The advantages of using triple-marker screening for chromosomal abnormalities

First trimester ultrasound tests alone or in combination with first trimester serum tests for Down's syndrome screening

BACKGROUND

Down's syndrome occurs when a person has three, rather than two copies of chromosome 21; or the specific area of chromosome 21 implicated in causing Down's syndrome. It is the commonest congenital cause of mental disability and also leads to numerous metabolic and structural problems. It can be life-threatening, or lead to considerable ill health, although some individuals have only mild problems and can lead relatively normal lives. Having a baby with Down's syndrome is likely to have a significant impact on family life.Non-invasive screening based on biochemical analysis of maternal serum or urine, or fetal ultrasound measurements, allows estimates of the risk of a pregnancy being affected and provides information to guide decisions about definitive testing.Before agreeing to screening tests, parents need to be fully informed about the risks, benefits and possible consequences of such a test. This includes subsequent choices for further tests they may face, and the implications of both false positive and false negative screening tests (i.e. invasive diagnostic testing, and the possibility that a miscarried fetus may be chromosomally normal). The decisions that may be faced by expectant parents inevitably engender a high level of anxiety at all stages of the screening process, and the outcomes of screening can be associated with considerable physical and psychological morbidity. No screening test can predict the severity of problems a person with Down's syndrome will have.

OBJECTIVES

To estimate and compare the accuracy of first trimester ultrasound markers alone, and in combination with first trimester serum tests for the detection of Down's syndrome.

SEARCH METHODS

We carried out extensive literature searches including MEDLINE (1980 to 25 August 2011), Embase (1980 to 25 August 2011), BIOSIS via EDINA (1985 to 25 August 2011), CINAHL via OVID (1982 to 25 August 2011), and The Database of Abstracts of Reviews of Effects (the Cochrane Library 2011, Issue 7). We checked reference lists and published review articles for additional potentially relevant studies.

SELECTION CRITERIA

Studies evaluating tests of first trimester ultrasound screening, alone or in combination with first trimester serum tests (up to 14 weeks' gestation) for Down's syndrome, compared with a reference standard, either chromosomal verification or macroscopic postnatal inspection.

DATA COLLECTION AND ANALYSIS

Data were extracted as test positive/test negative results for Down's and non-Down's pregnancies allowing estimation of detection rates (sensitivity) and false positive rates (1-specificity). We performed quality assessment according to QUADAS criteria. We used hierarchical summary ROC meta-analytical methods to analyse test performance and compare test accuracy. Analysis of studies allowing direct comparison between tests was undertaken. We investigated the impact of maternal age on test performance in subgroup analyses.

MAIN RESULTS

We included 126 studies (152 publications) involving 1,604,040 fetuses (including 8454 Down's syndrome cases). Studies were generally good quality, although differential verification was common with invasive testing of only high-risk pregnancies. Sixty test combinations were evaluated formed from combinations of 11 different ultrasound markers (nuchal translucency (NT), nasal bone, ductus venosus Doppler, maxillary bone length, fetal heart rate, aberrant right subclavian artery, frontomaxillary facial angle, presence of mitral gap, tricuspid regurgitation, tricuspid blood flow and iliac angle 90 degrees); 12 serum tests (inhibin A, alpha-fetoprotein (AFP), free beta human chorionic gonadotrophin (ßhCG), total hCG, pregnancy-associated plasma protein A (PAPP-A), unconjugated oestriol (uE3), disintegrin and metalloprotease 12 (ADAM 12), placental growth factor (PlGF), placental growth hormone (PGH), invasive trophoblast antigen (ITA) (synonymous with hyperglycosylated hCG), growth hormone binding protein (GHBP) and placental protein 13 (PP13)); and maternal age. The most frequently evaluated serum markers in combination with ultrasound markers were PAPP-A and free ßhCG.Comparisons of the 10 most frequently evaluated test strategies showed that a combined NT, PAPP-A, free ßhCG and maternal age test strategy significantly outperformed ultrasound markers alone (with or without maternal age) except nasal bone, detecting about nine out of every 10 Down's syndrome pregnancies at a 5% false positive rate (FPR). In both direct and indirect comparisons, the combined NT, PAPP-A, free ßhCG and maternal age test strategy showed superior diagnostic accuracy to an NT and maternal age test strategy (P < 0.0001). Based on the indirect comparison of all available studies for the two tests, the sensitivity (95% confidence interval) estimated at a 5% FPR for the combined NT, PAPP-A, free ßhCG and maternal age test strategy (69 studies; 1,173,853 fetuses including 6010 with Down's syndrome) was 87% (86 to 89) and for the NT and maternal age test strategy (50 studies; 530,874 fetuses including 2701 Down's syndrome pregnancies) was 71% (66 to 75). Combinations of NT with other ultrasound markers, PAPP-A and free ßhCG were evaluated in one or two studies and showed sensitivities of more than 90% and specificities of more than 95%.High-risk populations (defined before screening was done, mainly due to advanced maternal age of 35 years or more, or previous pregnancies affected with Down's syndrome) showed lower detection rates compared to routine screening populations at a 5% FPR. Women who miscarried in the over 35 group were more likely to have been offered an invasive test to verify a negative screening results, whereas those under 35 were usually not offered invasive testing for a negative screening result. Pregnancy loss in women under 35 therefore leads to under-ascertainment of screening results, potentially missing a proportion of affected pregnancies and affecting test sensitivity. Conversely, for the NT, PAPP-A, free ßhCG and maternal age test strategy, detection rates and false positive rates increased with maternal age in the five studies that provided data separately for the subset of women aged 35 years or more.

AUTHORS' CONCLUSIONS

Test strategies that combine ultrasound markers with serum markers, especially PAPP-A and free ßhCG, and maternal age were significantly better than those involving only ultrasound markers (with or without maternal age) except nasal bone. They detect about nine out of 10 Down's affected pregnancies for a fixed 5% FPR. Although the absence of nasal bone appeared to have a high diagnostic accuracy, only five out of 10 affected Down's pregnancies were detected at a 1% FPR.

First and second trimester serum tests with and without first trimester ultrasound tests for Down's syndrome screening

BACKGROUND

Down's syndrome occurs when a person has three copies of chromosome 21 (or the specific area of chromosome 21 implicated in causing Down's syndrome) rather than two. It is the commonest congenital cause of mental disability. Non-invasive screening based on biochemical analysis of maternal serum or urine, or fetal ultrasound measurements, allows estimates of the risk of a pregnancy being affected and provides information to guide decisions about definitive testing.  Before agreeing to screening tests, parents need to be fully informed about the risks, benefits and possible consequences of such a test. This includes subsequent choices for further tests they may face, and the implications of both false positive (i.e. invasive diagnostic testing, and the possibility that a miscarried fetus may be chromosomally normal) and false negative screening tests (i.e. a fetus with Down's syndrome will be missed). The decisions that may be faced by expectant parents inevitably engender a high level of anxiety at all stages of the screening process, and the outcomes of screening can be associated with considerable physical and psychological morbidity. No screening test can predict the severity of problems a person with Down's syndrome will have.

OBJECTIVES

To estimate and compare the accuracy of first and second trimester serum markers with and without first trimester ultrasound markers for the detection of Down's syndrome in the antenatal period, as combinations of markers.

