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Laboratory tests which have been used to screen for Down's syndrome include a low serum alpha-fetoprotein. If maternal serum AFP is used alone, it detects about 60% of cases. When this value is combined with measurements of total human chorionic gonadotropin (THCG) and unconjugated estriol (UE3), the initial screen is strengthened.

When this triple screening is used, about 80% of the infants with Down's syndrome may be detected. These results must be utilized with the gestational age, maternal weight, presence of multiple gestation, and other variables. If the risk of DS is 1:190 or greater, amniocentesis and chromosomal analysis may be offered.

Increases in AFP are not specific for neural tube defects and must be used in combination with other modalities such as ultrasonography, amniotic fluid AFP, and acetylcholinesterase. Measurements are reported in multiples of median (MoM). About 80% of patients with fetuses with open neural tube defects have elevated levels >2.5 MoM. This probability is dependent upon the prevalence in the geographic area. Thus if the prevalence is 1:1000, there is a 1:200 chance of an open spina bifida if the maternal serum AFP MoM is 2.5. Most measurements occur at 16 week gestation. There is maximal concentration within the fetal serum at this time, usually 3,000,000 ng/ml.


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The variation of risk estimates through pregnancy in second trimester maternal serum screening for Down syndrome.

Christiansen M, Hogdall EV, Larsen SO, Hogdall C.

Department of Clinical Biochemistry, Statens Serum Institut, Copenhagen, Denmark.

Prenat Diagn 2002 May;22(5):385-7 Abstract quote

The variation of risk estimates in second trimester maternal serum screening for Down's syndrome has been shown to be considerable in quality control schemes, i.e. UKNEQAS.

We studied the biological variation of risk estimates in 16 women through pregnancy. The maternal serum markers alpha-fetoprotein (AFP), human chorionic gonadotrophin (hCG), unconjugated estriol (uE3) and beta-hCG were determined six times during late first to late second trimester, and the associated likelihood ratios for Down syndrome were calculated.

The interpersonal variation of markers, as well as that of the likelihood ratio, was much greater than the intrapersonal variation. The average intrapersonal standard deviation (SD) of the triple test log likelihood ratio was 0.2291, corresponding to a central 95-percentile interval 0.36-2.81 of the likelihood ratio. The interpersonal SD of the log likelihood ratio was 0.5482, corresponding to a central 95-percentile interval 0.08-11.87 of the likelihood ratio.

The large difference between the intra- and interpersonal variation makes it unlikely that biological variation through pregnancy is a major contributor to the variation of risk estimates obtained several times in the same pregnancy. Rather, improvements in analytical quality and laboratory management must be expected to result in reduced variation and, in consequence, better performance of screening.



Down syndrome maternal serum marker screening after 18 weeks' gestation.

Muller F, Dreux S, Oury JF, Luton D, Uzan S, Uzan M, Levardon M, Dommergues M.

Service de Biochimie, Hopital Ambroise Pare, Boulogne, France.


Prenat Diagn 2002 Nov;22(11):1001-4 Abstract quote

Women having access to prenatal care late in pregnancy may still wish to benefit from maternal serum screening for Down syndrome.

Therefore, we established reference values for alpha-feto protein (AFP) and free beta-human chorionic gonadotrophin (beta-hCG), and assessed the diagnostic value of maternal serum marker screening at 18-35 weeks' gestation based upon a series of 4072 sera from unaffected pregnancies and 118 sera from pregnant women with fetuses affected by Down syndrome. Using a 1/250 risk cut-off, a detection rate of 72.9% (95% CI = 71.5-74.3%) was achieved with a false-positive rate of 7.51% (95% CI = 6.71-8.3%). This was not significantly different from the percentages observed in our 14-17 weeks routine screening (50 596 patients): 71.9% (95% CI = 71.5-72.3%) and 6.48% (95% CI = 6.28-6.68%), respectively.

Detection and screen-positive rates were, respectively, 51.3% (95% CI = 35.6-67.0%) and 5.95% (95% CI = 5.12-6.68%) in women aunder 35 years of age, and 84.8% (95% CI = 76.9-92.7%) and 24% (95% CI = 20.7-27.3%) in women aged 35 years and over.

In conclusion, maternal serum marker screening is feasible at 18 weeks' gestation and later, which may be of interest in selected cases.

