Statistical analysis
Data analysis was performed in several phases. Univariate analysis: the qualitative variables are described as number and percentage and the quantative data are expressed as mean, standard deviation (SD) and median and 25th and 75th percentiles. Bivariate analysis: Comparison were made using the Chi-square or Fisher exact test for qualitative variables and the Student’s t test for quantitative variables. For variables with a non normal distribution the corresponding non parametric tests were used. Survival analysis was performed by the Cox proportional hazards method calculating rates of risk using the backward step method applying the likelihood ratio to determine the significance of the model.
An exploratory model was previously used to preselect the variables based on the results of the Cox regression analysis with a single independent variable, and those with a p value < 0.05 were introduced into multivariate analysis. The dependent variable was mortality at 30 days and one year after the index episode and stepwise regression of the variables selected was made generating 6 different partial scaled models until a global model was obtained. The effect of anemia on monthly mortality from the day of inclusion until one year of follow-up was analyzed, only including the patients who were alive at each of the time periods. The statisical analyses were performed using SPSS 24.0.