A new screening protocol combining urine beta-core fragment and ultrasonography for Down syndrome detection.

OBJECTIVE: Our purpose was to ascertain the screening efficiency of a new midtrimester Down syndrome detection protocol that combines maternal urine testing and ultrasonographic examination. STUDY DESIGN: In a prospective study, beta-core fragment, the stable end product of human chorionic gonadotropin metabolism, was measured in maternal urine. The results were standardized for urine creatinine levels. The study was performed in women undergoing midtrimester genetic amniocentesis (15 to 24 weeks' gestation). Urine beta-core fragment values were expressed as multiples of the normal median for gestational age. The screening performance of a combination of ultrasonographic parameters and urine beta-core values for Down syndrome detection was determined. RESULTS: A total of 511 singleton pregnancies in women undergoing amniocentesis were studied, 18 of the women (3.5%) had a Down syndrome fetus. A urine beta-core fragment level > or = 97th percentile had a sensitivity of 61.1% and a false-positive rate of 3.2%. An abnormal prenatal screen was defined as a urine beta-core level > or = 97th percentile, increased nuchal thickness (> or = 5 mm), or the presence of gross structural defects. Corresponding values for the screening efficiency of an abnormal prenatal screen were sensitivity of 77.8% and a false-positive rate of 4.1%. With an abnormal prenatal screen the odds ratio is 82.8 (95% confidence interval 22.6 to 364.9) for having a Down syndrome fetus. CONCLUSION: The presence of an abnormal maternal urine beta-core level, a gross ultrasonographic anomaly, or increased nuchal thickness had a high detection rate and a low false-positive rate for Down syndrome. This novel screening algorithm is useful for further delineating the risk status in patients at high risk who are reluctant to undergo or decline genetic amniocentesis.