A novel method for the detection of Down syndrome with the use of four serum markers.

Azuma M, Yamamoto R, Wakui Y, Minobe S, Satomura S, Fujimoto S.

Department of Obstetrics and Gynecology, Hokkaido University School of Medicine, Sapporo, Japan.

Am J Obstet Gynecol 2002 Jul;187(1):197-201 Abstract quote

OBJECTIVE: The purpose of this study was to evaluate the use of the Lens culinaris agglutinin-reactive alpha-fetoprotein ratio for the detection of fetal Down syndrome in combination with traditional serum markers.

STUDY DESIGN: We obtained maternal serum from 530 women with unaffected pregnancies and 31 women who were pregnant with a fetus with Down syndrome at 14 to 20 weeks of gestation. Various combinations of Lens culinaris agglutinin-reactive alpha-fetoprotein ratio, alpha-fetoprotein, human chorionic gonadotropin, and unconjugated estriol were evaluated by score, without regard for the maternal age-related fetal Down syndrome risk.

RESULTS: The best combination of serum markers, according to our scoring method, was a combination of all 4 markers. This combination showed a sensitivity of 83.9%, with a 5.1% false-positive rate for Down syndrome. Moreover, in older women, high sensitivity was obtained without increasing the false-positive rate.

CONCLUSION: The screen using all 4 markers showed high sensitivity in all age groups without increasing the false-positive rate.

The comprehensive midtrimester test: high-sensitivity Down syndrome test.

Bahado-Singh R, Shahabi S, Karaca M, Mahoney MJ, Cole L, Oz UA.

Department of Obstetrics and Gynecology, Yale University School of Medicine, New Haven, Connecticut 06520-8063, USA.

Am J Obstet Gynecol 2002 Apr;186(4):803-8 Abstract quote

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

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

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

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

Second trimester screening for Down's syndrome: 7 years experience.

Beaman JM, Goldie DJ.

Department of Clinical Biochemistry, Southmead Hospital, Bristol, UK.

J Med Screen 2001;8(3):128-31 Abstract quote

OBJECTIVES: To evaluate the effectiveness of the first 7 years of a programme for second trimester antenatal screening for Down's syndrome, using alpha-fetoprotein (alphaFP) and total human chorionic gonadotropin (hCG) as maternal serum markers.

METHODS: A clinical biochemistry laboratory providing a screening service for four obstetric units. Women attending for antenatal care who accepted an offer of serum screening for Down's syndrome were tested between 15 and 20 weeks gestation. Down's risk estimates were calculated using maternal serum alphaFP and total hCG results as modifiers of the maternal age related risk. Outcome was determined in collaboration with the regional cytogenetics unit.

RESULTS: In 7 years 66,631 women were screened, in whom 108 Down's syndrome pregnancies were identified. Risks for Down's syndrome were reported without a specified cut off recommendation; however, at a cut off of 1 in 250, 72 (66.7%) of the affected pregnancies were screen positive, the false positive rate was 5.8%, and the uptake of amniocentesis 71.2%. The detection rate was higher in women screened before 17 weeks (70.5%) than in those screened at 17 weeks or later (56.7% overall and 20.0% in those under 30 years). The uptake of screening declined gradually from 84% in 1992 to 59.5% in 1998.

CONCLUSIONS: Two marker screening using aFP and total hCG is an effective way of screening for Down's syndrome and is widely accepted in the local community. Detection rates were comparable with other second trimester studies using two markers including both total and free beta hCG.

Maternal serum screening for down syndrome by using alpha-fetoprotein and human chorionic gonadotropin in an asian population. a prospective study.

Jou HJ, Shyu MK, Chen SM, Shih JC, Hsu JJ, Hsieh FJ.

Department of Obstetrics and Gynecology, Taiwan Adventist Hospital, Taipei, Taiwan.

Fetal Diagn Ther 2000 Mar-Apr;15(2):108-11 Abstract quote

The purpose of the present study is to evaluate the efficacy of second-trimester maternal serum screening program by using alpha-fetoprotein (AFP) and total human chorionic gonadotropin (hCG) in an Asian population.