SEARCH METHODS

We conducted a sensitive and comprehensive literature search of MEDLINE (1980 to 25 August 2011), Embase (1980 to 25 August 2011), BIOSIS via EDINA (1985 to 25 August 2011), CINAHL via OVID (1982 to 25 August 2011), the Database of Abstracts of Reviews of Effectiveness (the Cochrane Library 25 August 2011), MEDION (25 August 2011), the Database of Systematic Reviews and Meta-Analyses in Laboratory Medicine (25 August 2011), the National Research Register (Archived 2007), and Health Services Research Projects in Progress database (25 August 2011). We did not apply a diagnostic test search filter. We did forward citation searching in ISI citation indices, Google Scholar and PubMed 'related articles'. We also searched reference lists of retrieved articles SELECTION CRITERIA: Studies evaluating tests of combining first and second trimester maternal serum markers in women up to 24 weeks of gestation for Down's syndrome, with or without first trimester ultrasound markers, compared with a reference standard, either chromosomal verification or macroscopic postnatal inspection.

DATA COLLECTION AND ANALYSIS

Data were extracted as test positive/test negative results for Down's and non-Down's pregnancies allowing estimation of detection rates (sensitivity) and false positive rates (1-specificity). We performed quality assessment according to QUADAS criteria. We used hierarchical summary ROC meta-analytical methods to analyse test performance and compare test accuracy. Analysis of studies allowing direct comparison between tests was undertaken. We investigated the impact of maternal age on test performance in subgroup analyses.

MAIN RESULTS

Twenty-two studies (reported in 25 publications) involving 228,615 pregnancies (including 1067 with Down's syndrome) were included. Studies were generally high quality, although differential verification was common with invasive testing of only high risk pregnancies. Ten studies made direct comparisons between tests. Thirty-two different test combinations were evaluated formed from combinations of eight different tests and maternal age; first trimester nuchal translucency (NT) and the serum markers AFP, uE3, total hCG, free βhCG, Inhibin A, PAPP-A and ADAM 12. We looked at tests combining first and second trimester markers with or without ultrasound as complete tests, and we also examined stepwise and contingent strategies.Meta-analysis of the six most frequently evaluated test combinations showed that a test strategy involving maternal age and a combination of first trimester NT and PAPP-A, and second trimester total hCG, uE3, AFP and Inhibin A significantly outperformed other test combinations that involved only one serum marker or NT in the first trimester, detecting about nine out of every 10 Down's syndrome pregnancies at a 5% false positive rate. However, the evidence was limited in terms of the number of studies evaluating this strategy, and we therefore cannot recommend one single screening strategy.

AUTHORS' CONCLUSIONS

Tests involving first trimester ultrasound with first and second trimester serum markers in combination with maternal age are significantly better than those without ultrasound, or those evaluating first trimester ultrasound in combination with second trimester serum markers, without first trimester serum markers. We cannot make recommendations about a specific strategy on the basis of the small number of studies available.

Urine tests for Down's syndrome screening

BACKGROUND

Down's syndrome occurs when a person has three copies of chromosome 21, or the specific area of chromosome 21 implicated in causing Down's syndrome, rather than two. It is the commonest congenital cause of mental disability and also leads to numerous metabolic and structural problems. It can be life-threatening, or lead to considerable ill health, although some individuals have only mild problems and can lead relatively normal lives. Having a baby with Down's syndrome is likely to have a significant impact on family life. The risk of a Down's syndrome affected pregnancy increases with advancing maternal age.Noninvasive screening based on biochemical analysis of maternal serum or urine, or fetal ultrasound measurements, allows estimates of the risk of a pregnancy being affected and provides information to guide decisions about definitive testing. Before agreeing to screening tests, parents need to be fully informed about the risks, benefits and possible consequences of such a test. This includes subsequent choices for further tests they may face, and the implications of both false positive and false negative screening tests (i.e. invasive diagnostic testing, and the possibility that a miscarried fetus may be chromosomally normal). The decisions that may be faced by expectant parents inevitably engender a high level of anxiety at all stages of the screening process, and the outcomes of screening can be associated with considerable physical and psychological morbidity. No screening test can predict the severity of problems a person with Down's syndrome will have.

OBJECTIVES

To estimate and compare the accuracy of first and second trimester urine markers for the detection of Down's syndrome.

SEARCH METHODS

We carried out a sensitive and comprehensive literature search of MEDLINE (1980 to 25 August 2011), EMBASE (1980 to 25 August 2011), BIOSIS via EDINA (1985 to 25 August 2011), CINAHL via OVID (1982 to 25 August 2011), The Database of Abstracts of Reviews of Effectiveness (The Cochrane Library 2011, Issue 7), MEDION (25 August 2011), The Database of Systematic Reviews and Meta-Analyses in Laboratory Medicine (25 August 2011), The National Research Register (archived 2007), Health Services Research Projects in Progress database (25 August 2011). We studied reference lists and published review articles.

SELECTION CRITERIA

Studies evaluating tests of maternal urine in women up to 24 weeks of gestation for Down's syndrome, compared with a reference standard, either chromosomal verification or macroscopic postnatal inspection.

DATA COLLECTION AND ANALYSIS

We extracted data as test positive or test negative results for Down's and non-Down's pregnancies allowing estimation of detection rates (sensitivity) and false positive rates (1-specificity). We performed quality assessment according to QUADAS (Quality Assessment of Diagnostic Accuracy Studies) criteria. We used hierarchical summary ROC (receiver operating characteristic) meta-analytical methods to analyse test performance and compare test accuracy. We performed analysis of studies allowing direct comparison between tests. We investigated the impact of maternal age on test performance in subgroup analyses.

MAIN RESULTS

We included 19 studies involving 18,013 pregnancies (including 527 with Down's syndrome). Studies were generally of high quality, although differential verification was common with invasive testing of only high-risk pregnancies. Twenty-four test combinations were evaluated formed from combinations of the following seven different markers with and without maternal age: AFP (alpha-fetoprotein), ITA (invasive trophoblast antigen), ß-core fragment, free ßhCG (beta human chorionic gonadotrophin), total hCG, oestriol, gonadotropin peptide and various marker ratios. The strategies evaluated included three double tests and seven single tests in combination with maternal age, and one triple test, two double tests and 11 single tests without maternal age. Twelve of the 19 studies only evaluated the performance of a single test strategy while the remaining seven evaluated at least two test strategies. Two marker combinations were evaluated in more than four studies; second trimester ß-core fragment (six studies), and second trimester ß-core fragment with maternal age (five studies).In direct test comparisons, for a 5% false positive rate (FPR), the diagnostic accuracy of the double marker second trimester ß-core fragment and oestriol with maternal age test combination was significantly better (ratio of diagnostic odds ratio (RDOR): 2.2 (95% confidence interval (CI) 1.1 to 4.5), P = 0.02) (summary sensitivity of 73% (CI 57 to 85) at a cut-point of 5% FPR) than that of the single marker test strategy of second trimester ß-core fragment and maternal age (summary sensitivity of 56% (CI 45 to 66) at a cut-point of 5% FPR), but was not significantly better (RDOR: 1.5 (0.8 to 2.8), P = 0.21) than that of the second trimester ß-core fragment to oestriol ratio and maternal age test strategy (summary sensitivity of 71% (CI 51 to 86) at a cut-point of 5% FPR).

AUTHORS' CONCLUSIONS

Tests involving second trimester ß-core fragment and oestriol with maternal age are significantly more sensitive than the single marker second trimester ß-core fragment and maternal age, however, there were few studies. There is a paucity of evidence available to support the use of urine testing for Down's syndrome screening in clinical practice where alternatives are available.

First trimester serum tests for Down's syndrome screening

BACKGROUND

Down's syndrome occurs when a person has three, rather than two copies of chromosome 21; or the specific area of chromosome 21 implicated in causing Down's syndrome. It is the commonest congenital cause of mental disability and also leads to numerous metabolic and structural problems. It can be life-threatening, or lead to considerable ill health, although some individuals have only mild problems and can lead relatively normal lives. Having a baby with Down's syndrome is likely to have a significant impact on family life.Noninvasive screening based on biochemical analysis of maternal serum or urine, or fetal ultrasound measurements, allows estimates of the risk of a pregnancy being affected and provides information to guide decisions about definitive testing. However, no test can predict the severity of problems a person with Down's syndrome will have.

