268 women, Down syndrome screening was done in 200 (74.63%) (69.42-79.84, 95% Confidence Interval). Among them, 168 (84%) had a low risk for Down syndrome, and 32 (16%) had a high risk for Down syndrome.

Conclusions

The prevalence of Downs syndrome screening among pregnant women visiting the Department of Obstetrics and Gynecology of a tertiary care centre was found to be similar to other studies done in similar settings.

Keywords

down syndrome; nuchal translucency; screening.

Fetal myelomeningocele repair

Myelomeningocele is a congenital malformation that causes a spectrum of morbidity. With the standard of care now being in utero repair, the spectrum of morbidity has changed. The purpose of this article is to review the diagnosis, workup and treatment options of fetal myelomeningocele. We also review the obstetrical, neurological, gastrointestinal, urinary, and orthopedic outcomes of the in utero myelomeningocele repair.

Triple test screening for Down syndrome: an Egyptian-tailored study

BACKGROUND

The incidence of Down syndrome (DS) in Egypt varies between 1:555 and 1:770 and its screening by triple test is becoming increasingly popular nowadays. Results, however, seem inaccurate due to the lack of Egyptian-specific information needed for risk calculation and a clear policy for programme implementation. Our study aimed at calculation and validation of the triple marker medians used in screening Egyptian females as well as to recommend programme conventions to unify screening in this country.

METHODS

The study was conducted on 668 Egyptian women, in weeks 15-20 of pregnancy as proven by sonar. Chorionic gonadotropin (CG), α-fetoprotein (AFP) and unconjugated oestriol (uE3) were measured on Siemens Immulite analyzer. Medians of the three parameters were calculated, regressed against gestational age (GA) and weighted by the number of participants/week. Equations were derived to adjust each parameter to the maternal weight and were centered on the median Egyptian weight. Prisca software was fed with the above data, multiples-of-median (MoM) and DS risks were calculated and the screening performance was evaluated at a mid-trimester risk cutoff of 1:270.

RESULTS

Log-linear [AFP/uE3 = 10(A+B*GA)] and exponential equations [CG = A*e (B*GA)] were derived and the regressed medians were found to follow similar patterns to other Asian and Western medians. Oestriol was always lowest (even halved) while CG and AFP were intermediate. A linear reciprocal model best fitted weight distribution among Egyptians and successfully adjusted each parameter to a weight of 78.2 kg. Epidemiological monitoring of these recommendations revealed satisfactory performance in terms of 6.7% initial positive rate and 1.00 grand MoM.

CONCLUSIONS

Adoption of the above recommendations is hoped to pave the way to a successful DS screening programme tailored to Egyptian peculiarities.

Maternal Serum Screening Markers and Adverse Outcome: A New Perspective

There have been a number of studies evaluating the association of aneuploidy serum markers with adverse pregnancy outcome. More recently, the development of potential treatments for these adverse outcomes as well as the introduction of cell-free fetal DNA (cffDNA) screening for aneuploidy necessitates a re-evaluation of the benefit of serum markers in the identification of adverse outcomes. Analysis of the literature indicates that the serum markers tend to perform better in identifying pregnancies at risk for the more severe but less frequent form of individual pregnancy complications rather than the more frequent but milder forms of the condition. As a result, studies which evaluate the association of biomarkers with a broad definition of a given condition may underestimate the ability of such markers to identify pregnancies that are destined to develop the more severe form of the condition. Consideration of general population screening using cffDNA solely must be weighed against the fact that traditional screening using serum markers enables detection of severe pregnancy complications, not detectable with cffDNA, of which many may be amenable to treatment options.

Second and first trimester estimation of risk for Down syndrome: implementation and performance in the SAFER study

OBJECTIVES

Document patient choices and screening performance (false positive and detection rates) when three improved Down syndrome screening protocols were introduced coincidentally.

METHOD

Second-trimester 'triple marker' screening was expanded by adding second-trimester dimeric inhibin-A (four-marker), with or without first-trimester pregnancy-associated plasma protein-A (five-marker). Nuchal translucency (NT) measurements were included when available from accredited sonographers (six-marker). For assigning risk, two sets of marker distribution parameters were evaluated.