During June 1994 to July 1998, we conducted a prospective study of serum screening protocol for Down syndrome. The cut-off point for a positive result in this analysis was a risk of >/=1/270. A total of 17,742 pregnant women with singleton pregnancy were screened, and 1,153 (6.5%) had positive result. Sixteen of the 17,742 pregnancies had Down syndrome, and 10 of them had positive result. The positive rate and detective rate for Down syndrome were 6.5 and 62.5%, respectively. However, the detective rate will reduce to 47.6% after being adjusted by age-specific risk.

It is indicated that the double-marker test using AFP and total hCG is an effective screen strategy for second-trimester detection of Down syndrome in Asian women.

Maternal serum screening for Down syndrome in a teaching hospital in Hong Kong.

Law L, Lau T, Fung T, Rogers MS, Hjelm M.

Department of Chemical Pathology, Prince of Wales Hospital, Chinese University of Hong Kong, Shatin, NT, Hong Kong, China.

Chin Med J (Engl) 1999 Aug;112(8):754-7 Abstract quote

OBJECTIVE: To study prospectively the use of maternal serum alpha-fetoprotein (AFP) and total beta-human chorionic gonadotropin (hCG) concentrations for screening of Down syndrome in Hong Kong.

METHODS: AFP and total beta-hCG were measured in serum samples from 1638 singleton Chinese pregnancies at 14-22 weeks of gestation, recruited over a twelve-month period. Gestational ages were determined by ultrasonographic parameters measured at the same visit as the test for all cases. The gestational-age-specific and weight-adjusted medians for serum AFP and total beta-hCG were calculated. Risk for fetal Down syndrome (FDS) was derived by mathematical modeling of the medians together with maternal age. Amniocenteses were offered to women with a calculated FDS risk of 1:270 or greater.

RESULTS: The gestational-age-specific and weight-adjusted medians for maternal serum AFP were similar to previous studies while that of total beta-hCG were higher. A total of 101 patients (6.1%) were classified as being high risk for FDS, including 3.4% (48/1394) of those younger than 35 years of age and 21.7% (53/244) of those who were 35 or above. There were 4 cases of Down syndrome, 1 case of Tumer syndrome and 1 of Edward syndrome. Three out of the four cases of Down syndrome were screened positive, corresponding to a detection rate of FDS of 75%. A case of Tumer syndrome was also screened positive. A case of trisomy 18 was found to have very low levels of AFP [0.262 multiple of median (MoM)] and total beta-hCG (0.115 MoM).

CONCLUSIONS: Maternal serum screening using double biochemical markers (AFP and total beta-hCG) in combination with gestational dating by ultrasonography is effective in the detection of fetal Down syndrome and possibly other chromosomal disorders in Chinese pregnant women.

Down syndrome screening in an Asian population using alpha-fetoprotein and free beta-hCG: a report of the Taiwan Down Syndrome Screening Group.

Hsu JJ, Hsieh TT, Hsieh FJ.

Department of Obstetrics and Gynecology, Chang Gung Memorial Hospital, Taipei, Taiwan.

Obstet Gynecol 1996 Jun;87(6):943-7 Abstract quote

OBJECTIVE: To evaluate whether the strategy of maternal serum screening for Down syndrome, using alpha-fetoprotein (AFP) and free beta-hCG in combination with maternal age, a technique developed in western countries, is applicable to an Asian population.

METHODS: Alpha-fetoprotein and beta-hCG were measured in serum samples from 23 Down syndrome pregnancies and 1748 unaffected singleton Taiwanese (ethnically Chinese) pregnancies at 14-22 weeks' gestation. Gestational age-specific medians and a maternal weight correction formula were established for our own population. Likelihood ratio for Down syndrome pregnancies in relation to multiples of the median (MoM) levels of these analytes were derived from the overlapping gaussian frequency distribution curves for Down syndrome and unaffected pregnancies.

RESULTS: The serum AFP and free beta-hCG median MoM values of Down syndrome pregnancies were significantly abnormal in Asian subjects (0.77 and 2.91, respectively), and similar to those of affected pregnancies in white women. The median value of free beta-hCG:AFP MoM ratio (2.97) in Down syndrome pregnancies was significantly higher than that of unaffected pregnancies (1.09). The mean maternal weight during the second trimester in pregnant Asian women (55.2 kg) was markedly lighter than that of white women. At a 5.8% false-positive rate, free beta-hCG identified 47.8% of Down syndrome pregnancies (likelihood ratio 8.2), AFP detected only 13% of the cases (likelihood ratio 2.2), and free beta-hCG:AFP MoM ratio detected 43.5% of the cases (likelihood ratio 7.4). By using a multivariate risk algorithm involving the combination of AFP, free beta-hCG, and maternal age, 56.5% of Down syndrome cases could be detected with a 5.3% false-positive rate (likelihood ratio 10.7).

