Ordered factor of AKI stages
aki_stagesAn object of class ordered (inherits from factor) of length 4.
See https://kdigo.org/guidelines/ckd-evaluation-and-management/ for more details
aki_stages
#> [1] AKI Stage 1 AKI Stage 2 AKI Stage 3 No AKI
#> Levels: No AKI < AKI Stage 1 < AKI Stage 2 < AKI Stage 3