Ordered factor of GFR stages
GFR_stagesAn object of class ordered (inherits from factor) of length 6.
See https://kdigo.org/guidelines/ckd-evaluation-and-management/ for more details
GFR_stages
#> [1] G1 G2 G3a G3b G4 G5
#> Levels: G1 < G2 < G3a < G3b < G4 < G5