CONCLUSION: Maternal serum screening strategy using AFP and free beta-hCG in combination with maternal age is feasible in the detection of fetal Down syndrome among Asian women.


Very low second-trimester maternal serum alpha-fetoprotein: Association with high birth weight.

Baschat AA, Harman CR, Farid G, Chodirker BN, Evans JA.

Department of Obstetrics, University of Maryland, Baltimore, Maryland, USA.

Obstet Gynecol 2002 Apr;99(4):531-6 Abstract quote

OBJECTIVE:To investigate the relationship between very low maternal serum alpha-fetoprotein levels (MSAFP), neonatal size, and possible associations with obstetric complications.

\METHODS: This is a retrospective case-control study in a population managed prospectively by a standardized protocol. Perinatal outcomes were compared between patients with unexplained very low MSAFP (less than or equal to 0.25 multiples of the median) and control pregnancies with normal MSAFP, matched by precise gestational age, parity, maternal age within 1 year, and gender of the newborn.

RESULTS:Of the 84,909 women screened, 464 (0.55%) met the definition of very low MSAFP. On tertiary evaluation, 226 had dates reassigned by ultrasound. After exclusion of overt diabetics, patients who were not pregnant, invalidated MSAFP, and 17 patients lost to follow-up, 178 women (0.21% of the total) had true very low MSAFP. True very low MSAFP was associated with subsequent miscarriage in 67 women and with fetal aneuploidy and/or serious abnormalities in 12 patients, leaving a population of 97 women (1.14 per 1000 women screened) with unexplained very low MSAFP. Without obvious demographic or obstetric factors, these women had heavier babies, more babies above the 90th percentile, more delivery complications caused by large birth weight (41 versus 16, chi(2), P <.001) compared with gestational-age matched controls from the same screened population who had normal MSAFP.

CONCLUSION:Very low MSAFP predicts an unusually high rate of large birth weight infants, with increased fetal, intrapartum, and neonatal consequences. Maternal medical conditions or obvious demographic factors do not explain these consequences. These findings suggest a role for close fetal surveillance in the third trimester and extended efforts to assess maternal and neonatal glucose status.


The Preterm Prediction Study: the value of serum alkaline phosphatase, alpha-fetoprotein, plasma corticotropin-releasing hormone, and other serum markers for the prediction of spontaneous preterm birth.

Moawad AH, Goldenberg RL, Mercer B, Meis PJ, Iams JD, Das A, Caritis SN, Miodovnik M, Menard MK, Thurnau GR, Dombrowski M, Roberts JM; NICHD MFMU Network.

National Institute of Child Health and Human Development Maternal-Fetal Medicine Unit Network, USA.

Am J Obstet Gynecol 2002 May;186(5):990-6 Abstract quote

OBJECTIVE: High levels of a number of analytes are found in maternal blood; alkaline phosphatase,alpha-fetoprotein, and corticotropin-releasing hormone have been associated with spontaneous preterm birth. We investigated the relationship between 8 potential blood markers and subsequent spontaneous preterm birth in asymptomatic pregnant women.

STUDY DESIGN: We performed a nested case control study that involved 127 women who were enrolled in the preterm prediction study and who had a singleton spontaneous preterm birth at <35 weeks and 127 women who had a term birth and served as matched (age, parity, center) controls. Serum that was collected at 24 and 28 weeks was analyzed for alkaline phosphatase, alpha-fetoprotein, corticotropin-releasing hormone, and 5 other analytes.

