First-trimester Down syndrome screening using dried blood biochemistry andnuchal translucency

Objective: To assess the effectiveness of free beta-hCG, pregnancy-associat ed plasma protein A, and nuchal translucency in a prospective first-trimest er prenatal screening study for Down syndrome and trisomy 18. Methods: Risks were calculated for Down syndrome and trisomy 18 based on ma ternal age and biochemistry only (n = 10,251), nuchal translucency only (n = 5809), and the combination of nuchal translucency and biochemistry (n = 5 809). Results: The study population included 50 Down syndrome and 20 trisomy 18 c ases. Nuchal translucency measurement was done on 33 Down syndrome and 13 t risomy 18 cases. Down syndrome screening using combined biochemistry and ul trasound resulted in a false-positive rate of 4.5% (95% confidence interval [CI] 3.9%, 5.2%) and detection rate of 87.5% (95% CI 47%, 100%) in patient s under age 35 years. In older patients, the false-positive rate was 14.3% (95% CI 12.7%, 15.8%) and detection rate was 92% (95% CI 74%, 99%). For tri somy 18 screening, the false-positive rate was 0.4% (95% CI 0.24%, 0.69%) a nd detection rate was 100% (95% CI 40%, 100%) in younger patients, whereas in older patients the false-positive rate was 1.4% (95% CI 0.9%, 2.0%) and detection rate was 100% (95% CI 66%, 100%). Using modeling, at a fixed 5% f alse-positive rate, the Down syndrome detection rate was 91%. Conversely, a t a fixed 70% Down syndrome detection rate, the false-positive rate was 1.4 %. Conclusion: First-trimester screening for Down syndrome and trisomy 18 is e ffective and offers substantial benefits to clinicians and patients. (C) 20 00 by The American College of Obstetricians and Gynecologists.