A 52-year-old man with type 2 diabetes mellitus presents to the nephrology clinic for routine follow-up. His serum creatinine is 1.8 mg/dL (baseline 0.9 mg/dL 2 years ago), and his 24-hour urine protein is 2.5 g. His blood pressure is 148/92 mmHg. On examination, he has mild pedal edema. Laboratory investigations show: serum urea 45 mg/dL, serum potassium 5.2 mEq/L, and fasting blood glucose 180 mg/dL. Using the MDRD equation, his estimated GFR is calculated to be 38 mL/min/1.73 m². Which of the following best explains the discordance between his serum creatinine level and his actual renal function in this patient?