RESUMEN
PURPOSE: To establish fetal nasal bone length cut-off points for first trimester aneuploidy screening based on a normal curve of a Brazilian population. METHODS: The following tests were proposed: presence or absence of the nasal bone (NB); 2.5 and 5.0 NB percentiles relative to the normal curve; and 0.70, 0.75 and 0.80 multiples of the median (MoM) values defined in the receiver operating characteristic (ROC) curve. Nasal Bone tests were based on positive and negative likelihood ratio value detection rates (LR); the confidence interval was 95% in all tests. Cases in which ultrasonographic images of the NB were absent were not taken into account when evaluating the 2.5 and 5.0 percentiles and the 0.70, 0.75 and 0.80 MoM. RESULTS: The sample consisted of 571 fetuses (10-14 weeks). After exclusions (11) and loss of follow-up (53), the study sample was reduced to 507 patients. There were 23 Down syndrome patients among 41 aneuploid fetuses. The sensitivity of the qualitative NB test (absent vs. present) was 34.1%, and the specificity was 99.1% (+LR 37.89, -LR 0.66). An image of the nasal bone was absent in 52.2% of fetuses with the Down syndrome (+LR 58.00, -LR 0.48). CONCLUSIONS: The best tool for aneuploidy screening was the qualitative NB test (absent vs. present). Ultrasonography of the NB is a component of aneuploidy screening, and should not be used alone.