RESULTS

Over 3.5 years, 8571 women enrolled (median age 30.6 years). Uptake of the four-, five- and six-marker protocols was 18%, 46% and 36%, respectively. Of those selecting an integrated test (five or six markers), 9.7% did not provide the second trimester serum sample. False positive rates decreased with added markers (5.2%, 5.1% and 2.5%, respectively) and varied between the two parameter sets, while detection remained high. Overall, 21 of 23 cases were detected (91%, 95% CI 73-98%) at a 4.2% false positive rate (95% CI 3.3-5.1%).

CONCLUSIONS

Integrated screening protocols were chosen 4.6 times more often than four-marker screening (82% vs. 18% uptake). Overall detection was higher and false positives lower, consistent with recent guidelines. Important performance factors include gestational dating method, risk cut-off, and the parameter set used to assign risk.

Expected performance of second trimester maternal serum testing followed by a 'genetic sonogram' in screening of fetal Down syndrome

OBJECTIVE

To assess the efficacy of screening for fetal Down syndrome through the sequential provision of second trimester maternal serum tests and fetal ultrasound (the 'genetic sonogram').

METHODS

Monte Carlo modeling was used to generate typical results for second trimester serum screening. Serum test likelihood ratios were then multiplied by likelihood ratios associated with the provision of a genetic sonogram. The impact of adding the genetic sonogram on the detection rate (DR) and false-positive rate (FPR) were assessed for typical protocols that are currently in use. The effect of expanding the number of women who receive the genetic sonogram, variation in the marker frequencies in the genetic sonogram, and a multivariate Gaussian model that incorporated both serum and ultrasound measurements as continuous variables were also considered.

RESULTS

When the genetic sonogram is offered only to those women who are screen-positive by serum testing, there can be a substantial reduction in the number of women with an indication for amniocentesis but also some loss in detection. The extent of these changes will partially depend on the serum tests and cutoff used. Providing the genetic sonogram to more women can reduce loss in detection without resulting in high amniocentesis rates. As a sequential screening tool, the genetic sonogram can be improved by incorporating markers that have a high frequency in affected pregnancies and by using ultrasound measurements as continuous variables.

CONCLUSION

It should be possible to provide highly effective multistep screening protocols that maximize the benefits of both maternal serum and ultrasound while minimizing the amount of testing offered.

Evaluation of Down's syndrome screening population data sets by simulation: analyser-specific parameters may be superior to meta-analysis-derived parameters

OBJECTIVE

Choice of parameter sets used to calculate Down's syndrome risks is complicated. Published population statistics were compared with assay-specific parameters to optimise screening efficiency.

DESIGN

Weight-corrected Gaussian population statistics for alpha-fetoprotein (AFP), human chorionic gonadotropin (HCG) and unconjugated oestriol (uE(3)), expressed as log(10) multiples of median (MoM) were established for a Belgian cohort of 748 unaffected pregnancies. Using Cuckle's method and Access-specific data, Down's syndrome parameters were tailored to the Belgian cohort. Correlated marker triplets for affected and unaffected pregnancies were modelled and combined with maternal age to calculate term risks for Trisomy 21. Receiver-Operator-Curve (ROC) analysis was performed to identify the optimally-performing population set.

RESULTS

Log-normal distributions for the Access markers had geometric mean MoM values close to zero and standard deviation values equal to 0.1460 (AFP), 0.2185 (HCG) and 0.1317 (uE(3)). Correlation between AFP and other markers was significant (p < 0.001). Correlation between HCG and uE(3) was not significant (p = 0.4818). The median ratio between the lowest and highest risk outcomes for the test MoM set was 4.3. Areas under ROC curves differed significantly (p < 0.001) between the models and the analyser-assay specific parameters resulted in the largest area. At a 1 in 250 threshold, sensitivity and specificity were 69% and 96%. At false-positive rates (1-specificity) = 5%, sensitivity was 72.5%.

CONCLUSION

Population parameters significantly affect risk outcome and hence screening performance. Highest efficiency may be obtained with parameters tailored to an assay-specific population model. Consequently models from literature, without knowledge of the assay/analyser combination may lead to suboptimal performance.

[Prenatal serum marker screening in Korea: survey results]

BACKGROUND

Maternal serum triple marker screening has been covered by the medical insurance system in Korea since December 2004. The number of tests is on the increase, but an external quality control program and a basic survey have not been established yet. The aim of this study was to port the survey of prenatal screening tests.

