Toward the optimal protocol for Down syndrome screening

Midtrimester triple-test levels in women with chronic hypertension and altered renal function

OBJECTIVE

To study the association between midtrimester triple-test levels and renal function in pregnant women with chronic hypertension.

METHODS

Forty-three patients with chronic hypertension for whom renal function was evaluated by the creatinine clearance test between 14- and 20-weeks gestation were studied. Midtrimester triple-test values obtained as part of Down screening program were correlated with renal function.

RESULTS

hCG correlated inversely with creatinine clearance (r = -0.345; p = 0.002) and directly with alpha-fetoprotein (r = 0.499; p = 0.003).

CONCLUSIONS

In pregnant women with chronic hypertension, impaired renal function may alter midtrimester maternal serum hCG levels.

Maternal serum hyperglycosylated human chorionic gonadotrophin (HhCG) in the first trimester of pregnancies affected by Down syndrome, using a sialic acid-specific lectin immunoassay

In a series of 54 cases of pregnancies complicated by Down syndrome and 224 unaffected pregnancies we examined maternal serum levels of hyperglycosylated human chorionic gonadotrophin (HhCG) in samples collected in the first trimester (11-13 weeks) using a sialic acid-specific lectin immunoassay. We compared these levels with those of other potential first trimester serum markers [free beta-hCG, pregnancy-associated plasma protein A (PAPP-A) and total hCG (ThCG)] and modeled detection rates and false-positive rates of various biochemical markers in conjunction with fetal nuchal translucency (NT) and maternal age using an maternal age standardized population. Maternal serum HhCG in cases of Down syndrome were significantly elevated (median MoM 1.97) with 24/54 (44%) of cases above the 95th centile for unaffected pregnancies. Free beta-hCG was also elevated (median MoM 2.09) with 33% of cases above the 95th centile. PAPP-A levels were reduced (median MoM 0.47) with 38% below the 5th centile. ThCG levels, whilst elevated (median MoM 1.34), had only 20% of cases above the 95th centile. Maternal serum HhCG levels were not correlated with fetal NT but showed significant correlation with ThCG and free beta-hCG and with PAPP-A in the Down syndrome group (r=0.536). Maternal serum HhCG levels in cases with Down syndrome had a significant correlation with gestational age, increasing as the gestation increased. When HhCG was combined together with fetal NT, PAPP-A and maternal age, at a 5% false-positive rate the modeled detection rate was 83%, some 6% lower than when free beta-hCG was used and some 4% better than when ThCG was used. Maternal serum HhCG is unlikely to be of additional value when screening for Down syndrome in the first trimester.

Biochemical screening for Down syndrome in pregnancies following renal transplantation

OBJECTIVE

To assess the performance of the double marker test [free beta-human chorionic gonadotrophin (beta-hCG) and alpha-fetoprotein (AFP)] as a screening test for Down syndrome in pregnant patients who had a prior renal transplant.

DESIGN

A retrospective study.

SETTING

The Fetal Medicine Unit, Royal Free Hospital, London, UK.

METHODS

Detailed records of 14 post-renal transplant pregnancies were obtained from the Renal Unit of our hospital where the patients were followed up. The serum concentrations of urea, creatinine, free beta-hCG and AFP at the time of the double marker test were recorded, with a cut-off point of 1:250 for the double marker test. A control group of 14 normal pregnancies matched for age, parity and gestational age was used. The Mann-Whitney U-test and t-tests of unequal variance were applied to compare parameters of the study and the control groups.

RESULTS

Two patients in each group were high risk for Down syndrome and amniocentesis revealed normal karyotype. No babies with Down syndrome were delivered in either group. Regression analysis showed significant correlation between free beta-hCG and urea concentrations (p<0.001) and free beta-hCG and creatinine concentrations (p<0.001), but not for AFP.

CONCLUSIONS

The present study demonstrates that residual renal function alterations persisting after renal transplantation can affect the levels of free beta-hCG and AFP, thus resulting in false-positive screening for Down syndrome. First trimester nuchal translucency (NT) measurement in combination with second trimester ultrasonographic markers can be used in these patients, or alternatively the free beta-hCG levels should be corrected according to the serum creatinine levels.

The use of nuchal translucency measurement and second trimester biochemical markers in screening for Down's syndrome

OBJECTIVE

To assess the effectiveness of antenatal screening for trisomy 21 by first trimester sonography followed by second trimester biochemical screening.

DESIGN

Retrospective five-year review.

SETTING

Maternity unit of a university hospital.

POPULATION

An unselected group of 7447 pregnant women who had a first trimester scan and nuchal translucency measurement in our unit after January 1995 and had an estimated date of delivery before 1 January 2000. 11.9% were > or = 37 years old. A subgroup (n = 4,864) also had second trimester biochemical testing by alpha-fetoprotein and free beta-human chorionic gonadotrophin.

MAIN OUTCOME MEASURES

Prenatal and postnatal diagnosis of trisomy 21. Results There were 23 fetuses affected with trisomy 21. The overall prenatal detection rate was 87% (20/23; 95% CI 66% to 97%) and we performed invasive procedures in 8.5% of our population. First trimester sonography identified 74% (95% CI 51.6% to 89.8%) of affected fetuses. Second trimester biochemical screening detected half of the fetuses with trisomy 21 which were missed by first trimester screening, increasing the sensitivity to 90.5% (19/21; 95% CI 69.6% to 98.8%) for an invasive procedure rate of 4.2% performed in screened positive women. However, the positive predictive value of the biochemical test was very low (0.5%). In screen negative women, karyotyping for advanced maternal age did not detect any affected fetuses.

