Prenatal diagnosis policy in neural tube defects

Combined screening for open spina bifida at 11-13 weeks using fetal biparietal diameter and maternal serum markers

OBJECTIVE

Screening at 11-13 weeks with ultrasound biparietal diameter (BPD) can detect half of open spina bifida cases. Maternal serum α-fetoprotein (AFP) levels at 15-19 weeks are increased 3- to 4-fold, in open spina bifida. We assessed whether combined screening using BPD, AFP, and other serum markers at 11-13 weeks would increase detection.

STUDY DESIGN

Maternal AFP levels were measured on serum stored at 11-13 weeks in 44 open spina bifida and 182 unaffected pregnancies, and results were expressed in multiples of the median (MoM) for gestational age. All samples had been measured for free β-human chorionic gonadotropin (β-hCG) and pregnancy-associated plasma protein (PAPP)-A. A multivariate Gaussian model was used to predict screening performance from the serum data and BPD measurements on 80 cases, including 36 previously published.

RESULTS

The median AFP level in cases was 1.201 MoM, significantly higher than in unaffected pregnancies (P < .01, 1 tail). The median free β-hCG was significantly reduced to 0.820 MoM (P < .02), but the median PAPP-A was similar in cases and controls. Modeling predicted the following: BPD alone would detect 50% of cases for a 5% false-positive rate or 63% for 10%; adding AFP increases detection by 2%; and a combined test with BPD, AFP, and free β-hCG detects 58% for 5% or 70% for 10%.

CONCLUSION

Combining AFP and BPD with free β-hCG as part of first-trimester aneuploidy screening would also allow early detection about two-thirds of cases with open spina bifida.

The sensitivity of ultrasound and serum alpha-fetoprotein in population-based antenatal screening for neural tube defects. South Australia 1986-1991

OBJECTIVE

To determine the sensitivity of antenatal screening methods for neural tube defects in population-based screening in South Australia in 1986-1991, and whether ultrasound can replace serum alpha-fetoprotein screening in terms of achieving an equivalent level of sensitivity.

DESIGN AND SETTING

Ascertainment of all births and terminations of pregnancy with neural tube defects from multiple sources for 1986-1991 in South Australia. Serum and amniotic fluid alpha-fetoprotein results were obtained from the only laboratory performing the tests as a Statewide antenatal screening programme, and information on ultrasound screening from case notifications, hospital case records and medical practitioners who cared for the women.

SUBJECTS

All 243 births and terminations of pregnancy with neural tube defects in South Australia in 1986-1991.

MAIN OUTCOME MEASURES

The sensitivity of individual screening methods and of all methods used, particularly for spina bifida.

RESULTS

For pregnancies with neural tube defects screened by any method (serum alpha-fetoprotein, ultrasound or amniocentesis), 86% sensitivity was achieved. Ultrasound screening for anencephaly achieved 100% sensitivity even in low risk pregnancies, compared with 92% for serum alpha-fetoprotein. For spina bifida, the sensitivity of ultrasound screening increased with the level of risk in pregnancy: it was 60% in low risk pregnancies, which was equivalent to that of serum alpha-fetoprotein screening (64%); 89% in high risk pregnancies and 100% for women referred for confirmation of a suspected spina bifida by another ultrasonographer (chi 1(2) for trend = 23.49, P < 0.0001). Ultrasound screening in high risk pregnancies for spina bifida achieved higher sensitivity in teaching hospitals compared with other ultrasound services in the State (97% vs 65%), but sensitivity was equivalent for low risk pregnancies. It is estimated that, had the serum screening programme not been in place, the level of sensitivity achieved for spina bifida by ultrasound and amniocentesis would have been 62% compared with the actual situation of 76% with the programme in existence, a difference of nearly 15% (95% CI 2.5 to 26.7) (chi 1(2) = 5.45, P = 0.02).

CONCLUSIONS

Antenatal screening for neural tube defects in South Australia achieved a higher level of sensitivity with the maternal serum alpha-fetoprotein programme in place. We conclude that the serum screening programme should continue in South Australia pending a significant improvement in the sensitivity of routine ultrasound screening for spina bifida.