METHODS

Three different quality control specimens were prepared using the sera obtained from 100 women who were in the 15th to 20th week of pregnancy and visited Asan Medical Center during May 2005. We assumed that the three specimens belonged to the first day of 15 weeks, third day of 16 weeks, and second day of 19 weeks, respectively, and sent them to 10 laboratories. Nine laboratories replied to the survey. We analyzed concentrations, multiples of medians (MoMs), and risk estimates.

RESULTS

The coefficients of variance of MoM were 32.1-32.6% for alpha-fetoprotein, 15.3-19.8% for unconjugated estriol, 6.3-12.5% for human chorionic gonadotropin, and 12.9-18.2% for inhibin-A. In Down syndrome risk estimation for specimen-2, six of the eight laboratories that used the triple test reported the screen positive, but two laboratories reported negative. Three of five laboratories using the quadruple test reported the screen positive, and two laboratories reported negative. In case of neural tube defect, all laboratories except one reported all specimens the screen negative. In case of Edward syndrome, all laboratories reported all specimens the screen negative.

CONCLUSIONS

Since MoMs and risk estimates showed a wide variation among the participating laboratories in this survey, an external quality control and the standardization of the variables seemed warranted.

Reproducibility of Risk Figures in 2nd-Trimester Maternal Serum Screening for Down Syndrome: Comparison of 2 Laboratories

Background: Analytical error affects 2nd-trimester maternal serum screening for Down syndrome risk estimation. We analyzed the between-laboratory reproducibility of risk estimates from 2 laboratories.

Methods: Laboratory 1 used Bayer ACS180 immunoassays for α-fetoprotein (AFP) and human chorionic gonadotropin (hCG), Diagnostic Systems Laboratories (DSL) RIA for unconjugated estriol (uE3), and DSL enzyme immunoassay for inhibin-A (INH-A). Laboratory 2 used Beckman immunoassays for AFP, hCG, and uE3, and DSL enzyme immunoassay for INH-A. Analyte medians were separately established for each laboratory. We used the same computational algorithm for all risk calculations, and we used Monte Carlo methods for computer modeling.

Results: For 462 samples tested, risk figures from the 2 laboratories differed &gt;2-fold for 44.7%, &gt;5-fold for 7.1%, and &gt;10-fold for 1.7%. Between-laboratory differences in analytes were greatest for uE3 and INH-A. The screen-positive rates were 9.3% for laboratory 1 and 11.5% for laboratory 2, with a significant difference in the patients identified as screen-positive vs screen-negative (McNemar test, P &lt;0.001). Computer modeling confirmed the large between-laboratory risk differences.

Conclusion: Differences in performance of assays and laboratory procedures can have a large effect on patient-specific risks. Screening laboratories should minimize test imprecision and ensure that each assay performs in a manner similar to that assumed in the risk computational algorithm.

Medians for second-trimester maternal serum markers: geographical differences and variation caused by median multiples-of-median equations

OBJECTIVES

To establish gestational age-specific mid-trimester normal medians for the prenatal serum markers alpha fetoprotein (AFP), human chorionic gonadotropin (HCG) and unconjugated oestriol (uE3) for a Belgian population by using the Beckman Coulter Access chemiluminiscent immunoassays; to compare these data with data obtained from other geographical regions; to propose regression coefficients for regressed medians and analyse variation induced by different regression equations; to evaluate the effect of formulas used for gestation correction on estimating risk in Down's syndrome.

DESIGN

Data derived from 862 fresh serum samples from women being screened for Down's syndrome pregnancy, composed of selected pregnancies deemed to be normal, were examined in a retrospective study. Regressed medians were calculated by using a first-degree logarithmic-linear fit of the raw data. Multiples-of-median (MoM) values estimated by using a simple logarithmic-linear equation were compared with those calculated with higher-degree polynomials chosen with a goodness-of-fit analysis. Model-specific variation was estimated and the effect on risk for Down's syndrome was evaluated.

RESULTS

Regressed medians (Y) for Access serum markers AFP (IU/ml), HCG (IU/ml) and uE3 (nmol/l) for a Belgian population can be estimated with the equation Y = 10((A+BX)) with X = decimal weeks. The best fit was obtained with a third-degree and a second-degree polynomial for AFP and uE3, respectively. Differences between the medians and among the slopes of the geographical populations were found to be significant (analysis of covariance, p<0.001).