OBJECTIVES

The aim of this review was to estimate and compare the accuracy of first trimester serum markers for the detection of Down's syndrome in the antenatal period, both as individual markers and as combinations of markers. Accuracy is described by the proportion of fetuses with Down's syndrome detected by screening before birth (sensitivity or detection rate) and the proportion of women with a low risk (normal) screening test result who subsequently had a baby unaffected by Down's syndrome (specificity).

SEARCH METHODS

We conducted a sensitive and comprehensive literature search of MEDLINE (1980 to 25 August 2011), Embase (1980 to 25 August 2011), BIOSIS via EDINA (1985 to 25 August 2011), CINAHL via OVID (1982 to 25 August 2011), The Database of Abstracts of Reviews of Effectiveness (The Cochrane Library 25 August 2011), MEDION (25 August 2011), The Database of Systematic Reviews and Meta-Analyses in Laboratory Medicine (25 August 2011), The National Research Register (Archived 2007), Health Services Research Projects in Progress database (25 August 2011). We did forward citation searching ISI citation indices, Google Scholar and PubMed 'related articles'. We did not apply a diagnostic test search filter. We also searched reference lists and published review articles.  

SELECTION CRITERIA

We included studies in which all women from a given population had one or more index test(s) compared to a reference standard (either chromosomal verification or macroscopic postnatal inspection). Both consecutive series and diagnostic case-control study designs were included. Randomised trials where individuals were randomised to different screening strategies and all verified using a reference standard were also eligible for inclusion. Studies in which test strategies were compared head-to-head either in the same women, or between randomised groups were identified for inclusion in separate comparisons of test strategies. We excluded studies if they included less than five Down's syndrome cases, or more than 20% of participants were not followed up.

DATA COLLECTION AND ANALYSIS

We extracted data as test positive or test negative results for Down's and non-Down's pregnancies allowing estimation of detection rates (sensitivity) and false positive rates (1-specificity). We performed quality assessment according to QUADAS (Quality Assessment of Diagnostic Accuracy Studies) criteria. We used hierarchical summary ROC meta-analytical methods or random-effects logistic regression methods to analyse test performance and compare test accuracy as appropriate. Analyses of studies allowing direct and indirect comparisons between tests were undertaken.

MAIN RESULTS

We included 56 studies (reported in 68 publications) involving 204,759 pregnancies (including 2113 with Down's syndrome). Studies were generally of good quality, although differential verification was common with invasive testing of only high-risk pregnancies. We evaluated 78 test combinations formed from combinations of 18 different tests, with or without maternal age; ADAM12 (a disintegrin and metalloprotease), AFP (alpha-fetoprotein), inhibin, PAPP-A (pregnancy-associated plasma protein A, ITA (invasive trophoblast antigen), free βhCG (beta human chorionic gonadotrophin), PlGF (placental growth factor), SP1 (Schwangerschafts protein 1), total hCG, progesterone, uE3 (unconjugated oestriol), GHBP (growth hormone binding protein), PGH (placental growth hormone), hyperglycosylated hCG, ProMBP (proform of eosinophil major basic protein), hPL (human placental lactogen), (free αhCG, and free ßhCG to AFP ratio. Direct comparisons between two or more tests were made in 27 studies.Meta-analysis of the nine best performing or frequently evaluated test combinations showed that a test strategy involving maternal age and a double marker combination of PAPP-A and free ßhCG significantly outperformed the individual markers (with or without maternal age) detecting about seven out of every 10 Down's syndrome pregnancies at a 5% false positive rate (FPR). Limited evidence suggested that marker combinations involving PAPP-A may be more sensitive than those without PAPP-A.

AUTHORS' CONCLUSIONS

Tests involving two markers in combination with maternal age, specifically PAPP-A, free βhCG and maternal age are significantly better than those involving single markers with and without age. They detect seven out of 10 Down's affected pregnancies for a fixed 5% FPR. The addition of further markers (triple tests) has not been shown to be statistically superior; the studies included are small with limited power to detect a difference.The screening blood tests themselves have no adverse effects for the woman, over and above the risks of a routine blood test. However some women who have a 'high risk' screening test result, and are given amniocentesis or chorionic villus sampling (CVS) have a risk of miscarrying a baby unaffected by Down's. Parents will need to weigh up this risk when deciding whether or not to have an amniocentesis or CVS following a 'high risk' screening test result.

Beyond Trisomy 21: Additional Chromosomal Anomalies Detected through Routine Aneuploidy Screening

Prenatal screening is often misconstrued by patients as screening for trisomy 21 alone; however, other chromosomal anomalies are often detected. This study aimed to systematically review the literature and use diagnostic meta-analysis to derive pooled detection and false positive rates for aneuploidies other than trisomy 21 with different prenatal screening tests. Non-invasive prenatal testing had the highest detection (DR) and lowest false positive (FPR) rates for trisomy 13 (DR: 90.3%; FPR: 0.2%), trisomy 18 (DR: 98.1%; FPR: 0.2%), and 45,X (DR: 92.2%; FPR: 0.1%); however, most estimates came from high-risk samples. The first trimester combined test also had high DRs for all conditions studied (trisomy 13 DR: 83.1%; FPR: 4.4%; trisomy 18 DR: 91.9%; FPR: 3.5%; 45,X DR: 70.1%; FPR: 5.4%; triploidy DR: 100%; FPR: 6.3%). Second trimester triple screening had the lowest DRs and highest FPRs for all conditions (trisomy 13 DR: 43.9%; FPR: 8.1%; trisomy 18 DR: 70.5%; FPR: 3.3%; 45,X DR: 77.2%; FPR: 9.3%). Prenatal screening tests differ in their ability to accurately detect chromosomal anomalies. Patients should be counseled about the ability of prenatal screening to detect anomalies other than trisomy 21 prior to undergoing screening.

Measurement of Unconjugated Estriol in Serum by Liquid Chromatography–Tandem Mass Spectrometry and Assessment of the Accuracy of Chemiluminescent Immunoassays

BACKGROUND

Unconjugated estriol (uE3) is routinely analyzed in clinical laboratories as risk assessment for Down syndrome. Immunoassays of various types are the most commonly used methods. The accuracies of RIAs and ELISAs for uE3 have been questioned, and to date there have been no independent studies investigating the accuracy of the relatively new chemiluminescent immunoassays. We developed and validated a liquid chromatography–tandem mass spectrometry (LC-MS/MS) method for uE3 measurements in serum.

METHODS

Serum samples from patients in the second trimester of pregnancy were used, and uE3 concentrations were measured by LC-MS/MS and the Beckman Coulter Access® 2 and Siemens IMMULITE 2000 automatic chemiluminescent immunoassay analyzers.

RESULTS

The LC-MS/MS method was validated and showed limit of detection 0.05 ng/mL; limit of quantification 0.2 ng/mL; linearity of response to 32 ng/mL; total imprecision of 16.2%, 10.4%, and 8.2% for uE3 at 1.10, 4.18, and 8.32 ng/mL, respectively; and analytical recoveries of 95.9%–104.2%. ANOVA of the correlation for LC-MS/MS results vs chemiluminescent immunoassays results showed R2 = 0.9678 (Access 2 = 0.9305 LC-MS/MS + 0.2177, Sy|x = 0.1786, P &lt; 0.0001), and R2 = 0.9663 (IMMULITE 2000 = 0.8849 LC-MS/MS − 0.0403, Sy|x = 0.1738, P &lt; 0.0001). Bland–Altman plots of uE3 results revealed concentration-dependent immunoassay biases. Mock risk analysis for Down syndrome showed no apparent difference in the risk assessment outcomes if the adjusted method-specific multiples of the median were used, and the assay imprecision was &lt;10% CV.

