What GFR value indicates an absolute need to start a patient on dialysis?
a) GFR > 90 mL/min/1.73 m²
b) GFR > 60 mL/min/1.73 m²
c) GFR < 15 mL/min/1.73 m²
d) GFR < 30 mL/min/1.73 m²