GCK and type 1 diabetes mellitus: GFR was calculated from creatinine and cystatin C using the CKD-EPI creatinine equation (eGRF-cr) and CKD-EPI cystatin C equation (eGFR-cys), respectively. Results. Cystatin C levels were lower (p < 0.001) in the control (0.70 ± 0.13 mg/L), HNF1A (0.75 ± 0.21), and GCK (0.72 ± 0.16 mg/L) groups in comparison to those with either T1DM (0.87 ± 0.15 mg/L) or T2DM (0.9 ± 0.23 mg/L).