CONCLUSIONS

Standardization of immunoassay methods for uE3 analysis is needed to improve the accuracy of the measurements.

Second trimester serum tests for Down's Syndrome screening

BACKGROUND

Down's syndrome occurs when a person has three copies of chromosome 21 - or the specific area of chromosome 21 implicated in causing Down's syndrome - rather than two. It is the commonest congenital cause of mental retardation. Noninvasive screening based on biochemical analysis of maternal serum or urine, or fetal ultrasound measurements, allows estimates of the risk of a pregnancy being affected and provides information to guide decisions about definitive testing.  

OBJECTIVES

To estimate and compare the accuracy of second trimester serum markers for the detection of Down's syndrome.

SEARCH METHODS

We carried out a sensitive and comprehensive literature search of MEDLINE (1980 to May 2007), EMBASE (1980 to 18 May 2007), BIOSIS via EDINA (1985 to 18 May 2007), CINAHL via OVID (1982 to 18 May 2007), The Database of Abstracts of Reviews of Effectiveness (The Cochrane Library 2007, Issue 1), MEDION (May 2007), The Database of Systematic Reviews and Meta-Analyses in Laboratory Medicine (May 2007), The National Research Register (May 2007), Health Services Research Projects in Progress database (May 2007). We studied reference lists and published review articles.  

SELECTION CRITERIA

Studies evaluating tests of maternal serum in women at 14-24 weeks of gestation for Down's syndrome, compared with a reference standard, either chromosomal verification or macroscopic postnatal inspection.

DATA COLLECTION AND ANALYSIS

Data were extracted as test positive/test negative results for Down's and non-Down's pregnancies allowing estimation of detection rates (sensitivity) and false positive rates (1-specificity). We performed quality assessment according to QUADAS criteria. We used hierarchical summary ROC meta-analytical methods to analyse test performance and compare test accuracy. Analysis of studies allowing direct comparison between tests was undertaken. We investigated the impact of maternal age on test performance in subgroup analyses.

MAIN RESULTS

Fifty-nine studies involving 341,261 pregnancies (including 1,994 with Down's syndrome) were included. Studies were generally high quality, although differential verification was common with invasive testing of only high-risk pregnancies. Seventeen studies made direct comparisons between tests. Fifty-four test combinations were evaluated formed from combinations of 12 different tests and maternal age; alpha-fetoprotein (AFP), unconjugated oestriol (uE3), total human chorionic gonadotrophin (hCG), free beta human chorionic gonadotrophin (βhCG), free alpha human chorionic gonadotrophin (αhCG), Inhibin A, SP2, CA125, troponin, pregnancy-associated plasma protein A (PAPP-A), placental growth factor (PGF) and proform of eosinophil major basic protein (ProMBP).Meta-analysis of 12 best performing or frequently evaluated test combinations showed double and triple tests (involving AFP, uE3, total hCG, free βhCG) significantly outperform individual markers, detecting six to seven out of every 10 Down's syndrome pregnancies at a 5% false positive rate. Tests additionally involving inhibin performed best (eight out of every 10 Down's syndrome pregnancies) but were not shown to be significantly better than standard triple tests in direct comparisons. Significantly lower sensitivity occurred in women over the age of 35 years. Women who miscarried in the over 35 group were more likely to have been offered an invasive test to verify a negative screening results, whereas those under 35 were usually not offered invasive testing for a negative screening result. Pregnancy loss in women under 35 therefore leads to under ascertainment of screening results, potentially missing a proportion of affected pregnancies and affecting the accuracy of the sensitivity.

AUTHORS' CONCLUSIONS

Tests involving two or more markers in combination with maternal age are significantly more sensitive than those involving one marker. The value of combining four or more tests or including inhibin have not been proven to show statistically significant improvement. Further study is required to investigate reduced test performance in women aged over 35 and the impact of differential pregnancy loss on study findings.

Second trimester maternal serum biochemical screening for Down's syndrome: experience of a single obstetric unit

The objective of this study was to assess the effects of mid-trimester biochemical screening on the prenatal diagnosis of Down's syndrome. All patients booked for antenatal care at the Royal Free Hospital are offered a double marker biochemical screening test (alpha-fetoprotein and free beta human chorionic gonadotrophin). The results of double marker screening tests and the notes of all cases of trisomy 21 from 1 April 1993 to 31 March 1995 were reviewed. Of the 6170 women booked in the antenatal clinic during this 2-year period 4427 (71.8%) took up the offer of biochemical screening. Four hundred and twenty-nine (9.7% of those screened) were reported positive and 293 karyotypes were performed in this group (68.3% uptake) and 253 karyotypes were also performed for other indications. Thirteen of 21 affected pregnancies had biochemical screening. The detection rate of biochemical screening in women aged <37 years was 0% (0/2) with a 8.4% false positive rate. In women aged 37 years the detection rate was 100% (11/11) with a false positive rate of 23%. The overall detection rate for Down's syndrome was 84.6% and the false positive rate was 9.5%. In conclusion, biochemical screening successfully targeted affected pregnancies in women aged 37 years and was useful in reducing the fetal karyotyping rate. However, it was disappointing in the prenatal diagnosis of trisomy 21 in women <37 years old.

First trimester ultrasound screening for fetal aneuploidy and middle cerebral artery Doppler assessment for fetal alloimmunization

PURPOSE OF REVIEW

Traditional methods of fetal evaluation require invasive procedures (e.g. amniocentesis) with an inherent, albeit low, risk of complications and pregnancy loss. As medical knowledge and ultrasound technology have advanced, noninvasive modes of fetal evaluation have become increasingly used. Two disorders in which this has been most applied are fetal aneuploidy and alloimmunization to red blood cell antigens.

RECENT FINDINGS

First-trimester screening protocols for fetal aneuploidy combine ultrasound measurement of the fetal nuchal translucency with the measurement of two maternal serum hormones, free beta-human chorionic gonadotropin and pregnancy-associated plasma protein-A. This combination detects approximately 85% of women pregnant with fetuses with trisomy 21. An increase in the velocity of fetal middle cerebral artery blood flow occurs in the setting of moderate to severe fetal anemia and can be measured with Doppler ultrasound. Middle cerebral artery evaluation is equally sensitive in the prediction of anemia as amniocentesis for bilirubin breakdown products and has less associated risk. It has an additional benefit because it can also be applied to causes of anemia other than hemolysis.

SUMMARY

Noninvasive methods of fetal assessment are becoming increasingly commonplace. There has been a dramatic decrease in the use of prenatal diagnostic services as screening strategies have become available. Doppler assessment of the velocity of blood flow through the fetal middle cerebral artery will allow for more judicious application of invasive procedures to pregnancies at significant risk for anemia.

Prenatal diagnosis of 47,XXX

We report 2 cases of 47,XXX that were diagnosed prenatally and were screened positive for trisomy 21 by biochemical and ultrasound markers. These cases underline the importance of discussing the sex chromosome abnormalities during the genetic counseling after an abnormal triple screen test or ultrasound examination.

An outcomes analysis of five prenatal screening strategies for trisomy 21 in women younger than 35 years

OBJECTIVE

This study was undertaken to examine the cost-effectiveness and procedural-related losses associated with 5 prenatal screening strategies for fetal aneuploidy in women under 35 years old.

