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.