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

Abstract

Abstract Background: 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. Methods: 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. Results: 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. Conclusions: 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.