Sample size and statistical analysis
From a previous cohort of women with severe early PE (31) we estimated the interval from admission to delivery to be 12 days in non-complicated and 5 days in complicated pregnancies. Assuming this difference between women in the highest quartile of angiogenic delta values and those in the 1st-3rd quartiles, for an alpha-risk of 5% and a power of 80%, 56 women would be needed (log-rank test)(32).
Variables were checked for normal distribution by Kolmogorov-Smirnov test. Paired comparisons of the angiogenic values between admission and delivery were performed by non-parametric Wilcoxon sign test. Comparisons between uncomplicated and complicated pregnancies were performed by Student-T (assuming unequal variances), Mann-Whitney U, Pearson Chi-squared and Fisher-F, as appropriate.
The likelihood of maternal complications was modelled by logistic regression (with robust estimation of the standard errors). Levels of angiogenic factors were log-transformed. The predictive performance was determined by receiver–operating characteristic (ROC) curve analysis.
The interval from admission to delivery was graphed by Kaplan-Meier procedure and tested for differences in the survival time by the angiogenic delta values quartile using Log Rank (Mantel-Cox) test. The interval to delivery was modelled by multivariate Cox regression checking for the assumptions of proportionality and log-linear relationship.
Statistical analyses and graph constructions were performed using STATA 13.0 (StataCorp LT, Texas, USA) and R 3.1.2 (The R Foundation for Statistical Computing) [package “pROC”].