A 58-year-old man with type 2 diabetes presents to the nephrology clinic with a serum creatinine of 1.8 mg/dL and eGFR of 42 mL/min/1.73m². Urinalysis shows 2+ proteinuria. His blood pressure is 148/92 mmHg. He is currently on metformin 1000 mg twice daily. What is the most appropriate next step in management?

Explanation

## Clinical Context This patient has stage 3b chronic kidney disease (eGFR 30–44 mL/min/1.73m²) with evidence of diabetic nephropathy (proteinuria and hypertension). Metformin is contraindicated at this level of renal function due to lactic acidosis risk. ## Key Point: **Metformin is contraindicated when eGFR < 45 mL/min/1.73m².** Immediate discontinuation is mandatory to prevent metformin-associated lactic acidosis (MALA). ## High-Yield: **ACE inhibitors (or ARBs) are the cornerstone of renoprotection in diabetic nephropathy.** They reduce intraglomerular pressure, slow GFR decline, and reduce proteinuria — independent of blood pressure lowering. ## Clinical Pearl: **24-hour urinary protein quantification (or urine protein-to-creatinine ratio) is essential** to: - Establish baseline proteinuria severity - Assess response to ACE inhibitor therapy (target: >30% reduction in proteinuria) - Guide prognosis and future management decisions ## Management Algorithm ```mermaid flowchart TD A[eGFR 30-44 mL/min/1.73m² + Diabetes + Proteinuria]:::outcome --> B{Metformin status?}:::decision B -->|On metformin| C[DISCONTINUE immediately]:::urgent B -->|Not on metformin| D[Proceed to renoprotection] C --> E[Initiate ACE-I/ARB]:::action D --> E E --> F[Quantify proteinuria: 24h urine or UPCR]:::action F --> G[Monitor BP target < 130/80 mmHg]:::action G --> H[Repeat eGFR and proteinuria in 2-4 weeks]:::action ``` ## Dosing Note ACE inhibitors should be started at low doses and titrated cautiously; expect a transient 10–20% rise in serum creatinine in the first 1–2 weeks (due to efferent arteriolar vasodilation) — this is NOT an indication to stop the drug. [cite:Harrison 21e Ch 297]