Maternal serum screening for fetal trisomy 18: A comparison of fixed cutoff and patient-specific risk protocols

Objective: To compare the effectiveness of two widely used protocols for se cond-trimester screening for fetal trisomy 18. Methods: Second-trimester screening results for 41,565 women were reviewed to determine whether pregnancies could be considered to be at high risk for trisomy IS. The screening test was considered positive if either maternal serum concentrations of alpha-fetoprotein (MSAFP), hCG, and unconjugated es triol (E3) fell below defined levels, or the second-trimester patient-speci fic risk (based on maternal age and serum analytes) was greater than 1:100. Detection rates, false-positive rates, and pregnancy outcomes for the two protocols were compared. Results: The fixed-cutoff method showed a 23% detection rate and a 0.19% fa lse-positive rate for trisomy 18. These low rates were in close agreement w ith a theoretical expectation for fixed-cutoff trisomy 18 screening. The ri sk-based approach resulted in a 69% detection rate and a 0.45% false-positi ve rate. Both methods identified pregnancies with other fetal anomalies. Conclusion: Overall, the risk-based method is more effective than the fixed -cutoff approach to trisomy 18 screening. (C) 1999 by The American College of Obstetricians and Gynecologists.