STUDY DESIGN

Five prenatal screening strategies were compared in a decision analysis model: triple screen: maternal age and midtrimester serum alpha-fetoprotein, human chorionic gonadotropin (hCG), and unconjugated estriol; quad screen: triple screen plus serum dimeric inhibin A; first-trimester screen: maternal age, serum pregnancy-associated plasma protein A and free beta-hCG and fetal nuchal translucency at 10 to 14 weeks' gestation; integrated screen: first-trimester screen plus quad screen, but first-trimester results are withheld until the quad screen is completed when a composite result is provided; sequential screen: first-trimester screen plus quad screen, but the first-trimester screen results are provided immediately and prenatal diagnosis offered if positive; later prenatal diagnosis is available if the quad screen is positive. Model estimates were literature derived, and cost estimates also included local sources. The 5 strategies were compared for cost, the numbers of Down syndrome fetuses detected and live births averted, and the number of procedure-related euploid losses. Sensitivity analyses were performed for parameters with imprecise point estimates.

RESULTS

In the baseline analysis, sequential screening was the least expensive strategy ($455 million). It detected the most Down syndrome fetuses (n=1213), averted the most Down syndrome live births (n=678), but led to the highest number of procedure-related euploid losses (n=859). The integrated screen had the fewest euploid losses (n=62) and averted the second most Down syndrome live births (n=520). If fewer than 70% of women diagnosed with fetal Down syndrome elect to abort, the quad screen became the least expensive strategy.

CONCLUSION

Although sequential screening was the most cost-effective prenatal screening strategy for fetal trisomy 21, it had the highest procedure-related euploid loss rate. The patient's perspective on detection versus fetal safety may help define the optimal screening strategy.

Robertsonian translocations: mechanisms of formation, aneuploidy, and uniparental disomy and diagnostic considerations

Robertsonian translocations (ROBs) are rearrangements of the acrocentric chromosomes 13-15 and 21-22. Cytologically, ROBs between homologous chromosomes cannot be distinguished from isochromosomes that originate through duplication of a single homologue. Both types of rearrangements can be involved in aneuploidy. A conceptus with a trisomy or a monosomy can be rescued, and in a proportion of cases, a uniparental disomy (UPD) would result. If there are regions of genome imprinting on a uniparental chromosome pair, phenotypic consequences can result. Chromosomes 14 and 15 are imprinted, and UPD of these are known to result in abnormalities. Thus, prenatal testing should be considered in all pregnancies when one of the parents is a balanced carrier of a ROB because of the risk for aneuploidy, and UPD testing should be considered in fetuses found to carry a balanced ROB or isochromosome that involves chromosomes 14 or 15. Additionally, infants or children with congenital anomalies who carry a ROB should also be considered for UPD testing.

Combined first-trimester versus second-trimester serum screening for Down syndrome: a cost analysis

OBJECTIVE

The purpose of this study was to compare the cost-effectiveness of combined first-trimester screening for fetal Down syndrome with second-trimester maternal serum triple screening.

STUDY DESIGN

A first-trimester screening approach that used nuchal translucency measurement and maternal serum screening was evaluated against second-trimester maternal serum triple screening in a hypothetic population. Screening sensitivities and screen-positive rates were 91% and 5% for the first-trimester approach and 70% and 7.5% for the second-trimester approach, respectively. The costs of fetal Down syndrome, live-born Down syndrome cost, and total costs (screening plus live-born costs) were calculated for each screening program.

RESULTS

First-trimester screening was associated with lower screening and live-born Down syndrome costs versus second-trimester serum screening. Total Down syndrome screening costs were 29.1% lower with first-trimester screening.

CONCLUSION

In this hypothetic model, combined first-trimester screening for fetal Down syndrome was more cost-effective than universal second-trimester triple serum screening.

Second trimester maternal serum analytes in triploid pregnancies: correlation with phenotype and sex chromosome complement

Second trimester maternal serum alpha-fetoprotein (MS-AFP), human chorionic gonadotrophin (hCG), unconjugated estiol (uE3), and inhibin-A (INH-A) levels were evaluated in pregnancies complicated by triploidy. In addition to seven new triploid pregnancies, the results for 67 published cases were reviewed. All cases appear to fall into two major groups. First, those identifiable as screen-positive for both Down syndrome and an open neural tube defect (ONTD) with elevated MS-AFP, grossly elevated hCG, low/normal uE3, and probably elevated INH-A. Pregnancies in the second group are identifiable as screen-positive for trisomy 18 with low/normal MS-AFP, and very low hCG, uE3 and INH-A. Triploid pregnancies with high maternal serum hCG nearly always show a placenta with partial mole (25/27 or 93%), a high frequency of ONTDs or ventral wall defects (VWDs) (8/28 or 29%) and have either an XXX or XXY karyotype (observed ratio 6:10, respectively). Low hCG is infrequently associated with a molar placenta (1/11 or 9%), does not appear to be associated with ONTDs or VWDs (0/29 or 0%), and shows an excess of XXX over XXY karyotypes (observed ratio 17:2). There were 16 cases with either a molar placenta, an ONTD or a VWD that received the MS-AFP and hCG tests. All 16 were screen-positive for an ONTD (MS-AFP> or =2 multiples of the median). In addition, all 31 cases that received MS-AFP, hCG, uE3 (and where available INH-A) were screen-positive for either Down syndrome or trisomy 18. The findings are discussed in the context of expected differences between digynic and diandric triploidy. It is suggested that the sex chromosome complement in triploidy is an important factor in determining risk for partial mole development and in utero survival.

Down's syndrome screening: a controversial test, with more controversy to come!

By 1998, most health authorities offered antenatal screening for Down's syndrome, usually by biochemical methods. To date, the development of this form of screening has not been coordinated by a national body and, consequently, there are wide variations in practice between localities. Fortunately, many of these variations have not led to any noticeable inequality of health provision, but the wide variation in risk cut offs used by different centres does. Other variations merely lead to potentially unnecessary expenditure; whereas it is believed that adding extra tests to the screening procedure is beneficial (such as double test to triple test), statistical evaluation of the confidence intervals for the detection rates quoted indicates that there is no evidence that the extra test provides an increase in detection. The cervical screening programme has progressively improved, partly through the auspices of a national framework. A similar national approach would benefit Down's screening and is only now being considered: the national screening committee (NSC) is currently drafting recommendations. To ensure optimum screening performance, the NSC should specify the risk thresholds applied, the screening protocols to be used--that is, an opt-in programme with a minimum (possibly even a maximum) of two biochemical analytes or a nuchal fold evaluation--and perhaps should even recommend national population parameters to be used for risk calculation. It might even be advisable for statistical work to be carried out to determine whether local derivation of medians is truly necessary. Furthermore, defined options for older women could be specified--for example, should all older patients have the option to proceed directly to amniocentesis if they wish or should National Health Service amniocentesis only be available for those with a "high risk" screening result. The difficulties that will face the NSC in deciding which screening policy to adopt are also considered; specifically, the lack of evidence to suggest that triple testing is superior to double testing, and the lack of evidence to prove the superiority of one analyte over another. This inadequacy of evidence is not from want of trying, but is caused by the problems of collecting enough data to provide statistical significance. Finally, there is one important difference between cervical and Down's syndrome screening that has a major impact on the advice given by any "expert"; namely, patents. Many aspects of Down's screening are subject to patents and, therefore, there is more potential for apparently uncontroversial decisions to rebound with future retrospective patent infringement claims. Thus, it would be sensible to insist that any member of a national body deciding upon Down's screening policy must fully disclose all potential conflicts of interest, both personal and family, before they are allowed to sit on the committee. Furthermore, if a national policy is decided upon, worldwide patent searches should be carried out to determine whether there are any possible unforeseen legal consequences of any recommendation.

