Serum Cystatin C for Estimation of GFR

An 80-year-old man with type 2 diabetes and hypertension was referred to the nephrology clinic for persistent hyperkalemia. The patient was frail and sedentary, with a BMI of 37. His estimated glomerular filtration rate calculated using creatinine was 64 mL/min/1.73 m2. How would you interpret these results?