CONCLUSIONS

Belgian marker medians versus gestational time are found to show a pattern that is similar to that in the literature. The log-linear equation is observed to give a good fit and can be suggested as a tool for calculating median MoM values for Belgian laboratories that use Access biochemical prenatal markers.

First-trimester or second-trimester screening, or both, for Down's syndrome

BACKGROUND

It is uncertain how best to screen pregnant women for the presence of fetal Down's syndrome: to perform first-trimester screening, to perform second-trimester screening, or to use strategies incorporating measurements in both trimesters.

METHODS

Women with singleton pregnancies underwent first-trimester combined screening (measurement of nuchal translucency, pregnancy-associated plasma protein A [PAPP-A], and the free beta subunit of human chorionic gonadotropin at 10 weeks 3 days through 13 weeks 6 days of gestation) and second-trimester quadruple screening (measurement of alpha-fetoprotein, total human chorionic gonadotropin, unconjugated estriol, and inhibin A at 15 through 18 weeks of gestation). We compared the results of stepwise sequential screening (risk results provided after each test), fully integrated screening (single risk result provided), and serum integrated screening (identical to fully integrated screening, but without nuchal translucency).

RESULTS

First-trimester screening was performed in 38,167 patients; 117 had a fetus with Down's syndrome. At a 5 percent false positive rate, the rates of detection of Down's syndrome were as follows: with first-trimester combined screening, 87 percent, 85 percent, and 82 percent for measurements performed at 11, 12, and 13 weeks, respectively; with second-trimester quadruple screening, 81 percent; with stepwise sequential screening, 95 percent; with serum integrated screening, 88 percent; and with fully integrated screening with first-trimester measurements performed at 11 weeks, 96 percent. Paired comparisons found significant differences between the tests, except for the comparison between serum integrated screening and combined screening.

CONCLUSIONS

First-trimester combined screening at 11 weeks of gestation is better than second-trimester quadruple screening but at 13 weeks has results similar to second-trimester quadruple screening. Both stepwise sequential screening and fully integrated screening have high rates of detection of Down's syndrome, with low false positive rates.

Prenatal screening for Down syndrome: current and future methods

Second-trimester serum screening for Down syndrome has had a relatively long clinical life, beginning in the mid-1980s and continuing to the present day. In the past few years, however, new screening methods that involve testing just a few weeks earlier and the integration of first-trimester and second-trimester markers have been proposed and are being used. These improved methods have begun the transition to better and, hopefully, safer prenatal screening. In the past, as many as 1 in 10 pregnant women learned that they were at increased risk of having a baby with a serious birth defect and had to decide whether to have an invasive diagnostic procedure. Now, screening methods are at the point where as few as 1 in 50 or 1 in 100 pregnant women are found to be at increased risk. The ultimate goal in screening is to make noninvasive testing methods so safe that only those few women who are found to be at the very highest risk will need to face the uncertainty of invasive procedures. In the next few years, that goal will probably be achieved.

Evaluation of cell-free fetal DNA as a second-trimester maternal serum marker of Down syndrome pregnancy

BACKGROUND

Second-trimester cell-free fetal DNA (studied only in pregnancies with male fetuses) is higher in maternal serum samples from women carrying Down syndrome fetuses than in unaffected pregnancies. In this study we evaluated the potential performance of fetal DNA as a screening marker for Down syndrome.

METHODS

Data on maternal serum fetal DNA concentrations and the corresponding concentrations of the quadruple serum markers were available from 15 Down syndrome cases, each matched for gestational age and length of freezer storage, with 5 control samples. Analyte values were expressed as multiple(s) of the control or population median. Screening performance of fetal DNA, both alone and when added to estimates of quadruple marker performance, was determined after modeling using univariate and multivariate gaussian distribution analysis.

RESULTS

The median fetal DNA concentration in Down syndrome cases was 1.7 times higher than in controls. In univariate analysis, fetal DNA gave a 21% detection rate at a 5% false-positive rate. When added to quadruple marker screening, fetal DNA increased the estimated detection rate from 81% to 86% at a 5% false-positive rate.

CONCLUSIONS

Cell-free fetal DNA, measured in maternal serum, can modestly increase screening performance above what is currently available in the second trimester. If and when maternal serum fetal DNA can be measured in pregnancies with both male and female fetuses, the utility and cost-effectiveness of adding it as a Down syndrome screening marker should be assessed.