Six year survey of screening for Down's syndrome by maternal age and mid-trimester ultrasound scans

OBJECTIVE

To assess the effectiveness of antenatal screening for Down's syndrome by maternal age and routine mid-pregnancy ultrasound scanning.

DESIGN

Retrospective six year survey.

SETTING

Maternity units of a district general hospital.

SUBJECTS

Pregnant women booked for delivery in hospital between 1 January 1993 and 31 December 1998.

MAIN OUTCOME MEASURES

All cases of Down's syndrome occurring in district identified from regional congenital anomaly register and cytogenetic laboratory records. Women's case notes were examined to identify indication for karyotyping, gestation at diagnosis, and outcome of pregnancy.

RESULTS

31 259 deliveries occurred during study period, and 57 cases of Down's syndrome were identified, four in failed pregnancies and 53 in ongoing pregnancies or in neonates. The analysis was confined to ongoing pregnancies or liveborn children. Invasive antenatal tests were performed in 6.6% (2053/31 259), and 68% (95% confidence interval 56% to 80%) of cases of Down's syndrome were detected antenatally, giving a positive predictive value of 1.8%. There were 17 undetected cases, and in seven of these the women had declined an offer of invasive testing. In women aged less than 35 years the detection rate was 53% (30% to 76%). Most of the cases detected in younger women followed identification of ultrasound anomalies.

CONCLUSIONS

The overall detection rate was considerably higher than assumed in demonstration projects for serum screening. As a result, the benefits of serum screening are much less than supposed. Before any new methods to identify Down's syndrome are introduced, such as nuchal translucency or first trimester serum screening, the techniques should be tested in properly controlled trials.

Second trimester maternal serum hCG level in an Asian population: normal reference values by ultrasound dating

OBJECTIVE

To establish normative data of maternal serum chorionic gonadotropin (hCG) during the second trimester in an Asian population.

METHODS

We measured the maternal serum hCG levels in 17,955 normal singleton pregnancies between 15 and 21 weeks of gestation. The gestation age was estimated by measurement of fetal biparietal distance (BPD) in all cases. Median values of hCG at various gestational weeks were calculated and the values of hCG were converted to multiple of median (MoM). The incidences of low MoM value and high MoM value were also calculated.

RESULTS

The mean and median values of hCG were 57,153 mIU/ml and 50,120 mIU/ml, respectively, at 15 weeks of gestation and then decreased to 30,898 mIU/ml and 26,226 mIU/ml, respectively, at 21 weeks. We found 8.6% and 9.4% of normal singleton pregnancies have hCG MoM values >2.0 MoM and <0.5 MoM, respectively.

CONCLUSIONS

Our report provides a normal reference data of second trimester maternal hCG levels by ultrasound dating in an Asian population.

The triple-marker test in predicting fetal aneuploidy: a compromise between sensitivity and specificity

OBJECTIVE

to review the contribution of unconjugated estriol in Down's syndrome detection, and influence of maternal age, cut-off choice, and population specificity on the balance between triple-marker test sensitivity and specificity.

STUDY DESIGN

Prenatal karyotyping was performed in 2833 pregnant women, 73% of them over the age of 34. Duration of gestation was determined by ultrasound in 98% of women. Prior to amniocentesis, the serum levels of alpha-fetoprotein, human chorionic gonadotropin and unconjugated estriol were evaluated and corrected for weight. The risk of trisomy 21 was calculated for the first 986 subjects using default medians, and for 1847 by our population-specific medians. The cut-off was initially 1:300 at term, but the 1:100 and 1:200 risks were also tested. Down syndrome risk was calculated with alpha-fetoprotein and human chorionic gonadotropin combination as well. Linear logistic regression model was performed to test the ability of aneuploidy markers to classify patients into normal and trisomic groups.

RESULTS

There were twelve cases of Down's syndrome, seven of trisomy 18, four of trisomy 13, and one trisomy 22. Four cases of aneuploidy (16.7%) referred to women younger than 35. With the cut-off risk of 1:300, detection rate was 87.5% and specificity 63.3%, and with 1:100, 66.7% and 79.5%, respectively. The sensitivity for Down's syndrome was from 75% for cut-off=1:100 to 92% for cut-off=1:300, while detection of other trisomies was less successful (58% and 83%, respectively). Exclusion of unconjugated estriol MoM from the risk calculations reduced detection rate by 33% and improved specificity by 4% independently of cut-off choice. Linear logistic regression analysis showed that only unconjugated estriol was able to correctly classify patients between normal and trisomy 21 (p=0.011, odds ratio=1.445), and normal and trisomy 18 (p=0.0023, odds ratio=1.96) groups.

CONCLUSIONS

The unconjugated estriol significantly contributes in Down's syndrome detection with cost of slightly reduced specificity. The 1:300 risk caused an unfavorable compromise between sensitivity and specificity. A higher cut-off, 1:100, would indicate performance of amniocentesis in women aged 39 years and older, and in those aged 35-39 only after the screen-positive result.

Fetal Down syndrome screening: a cost effectiveness analysis of alternative screening programs

OBJECTIVE

To compare the efficacy and cost effectiveness of different screening programs for fetal Down syndrome (DS).

METHODS

Screening tools evaluated included maternal age, triple screening (TS), and ultrasound (U/S) for fetal markers of DS. Sensitivities used were TS: 55% <35 years, 80% > or = 35 years; U/S: 70%. Average regional fees used were TS: $80, U/S: $200, amniocentesis (AM): $1,000. Five screening programs were evaluated: 1) <35 years, no screening; > or = 35 years, AM; 2) <35 years, TS with AM for screen-positive subjects; > or = 35 years, AM; 3) all patients, TS with AM for screen-positive subjects; 4) <35 years, TS followed by U/S for screen-positive women, AM for women with fetal markers of DS on U/S; > or = 35 years TS with AM for screen-positive subjects; 5) all women, TS followed by U/S for screen-positive women, AM for women with fetal markers of DS on U/S. The sensitivity, total cost, cost/case DS detected (Cost/DS), AM losses, and residual risk (RR, undetected DS fetuses/women not receiving AM) were calculated for each screening program. Population analysis was performed using 1988 IL delivery statistics.

RESULTS

It was estimated that 260 cases of DS would occur in the population of 167,654 women (8.4% > or = 35 years at delivery). Sensitivity for programs 1-5 was 30, 69, 62, 51, and 36 percent, respectively, and cost/DS was 181,000, 203,000, 162,000, 151,000, and 194,000 dollars, respectively.

CONCLUSIONS

DS screening incorporating TS in all patients with program #4 and without program #3 selective U/S in women <35 years yield the best combination of sensitivity and cost effectiveness while minimizing the number of AM-related losses.

A retrospective evaluation of second-trimester serum screening for fetal trisomy 18: experience of two laboratories

A retrospective study on screening methods for fetal trisomy 18 has been carried out in two different laboratories using the serum parameters: total human chorionic gonadotropin (hCG), unconjugated oestriol (uE3), and alpha-fetoprotein (AFP) in different combinations and in single marker protocols. Laboratory A (L(A)) utilized a radio-immunoassay to examine 38 fetal trisomy 18 cases and laboratory B (L(B)) utilized an enzyme-immunoassay to examine 33 trisomy 18 cases. As unaffected references the whole routine cohorts of each laboratory were used (L(A): 29 043; L(B): 4264). In both trisomy 18 study groups the median hCG and uE3 multiples of the median (MoM) values were markedly declined (L(A): 0.21 MoM, 0.37 MoM; L(B): 0.31 MoM, 0.44 MoM). Even after exclusion of trisomy 18 cases with combined neural tube or ventral wall defects the medians of AFP MoM values were only moderately declined (L(A): 0.73 MoM; L(B): 0.8 MoM). Receiver-operator characteristic (ROC) curves after multivariate discriminance analysis and single marker evaluation demonstrated that the difference of efficiency between a combination of hCG, uE3 and AFP, and a combination of hCG and uE3 is small but that any of these combinations are more efficient than a combination of hCG and AFP or single marker protocols, respectively. At a risk cut-off generating a false-positive rate of one per cent the most effective marker combination detected 31 of 38 (81.6 per cent) affected pregnancies in L(A) and 25 of 33 (75.8 per cent) in L(B). The differences in sensitivity and specificity seem to be due to the different analytical systems being utilized by the two laboratories.

