Improvement of a prenatal screening program for trisomy 18 in the first trimester of gestation

Abstract

The aim of this study was to evaluate population parameters (medians, standard deviations and coefficients of correlation) different from those used by the commercial software Elipse(®) v3.0 (Perkin Elmer) in the calculation of prenatal risk of trisomy 18. Moreover, the truncation limits used for extreme values of free β-human chorionic gonadotropin (fβ-hCG), pregnancy associated plasma protein-A (PAPP-A) and nuchal translucency (NT) were revised. A calculation engine for the prenatal risk of trisomy 18 was developed [called FMF (Fetal Medicine Foundation) calculator]. Recently, published population parameters for fβ-hCG and PAPP-A as well as new truncation limits were included in this calculator. The patient-specific risks obtained by Elipse(®) v3.0 and FMF calculators, were compared in 18,801 pregnant women, including 13 cases of trisomy 18, four cases of trisomy 13 and one case of triploidy. Using a cut-off point of 1:250, FMF calculator increased the detection rate of trisomy 18 from 62% to 100% with a 0.31% increase in the false-positive rate (FPR). When the detection rate was fixed at 100%, the FPR generated by Elipse v3.0 (1.52%) was significantly higher (p<0.0001) than that generated by the FMF calculator (0.36%). Moreover, an improved detection in cases of trisomy 13 and triploidy was observed. It is recommended that each laboratory reviews the population parameters and truncation limits used in the risk calculation of trisomy 18, in order to obtain an adequate performance in the screening.