Outcome validation of the Beckman Coulter access analyzer in a second-trimester Down syndrome serum screening application

BACKGROUND

Mid-trimester maternal serum alpha-fetoprotein (AFP) and unconjugated estriol (uE3) are 30% lower and human chorionic gonadotropin (hCG) is twofold higher in Down syndrome pregnancies compared with unaffected pregnancies. In maternal serum screening, patient-specific risks are calculated using published gaussian frequency distribution parameters for these three markers obtained with previously available immunoassays. New immunoassays must generate similar distribution parameters if the accuracy of assigned risks and overall performance of prenatal screening are to be maintained.

METHODS

Agreement between the Beckman Coulter Access and the Bayer Immuno 1 assays for AFP and hCG and the Amersham Amerlex-M RIA for uE3 was assessed in 558 fresh sera. Precision was measured over 6 weeks. Median concentrations were calculated by regression of 568 Caucasian singleton pregnancy samples against gestational age in days. Frozen mid-trimester sera from 44 confirmed Down syndrome singleton pregnancies (cases) were selected without conscious bias for reanalysis, and each case was matched with five control specimens from unaffected pregnancies. Serum markers were expressed as the multiple of the median (MoM) concentration derived from the control samples, corrected for maternal weight and converted to their log-equivalent values. Normality was assessed using probability plots and the Shapiro-Wilk W-test. Gaussian distribution parameters were compared with established values, and Down syndrome risk calculations were assessed with a commonly used risk algorithm.

RESULTS

The Access AFP and hCG assays had consistent proportional agreement with the established assays, whereas agreement between the uE3 methods was less consistent. Analytical imprecision was 3-6% at mid-trimester concentrations. Normal distributions were obtained for the log MoM values of all three markers in both the Down syndrome and unaffected populations, and their gaussian distribution parameters compared well with established values. The performance of the Access assays in an established trivariate risk algorithm for Down syndrome was equal to the performance exhibited by traditional methods.

CONCLUSION

The Beckman Coulter Access analyzer provides valid mid-trimester serum AFP, uE3, and hCG results and risk assessments when applied in a prenatal Down syndrome screening service.

Advances in prenatal screening for Down syndrome: II first trimester testing, integrated testing, and future directions

BACKGROUND

The acceptability of prenatal screening and diagnosis of Down syndrome is dependent, in part, on the gestational age at which the testing is offered. First trimester screening could be advantageous if it has sufficient efficacy and can be effectively delivered.

ISSUES

Two first trimester maternal serum screening markers, pregnancy-associated plasma protein-A (PAPP-A) and free beta-human chorionic gonadotropin (beta-hCG), are useful for identifying women at increased risk for fetal Down syndrome. In addition, measurement of an enlarged thickness of the subcutaneous fluid-filled space at the back of the neck of the developing fetus (referred to as nuchal translucency or NT) has been demonstrated to be an indicator for these high-risk pregnancies. When these three parameters are combined, estimates for Down syndrome efficacy exceed those currently attainable in the second trimester. Women who are screen-positive in the first trimester can elect to receive cytogenetic testing of a chorionic villus biopsy. The first trimester tests could also, theoretically, be combined with the second trimester maternal serum screening tests (integrated screening) to obtain even higher levels of efficacy. There are, however, several practical limitations to first trimester and integrated screening. These include scheduling of testing within relatively narrow gestational age intervals, availability of appropriately trained ultrasonographers for NT measurement, risks associated with chorionic villus biopsy, and costs. There is also increasing evidence that an enlarged NT measurement is indicative of a high risk for spontaneous abortion and for fetal abnormalities that are not detectable by cytogenetic analysis. Women whose fetuses show enlarged NT, therefore, need first trimester counseling regarding their Down syndrome risks and the possibility of other adverse pregnancy outcomes. Follow-up ultrasound and fetal echocardiography in the second trimester are also indicated.

CONCLUSION

First trimester screening appears to be a highly effective method to screen for Down syndrome. Women with screen-positive results based on NT measurement appear to be at increased risk for diverse fetal abnormalities. The finding of a normal fetal karyotype may not, therefore, carry a high level of reassurance for a normal baby.

Influence of maternal smoking on the birth prevalence of Down syndrome and on second trimester screening performance

OBJECTIVES

To determine the influence of maternal smoking on the birth prevalence of Down syndrome and on second trimester screening performance.