Prenatal diagnosis of de novo interstitial deletion of proximal 4q by maternal serum screening for Down syndrome

Deletion of the proximal portion of chromosome 4q is apparently rare. To our knowledge, prenatal diagnosis of the interstitial deletion of 4q12-21.1 has never been reported. We present a prenatal case of 4q deletion in association with a positive Down syndrome screening test of an elevated maternal serum free beta human chorionic gonadotrophin (beta-hCG) level. The prenatal sonogram revealed intra-uterine growth retardation (IUGR) and shortening of the femur. Facial dysmorphism included micrognathia, depressed nasal bridge and low-set ears, these anomalies were evident at the postnatal examination. All of the anomalies were consistent with those described in proximal 4q deletion syndrome. Our case suggests that chromosome studies may be indicated for patients with high maternal serum free beta-hCG and IUGR in the early second trimester.

Triple-marker test as screening for Down syndrome: a meta-analysis

The effectiveness of triple-marker testing as screening for Down syndrome needs to be evaluated by means of formal meta-analytic techniques. We did a MEDLINE search to identify studies evaluating the detection of Down syndrome by use of the triple-marker test. Reference lists of articles were also checked. Papers published in either English, French, or German from 1966 to November 1996 were eligible for this review. Twenty cohort studies were identified. Results of sensitivities and false-positive rates from different subgroups of the study sample were compared by using summary receiver-operating characteristic curve analysis. Medians of sensitivities and false-positive rates were also estimated. A total of 194,326 patients were included. In women of all ages, the medians for sensitivities were 67, 71, and 73 percent when the cutoffs used were 1:190-200, 1:250-295, and 1:350-380, respectively. The median false-positive rates fluctuated between 4 and 8 percent. For women at or above 35 years old, the medians of sensitivity and false-positive rate were 89 and 25 percent, respectively, when the chosen cutoff was 1:190-200. In patients below 35 years old, the median sensitivity was 57 percent if the cutoff used was 1:250-295. Summary receiver-operating characteristic curves showed that 1:190 was the best cutoff for predicting Down syndrome. The triple-marker testing is an effective screening method of detecting Down syndrome pregnancies. It is less effective in younger than in older age groups and may be offered as an alternative to amniocentesis to pregnant women over 35.

Antenatal screening for Down's syndrome

BACKGROUND

In 1968 the first antenatal diagnosis of Down's syndrome was made and screening on the basis of selecting women of advanced maternal age for amniocentesis was gradually introduced into medical practice. In 1983 it was shown that low maternal serum alpha fetoprotein (AFP) was associated with Down's syndrome. Later, raised maternal serum human chorionic gonadotrophin (hCG), and low unconjugated oestriol (uE3) were found to be markers of Down's syndrome. In 1988 the three biochemical markers were used together with maternal age as a method of screening, and this has been widely adopted. PRINCIPLES OF ANTENATAL SCREENING FOR DOWN'S SYNDROME: Methods of screening need to be fully evaluated before being introduced into routine clinical practice. This included choosing markers for which there is sufficient scientific evidence of efficacy, quantifying performance in terms of detection and false positive rates, and establishing methods of monitoring performance. Screening needs to be provided as an integrated service, coordinating and managing the separate aspects of the screening process. SERUM MARKERS AT 15-22 WEEKS OF PREGNANCY: A large number of serum markers have been found to be associated with Down's syndrome between 15 and 22 weeks of pregnancy. The principal markers are AFP, hCG or its individual subunits (free alpha- and free beta-hCG), uE3, and inhibin A. Screening performance varies according to the choice of markers used and whether ultrasound is used to estimate gestational age (table 1). When an ultrasound scan is used to estimate gestational age the detection rate for a 5% false positive rate is estimated to be 59% using the double test (AFP and hCG), 69% using the triple test (AFP, hCG, uE3), and 76% using the quadruple test (AFP, hCG, uE3, inhibin A), all in combination with maternal age. Other factors that can usefully be taken into account in screening are maternal weight, the presence of insulin dependent diabetes mellitus, multiple pregnancy, ethnic origin, previous Down's syndrome pregnancy, and whether the test is the first one in a pregnancy or a repeat. Factors such as parity and smoking are associated with one or more of the serum markers, but the effect is too small to justify adjusting for these factors in interpreting a screening test.

URINARY MARKERS AND FETAL CELLS IN MATERNAL BLOOD

Urinary beta-core hCG has been investigated in a number of studies and shown to be raised in pregnancies with Down's syndrome. This area is currently the subject of active research and the use of urine in future screening programmes may be a practical possibility. Other urinary markers, such as total oestriol and free beta-hCG may also be of value. Fetal cells can be identified in the maternal circulation and techniques such as fluorescent in situ hybridisation can be used to identify aneuploidies, including Down's syndrome and trisomy 18. This approach may, in the future, be of value in screening or diagnosis. Currently, the techniques available do not have the performance, simplicity, or economy needed to replace existing methods.

DEMONSTRATION PROJECTS

Demonstration projects are valuable in determining the feasibility of screening and in refining the practical application of screening. They are of less value in determining the performance of different screening methods. Several demonstration projects have been conducted using the triple and double tests. In general, the uptake of screening was about 80%. The screen positive rates were about 5-6%. About 80% of women with positive screening results had an invasive diagnostic test, and of those found to have a pregnancy with Down's syndrome, about 90% chose to have a termination of pregnancy. ULTRASOUND MARKERS AT 15-22 WEEKS OF PREGNANCY: There are a number of ultrasound markers of Down's syndrome at 15-22 weeks, including nuchal fold thickness, cardiac abnormalities, duodenal atresia, femur length, humerus length, pyelectasis, and hyperechogenic bowel. (ABSTRA

Prospective evaluation of prenatal maternal serum screening for trisomy 18

OBJECTIVE

Our goal was to evaluate the performance of prenatal serum screening for trisomy 18.

STUDY DESIGN

All 40,762 samples for maternal serum testing (August 1991 to June 1994) with a trisomy 18-positive screen (n = 175, alpha-fetoprotein < or =0.75 multiples of the median, unconjugated estriol < or =0.60 multiples of the median, human chorionic gonadotropin < or =0.55 multiples of the median) were analyzed. Results of all amniocenteses, ultrasonographic studies, and birth or death certificate information were obtained from the Iowa Expanded Serum Screening Program, the Iowa Department of Public Health, and the Iowa Birth Defects Registry.

RESULTS

We obtained the expected screen-positive rate for trisomy 18 (0.43%, 175/40,762). Fourteen samples from outside the state were excluded, which left 161 cases with outcome data obtained through amniocentesis (n = 121), birth certificates (n = 34), telephone contact (n = 2), or a sonogram indicating a nonviable gestation (n = 4). Of 121 screen-positive women undergoing amniocentesis, 119 had a normal karyotype and 2 had an abnormal karyotype: 69,XXY and 47,XY,+18. Of 36 who declined amniocentesis, none had findings consistent with aneuploidy on clinical neonatal examination. Of the 103 patients who had a detailed ultrasonographic study at the University of Iowa, 27 had a subtle fetal abnormality or growth alteration. Both cases with aneuploidy were in this group. An additional 7 cases of trisomy 18 without the typical trisomy 18 maternal serum screening pattern were diagnosed during this period either at amniocentesis performed because of increased Down syndrome risk indicated by serum screening (n = 1), by elevated alpha-fetoprotein level (n = 1), or by advanced maternal age (n = 2) with serum for screening drawn coincidentally, or they were diagnosed postnatally (n = 3). Three of the 7 cases had early second-trimester ultrasonographic examinations, and all showed abnormalities.