RESULTS: Alkaline phosphatase, alpha-fetoprotein, and corticotropin-releasing hormone, but not other analytes, were significantly elevated in pregnancies that ended in spontaneous preterm birth. For alkaline phosphatase at 24 weeks, the odds ratio for spontaneous preterm birth at <32 weeks was 6.8 (range, 1.4-32.8) and for spontaneous preterm birth at <35 weeks 5.1 (range, 1.7-15.6). Similar results were found at 28 weeks. For alpha-fetoprotein at 24 weeks, the odds ratio for spontaneous preterm birth at <32 weeks was 8.3 (range,2.2-30.9) and for spontaneous preterm birth at <35 weeks was 3.5 (range, 1.8-6.7). The levels at 28 weeks were still predictive but less so than at 24 weeks. Corticotropin-releasing hormone, at 28 weeks but not at 24 weeks, was predictive for spontaneous preterm birth at <35 weeks, with an odds ratio 3.4 (range, 1.0-10.9).

CONCLUSION: Elevated alkaline phosphatase and alpha-fetoprotein are associated with subsequent spontaneous preterm birth in asymptomatic pregnant women at 24 and 28 weeks. Elevated corticotropin-releasing hormone levels at 28 weeks are associated with spontaneous preterm birth at <35 weeks.


Conditions Associated with Increased Maternal Serum AFP Gestational age underestimated
Mulitple pregnancy
Growth retardation
Open neural tube defects
Intrauterine death (early)
Fetal distress
Congenital nephropathies
Sacrococcygeal teratoma
Congenital neoplasms
Duodenal atresia
Abdominal pregnancy
Maternal alpha-feto protein producing neoplasm
Tetralogy of Fallot
Hydrops fetalis
Turner's syndrome without hygroma
Cystic hygroma
Placental defects
Conditions Associated with Decreased Maternal Serum AFP Some normal pregnancies
Fetal death (Late)
Molar pregnancy
Down syndrome
Gestational age overestimated
Spontaneous, missed abortion
Edward's syndrome (Trisomy 18)

Correction for insulin-dependent diabetes in maternal serum alpha-fetoprotein testing has outlived its usefulness.

Evans MI, Harrison HH, O'Brien JE, Dvorin E, Huang X, Krivchenia EL, Reece EA.

Department of Obstetrics and Gynecology, MCP/Hahnemann University, 245 N. 15th Street, MS 495, Philadelphia, PA 19102, USA.

Am J Obstet Gynecol 2002 Oct;187(4):1084-6 Abstract quote

OBJECTIVE: Historically, alpha-fetoprotein (AFP) levels in insulin-dependent diabetes (IDDM) have shown an approximately 20% decrement, and a correction factor is used to standardize multiples of the median (MOMs). With new laboratory methods and improved precision, we sought to re-evaluate the correctness of this approach.

STUDY DESIGN: Consecutive biochemical screens were conducted among 60,287 nondiabetic patients and 307 patients with IDDM. Analyses were conducted in one laboratory, and comparisons were made with use of standard formula weight adjustments including a 20% correction factor for IDDM. Patients were then stratified according to maternal weight.

RESULTS: Nondiabetic patients averaged 1.00 MOM, IDDM patients 0.91 MOM with no adjustments, 0.96 MOM adjusting for weight only, and 1.20 MOM adjusting for weight and diabetes status. To explain the "overcorrection," analysis by maternal weight showed significant overrepresentation of IDDM patients at 175 pounds or above. In fact, the mean weight in pounds for nondiabetic subjects was 151 +/- 35 and for those with IDDM 174 +/- 52 (P <.001). With use of an upper limit weight cutoff of 200 pounds, results are within 4% of normal.

CONCLUSIONS: With current methodologies, the 20% correction factor for IDDM erroneously overcorrects. Two hundred pounds is sufficient, the weight correction for diabetic status should be abandoned.

Maternal smoking: age distribution, levels of alpha-fetoprotein and human chorionic gonadotrophin, and effect on detection of Down syndrome pregnancies in second-trimester screening.

Crossley JA, Aitken DA, Waugh SM, Kelly T, Connor JM.

Institute of Medical Genetics, Yorkhill NHS Trust, Glasgow, G3 8SJ, UK.

Prenat Diagn 2002 Mar;22(3):247-55 Abstract quote

OBJECTIVES: To study the levels of maternal serum alpha-fetoprotein (AFP) and human chorionic gonadotrophin (hCG) in the second trimester in smokers and non-smokers with unaffected and Down syndrome pregnancies; to examine the rate of smoking in different maternal age groups in a population having routine prenatal screening; and to assess the effect of smoking on the detection rates for Down syndrome and corresponding false-positive rates, both overall and in different maternal age groups.