CONCLUSION

First trimester nuchal translucency measurement is an effective screening test for the prenatal detection of fetuses with Down's Syndrome. Although the measurement of biochemical markers in the second trimester can detect additional affected fetuses this may be outweighed by the delay in diagnosis, the extra visits and cost so that the right time for biochemical screening is most likely to be in the first trimester.

Is maternal serum total hCG a marker of trisomy 21 in the first trimester of pregnancy?

In a study of 130 first trimester cases of trisomy 21 and 959 controls we have shown that the median MoM for alpha-fetoprotein (AFP) is lower (0.82) and that for total human chorionic gonadotrophin (hCG) is higher (1.31) than in the control group. For AFP 15.3% of cases were below the 5th centile and for total hCG 19. 8% were above the 95th centile. The median shift observed for AFP and total hCG is poorer than that for pregnancy associated plasma protein-A (PAPP-A) or free beta-hCG and together with maternal age, AFP and total hCG could only be expected to detect 40% of cases. In combination with PAPP-A, total hCG would identify 52% of cases, somewhat less than the 67% observed with free beta-hCG and PAPP-A. However, we have demonstrated for total hCG a significant temporal change in median MoM with gestational age. Before 70 days the median MoM was less than 0.5, between 70 and 83 days this increased to 1.13, and between 84 and 97 days this increased to 1.52. This median shift has significant implications for interpreting previous studies and even more significant implications for detection rates. When population parameters specific to the gestational age in question are used, detection rates with total hCG and PAPP-A increase from 47% at 70-83 days to 60% at 84-97 days. This observation explains much of the confusion around total hCG in the first trimester and shows the importance of selecting analyte pairs and population parameters appropriate to the time in gestation when screening is performed.

Screening for trisomy 21 in twin pregnancies in the first trimester using free beta-hCG and PAPP-A, combined with fetal nuchal translucency thickness

In the first trimester of pregnancy the biochemical markers free beta-hCG and pregnancy associated plasma protein-A (PAPP-A) are used for the prenatal screening of trisomy 21, either alone or in combination with nuchal translucency (NT) thickness. In this study, I have analysed the distribution of these biochemical markers in 159 twin pregnancies and compared this with 3466 singleton pregnancies. On average free beta-hCG values are 2.099 times greater in twins than in singletons and PAPP-A some 1.86 times greater. The width of the analyte distribution in twins is very similar to that in singleton pregnancies. Using statistical modelling techniques I have predicted that at a 5% false positive rate the detection rate in twins discordant for trisomy 21 will be 52% and in twins concordant for trisomy 21 will be 55%, if correction for twin pregnancy is carried out using the 'pseudo risk' approach. The detection rate using biochemical parameters is less than that achievable for twins using NT (75%). However, the combination of NT and maternal serum biochemistry will give detection rates approaching 80%. These rates are some 10% less than in singleton pregnancies, but nevertheless combining NT and biochemistry will allow high rates of detection of affected twins with the benefit of ultrasound and NT being able to specifically locate the affected twin. Twin screening using both modalities should be considered when introducing first trimester screening.

Second trimester prenatal screening for Down's syndrome using alpha-fetoprotein and free beta hCG: a seven year review

OBJECTIVE

To determine the value and impact over a seven year period of a second trimester screening programme for trisomy 21 and trisomy 18, using the two maternal serum markers alpha-fetoprotein and free beta human chorionic gonadotrophin.

DESIGN

Retrospective review.

SETTING

A biochemical screening laboratory serving three health districts with three antenatal clinics in both teaching and nonteaching hospitals.

POPULATION

67,904 pregnancies in women of all ages screened between 14 and 22 weeks of gestation between 1 April 1991 and 31 March 1998.

METHODS

All women booked into three major antenatal clinics were offered biochemical screening. Women at increased risk of trisomy 21 or trisomy 18 (> or =1 in 250 at term) were offered an invasive diagnostic procedure. Follow up of the outcome of all pregnancies was performed.

MAIN OUTCOME MEASURES

Detection rate for trisomy 21 and trisomy 18, false positive rates, uptake of screening, uptake of amniocentesis in women identified at increased risk, prevalence of trisomy 21 at birth, detection and false positive rates by maternal age, fetal loss rate after amniocentesis, report turn around time, and identification of other anomalies.

RESULTS

Overall, 87% (67,904/78,501) of women underwent screening. The rate of detection of trisomy 21 was 75% (80/107; 95% CI 66 to 83) with a 5.1% false positive rate (3466/67,904; CI 4.9 to 5.3%). In women under 30 years of age the detection rate was 60% (18/30; CI 41 to 77) with a 2.6% false positive rate (956/36,371; CI 2.5 to 2.8). The rate of detection of trisomy 18 was 57% (8/14; CI 29 to 82) with a 0.7% false positive rate (475/67,904; CI 0.64 to 0.76). Uptake of amniocentesis was 83% (2912/3508). Women were 3.3 times more likely to refuse amniocentesis if the risk was close to the cutoff (1 in 250) than if the risk was > or =1 in 50. Fetal loss within 28 days of amniocentesis was 0.9% (25/2912). Prenatal screening identified 84 other anomalies in addition to 41 cases of impending fetal death.

CONCLUSION

Second trimester prenatal screening for trisomy 21 and trisomy 18 using a simple two marker approach incorporating free beta hCG can achieve high detection rates over a long period of time. Health authorities who still have not introduced trisomy 21 screening should be encouraged by what can be achieved and should consider making such screening available to all women. Established second trimester detection rates of 75% for a 5% false positive rate will be the benchmark by which first trimester screening using nuchal translucency, PAPP-A and free beta hCG will be judged.