CONCLUSIONS

The detection rate of trisomy 18 among patients offered amniocentesis was significantly lower (p < 0.05) than the expected rate (10/161 on the basis of published data). Combining serum screening with detailed ultrasonographic evaluations may improve predictive value by more precisely targeting amniocentesis toward those at highest risk.

Risk of Down syndrome and any clinically significant chromosome defect in pregnancies with abnormal triple-screen and normal targeted ultrasonographic results

OBJECTIVE

Our purpose was to study prospectively the use of ultrasonographic biometry to refine the risk estimates for both Down syndrome and any clinically significant chromosome defect in women with abnormal biochemical triple-screen results.

STUDY DESIGN

Ultrasonographic biometry and anatomic survey were performed on study and control cases. Expected values for humerus, femur, combined humerus plus femur lengths, and abdominal circumference were generated on the basis of biparietal diameter obtained from a normal group. Threshold observed/expected values of each measurement for screening for Down syndrome and clinically significant chromosome defects were determined with receiver-operator characteristic curves. By stepwise logistic regression analysis the optimal screening parameters, including nuchal thickness, for detection of Down syndrome and clinically significant chromosome defect were determined. Risk tables for chromosome anomalies were developed on the basis of ultrasonography and triple-screen values.

RESULTS

Of 1034 cases at risk for Down syndrome (risk > or = 1/270) or trisomy 18 on the basis of triple-screen results, there were 11 cases of Down syndrome, 1 of trisomy 18, and 17 clinically significant chromosome defects. Abnormal nuchal thickness or observed/expected humerus length < 0.92 was the most sensitive parameter for Down syndrome detection. Abnormal nuchal thickness or observed/expected combined femur and humerus length < 0.90 was the most sensitive for significant chromosome defects. With abnormal biometry or anatomy the Down syndrome risk was 8 of 127 versus 1 of 753 in normals, odds ratio 50.4 (95% confidence interval 6.4 to 90.2), p < 0.00001, and the risk of significant defects was 11 of 90 versus 6 of 830 in normals, odds ratio 19.3 (95% confidence interval 6.4 to 60.5), p < 0.00001. In a pregnancy with a 1 in 270 triple-screen risk for Down syndrome, normal biometric and anatomic results reduce the risk to 1 in 2100.

CONCLUSION

Normal ultrasonographic anatomy and biometry significantly reduces the risk of both Down syndrome and any significant chromosome defects in pregnancies with abnormal triple-screen results.

Can reliable Down's syndrome detection rates be determined from prenatal screening intervention trials?

OBJECTIVES - To develop a standardised approach for analysing Down's syndrome screening performance in clinical practice and to apply it to published intervention trials in order to estimate detection and false positive rates more accurately. METHODS - Peer reviewed intervention trials, grouped by specific combination of analytes, were reanalysed. Revised detection rates were calculated for each study, taking into account both the high spontaneous loss during the last half of pregnancy and the possible under ascertainment of Down's syndrome live births not detected by screening. Collective screening performance was estimated, when possible, using a published methodology based on fitting receiver-operator characteristic curves. RESULTS - Sixteen trials were analysed; 11 using three, and five using two, analytes. Collective screening performance for the triple analyte trials was Down's syndrome detection rates of 57, 64, and 69% at amniocentesis referral rates of 3, 5, and 7% respectively. Four of the five studies involving two analytes performed less well, individually, when compared with the overall performance of the three analyte studies. It was not possible to estimate collective performance for the two analyte studies because there were too few. CONCLUSIONS - Accurate Down's syndrome detection rates are difficult to obtain in intervention trials owing to two potential biases, both of which tend to produce overestimates of the true rates. These sources of bias need to be taken into account when analysing and reporting Down's syndrome intervention trials. The methodology presented here offers the opportunity to achieve a more reliable, standardised estimate of both individual and collective intervention trial screening performance.

Triple marker (alpha-fetoprotein, unconjugated estriol, human chorionic gonadotropin) versus alpha-fetoprotein plus free-beta subunit in second-trimester maternal serum screening for fetal Down syndrome: a prospective comparison study

OBJECTIVE

Our purpose was to compare the efficacy of triple-marker screening (alpha-fetoprotein, unconjugated estriol, human chorionic gonadotropin) with alpha-fetoprotein plus free beta-human chorionic gonadotropin.

STUDY DESIGN

Free beta-human chorionic gonadotropin was concurrently assayed in 2349 maternal serum samples. Trivariate and bivariate algorithms were used to calculate the risk for fetal Down syndrome by the two protocols. Free beta-human chorionic gonadotropin from 12 cases of fetal Down syndrome previously screened with the triple marker was retrospectively assayed.

RESULTS

Mean maternal age of our study was 29.8 years (range 14 to 51 years). The initial screen-positive rate with the triple marker was 8.0% compared with 12.8% for alpha-fetoprotein plus free beta-human chorionic gonadotropin. All three cases of fetal Down syndrome ascertained in our prospective study were detected by the triple marker; in contrast, one of three was detected by alpha-fetoprotein plus free beta-human chorionic gonadotropin. By adding 12 additional cases of fetal Down syndrome, 12 of 15 (80%) were screen positive with triple marker and nine of 15 (60%) were screen positive with alpha-fetoprotein plus free beta-human chorionic gonadotropin.

CONCLUSION

The detection rate of fetal Down syndrome was greater by use of a triple marker screen than when using alpha-fetoprotein plus free beta-human chorionic gonadotropin. Our data do not support the claims of other studies that suggest that alpha-fetoprotein plus free beta-human chorionic gonadotropin is superior to triple markers.

Screening for Down's syndrome

Down's syndrome (DS) is the commonest cause of severe mental retardation in children. It is the result of trisomy of chromosome 21 which is usually a random event though it is commoner in older mothers. DS can be diagnosed by chorionic villus sampling (CVS) and amniocentesis followed by karyotyping. Because of the risks associated with these invasive procedures, they can only be offered to a high-risk group. At one time the sole basis for identifying this increased risk was maternal age, but within the past ten years a series of biochemical and ultrasound abnormalities have been shown in DS pregnancies. The biochemical abnormalities include changes in the levels of most fetal and placental products in the maternal circulation. The best-known of these changes are the reduced levels of alphafetoprotein (AFP) and oestriol (E3) and increased levels of human chorionic gonadotrophin (hCG). The mechanism underlying these biochemical phenomena is unknown. Screening programmes involving the measurement of hCG and AFP, with or without additional parameters such as E3, at 15-18 weeks of pregnancy can typically identify 60% or more of cases of DS with a screen-positive rate of 5%. The combined risk derived from the various biochemical parameters, together with maternal age, is calculated by one of a number of computer programmes which have been developed for this purpose. There has been considerable discussion as to the exact biochemical tests which should be used for DS screening. This had led to controversy as to whether measurement of E3 has a place, and whether or not measurement of the free beta-subunit of hCG should replace measurement of the intact molecule. A notable recent development is the suggestion that measurement of the urinary beta-core of the hCG could be a highly discriminatory marker. A number of factors can affect the results of biochemical screening for DS. These include maternal weight, gestational age, ethnic origin, smoking, and diabetes. In addition, abnormal levels of the biochemical products may be found in other chromosome abnormalities.