METHODS

First, a meta-analysis of cohort and case-control studies was performed to estimate the effect of maternal smoking on the live birth prevalence of Down syndrome. Then, data from 8779 women screened using the quadruple test (alphafetoprotein (AFP), unconjugated estriol (uE(3)), human chorionic gonadotrophin (free beta-hCG), and inhibin-A levels with maternal age) were used to determine the effect of smoking on the serum markers. A Monte Carlo simulation was used to assess the impact of adjusting for smoking status on screening performance.

RESULTS

The relative risk of Down syndrome in smokers (versus non-smokers) was 0.95 (95% confidence interval (CI) 0.87 to 1.03). Serum marker levels were determined as multiples of the median for non-smokers of the same gestational age and adjusted for maternal weight (MoM). The MoM values for AFP were 5% higher (95% CI 2-7%), uE(3) 4% lower (95% CI 2-5%), free beta-hCG 20% lower (95% CI 17-23%) and inhibin-A 62% higher (95% CI 57-67%) in smokers compared with non-smokers. Adjusting marker levels for smoking resulted in less than a 1 percentage point increase in the detection rate for a 5% false positive rate with the double, triple or quadruple tests.

CONCLUSIONS

There is no evidence of an association between the birth prevalence of Down syndrome and maternal smoking. The case for adjusting screening marker levels for smoking is not compelling. But if smoking data are collected routinely adjustment could be made and this would yield similar detection and false positive rates in smokers and non-smokers.

Advances in prenatal screening for Down syndrome: I. general principles and second trimester testing

BACKGROUND

Down syndrome is one of the most important causes of mental retardation in the population. In the absence of prenatal screening and diagnosis, prevalence at birth in the United States would currently exceed 1:600. The purpose of prenatal screening is to identify those women at the increased risk for an affected pregnancy and to maximize the options available to these women.

TESTS AVAILABLE

Second trimester serum screening involves combining the maternal age-specific risk for an affected pregnancy with the risks associated with the concentrations of maternal serum alpha-fetoprotein (MSAFP), unconjugated estriol (uE3), and human chorionic gonadotropin (hCG) (triple testing). A forth analyte, inhibin-A (INH-A), is increasingly being utilized (quadruple testing). Optimal second trimester screening requires the integration of a number of clinical variables, the most important of which is an accurate assessment of gestational age. In addition to Down syndrome, the triple and quadruple tests preferentially identify fetal trisomy 18, Turner syndrome, triploidy, trisomy 16 mosaicism, fetal death, Smith-Lemli-Opitz syndrome, and steroid sulfatase deficiency. Some programs modify the Down syndrome risks generated through maternal serum screening tests with fetal biometric data obtained by ultrasound. Other second trimester tests have shown promise, including the analysis of maternal urine and fetal cells in the maternal circulation, but none are in routine clinical use.

CONCLUSION

The second trimester triple and quadruple tests provide benchmarks for evaluating new screening protocols. The combination of fetal biometry, new test development as well as clarification of the role of co-factors that affect the concentrations of analytes in existing tests should lead to greater efficacy in second trimester screening for Down syndrome.

Assessment of the value of reporting partial screening results in prenatal screening for Down syndrome

Prenatal screening for Down syndrome can be performed using the first trimester Combined Test [nuchal translucency (NT), pregnancy-associated plasma protein A (PAPP-A), free beta-human chorionic gonadotrophin (hCG) and maternal age] or the Integrated Test (for example, NT and PAPP-A in the first trimester and two or more serum markers in the second trimester, all with maternal age). We investigated the value of providing partial results when using the Combined Test or Integrated Test to identify women with a high enough risk of having an affected pregnancy based on NT and maternal age alone such that there would be little advantage in combining this information with data on the serum markers. We also assessed whether in programmes using the Integrated Test it is worthwhile reporting partial results based on risk using first trimester markers and not obtaining a second trimester blood sample. Published data based on 480 affected and 96 839 unaffected pregnancies were used for the present study. Using NT and age alone, about 0.14% of all women screened would have such a high risk that they would always remain screen-positive after the Combined Test and only 0.06% would remain screen-positive after the Integrated Test. Similarly, about 0.07% of all women screened who have a high risk based on NT, PAPP-A and age would remain screen-positive after the Integrated Test. These percentages are too small to justify reporting two risk estimates for all women, given the confusion this would generate. It is therefore not worthwhile reporting partial risk estimates in screening programmes using the Combined Test or Integrated Test.