METHODS: Information on maternal smoking status, maternal age and serum marker levels was collected from case note searches and the screening programme database on 2272 unaffected singleton pregnancies, 36 unaffected twin pregnancies and 103 singleton Down syndrome pregnancies.

RESULTS: In unaffected pregnancies the smokers had a median age 3.3 years less than the non-smokers, while in the Down syndrome cases the corresponding age difference was 2.0 years. Median analyte levels in multiples of the median (MoM) in the unaffected singleton pregnancies were, for non-smokers: AFP=0.97, hCG=1.04; and for smokers, AFP=1.04, hCG=0.80. In the Down syndrome pregnancies the medians were, for non-smokers: AFP=0.69, hCG=2.49; and for smokers, AFP=0.70, hCG=1.53. Correction for smoking status gave median MoMs of 1.0 for both AFP and hCG in the unaffected pregnancies in both smokers and non-smokers. In the Down syndrome cases the corrected medians were, for non-smokers: AFP=0.67, hCG=2.29; and for smokers, AFP=0.73, hCG=1.99. Before correction for maternal smoking the overall detection rate for Down syndrome was 66.7% with a false-positive rate of 6.2%. After correction the detection rate was 67.7% with a false-positive rate of 4.9%. Between the smoking and non-smoking groups there was a significant difference in the detection rate (37.5% versus 76.0%) and the false-positive rate (1.8% versus 8.1%), which disappeared after correction for smoking status (detection rate 62.5% versus 69.3%, false-positive rate 3.9% versus 5.4%). No evidence of a lower incidence of Down syndrome in smokers was found.

CONCLUSIONS: While correcting AFP and hCG results for maternal smoking status will have little impact on the overall detection rate for Down syndrome, it may reduce the false-positive rate and will improve the accuracy of the risks given to individual women.

Detection and false-positive rates of maternal serum markers for Down syndrome screening according to maternal age in women over 35 years of age. A study of the agreement of eight dedicated software packages.

Muller F, Thalabard JC, Ngo S, Dommergues M.

Biochimie, Hopital Ambroise Pare, AP-HP, 92104 Boulogne, France.

Prenat Diagn 2002 May;22(5):350-3 Abstract quote

Maternal serum markers for trisomy 21 screening (MSS) can be assayed in women > or =35 years in an attempt to reduce the need for invasive procedures and thereby avoid their side effects.

Our objective was to compare, in women > or =35, eight different software packages dedicated to second trimester MSS, thus providing reliable data for patient counselling. A simulation study was carried out on 189 sera from women with Down syndrome fetuses and 11 962 sera from mothers of unaffected babies.

The first step was to estimate the joint distribution of alpha-fetoprotein (AFP) and free beta-human chorionic gonadotrophin (beta-hCG). The second step was to calculate trisomy 21 detection and false-positive rates for each software according to maternal age (35-45 years), using the usual 1:250 risk threshold. Agreement between software packages was measured using 2 x 2 kappa coefficients. Detection rates and false-positive rates increased with maternal age. Depending on the software, 57-71% detection rates were achieved at 35 years with 12-18% false-positive rates. At 45 years, 61-100% detection rates were achieved with 66-95% false-positive rates.

Up to 39 years, all softwares were concordant (kappa coefficients >0.75). In the range 35-45 years, false-positive and detection rates increased substantially with maternal age and differences between software packages are observed.

Henry JB. Clinical Diagnosis and Management by Laboratory Methods. Twentieth Edition. WB Saunders. 2001.

Commonly Used Terms

MoM-Multiple of the median. This is determined by each screening program collecting the AFP medians for each week of pregnancy from 14-18 weeks by using at least 100 patients at each completed week (a patient at 16 weeks and 6 days will be included in the 16 week group). Patients whose results are greater than 3.5 standard deviations (SD) from the log mean are excluded. The estimates of the medians are plotted as a log-linear regression of AFP versus completed weeks of gestation. At 16 weeks of gestation, the typical median is 35 ng/ml. The patient's AFP concentration is divided by the median value appropriate for the patient's gestational age. Additional adjustments are made for insulin-requiring diabetes, maternal weight, maternal race, and number of fetuses. These AFP medians should be re-evaluated annually.

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