A 58-year-old male with a 20-year history of poorly controlled type 2 diabetes mellitus (HbA1c 9.2%) and chronic kidney disease stage 3b (eGFR 38 mL/min/1.73m²) is scheduled for elective total knee arthroplasty under spinal anesthesia. He is on metformin 1500 mg daily, lisinopril, and atorvastatin. Preoperative assessment reveals normal renal function trend over the past 3 months. Which of the following is the most appropriate perioperative management of his metformin?

Explanation

## Metformin Perioperative Management in CKD Stage 3b **Key Point:** Metformin discontinuation is indicated in patients with eGFR 30–44 mL/min/1.73m² (CKD stage 3b) undergoing surgery due to increased risk of lactic acidosis, particularly with contrast exposure and perioperative hemodynamic changes. ### Rationale for Correct Answer (Option 2 – Discontinue morning of surgery; resume 48 hours postoperatively): - **Timing of discontinuation:** Metformin should be stopped on the morning of surgery (not 48 hours before) to minimize glycemic derangement and allow adequate perioperative glucose control with insulin if needed. - **Renal function confirmation:** Resumption requires confirmation of stable renal function (eGFR ≥45 mL/min/1.73m²) and restoration of oral intake to ensure adequate hydration and drug clearance. - **48-hour postoperative window:** This interval accounts for: - Potential perioperative acute kidney injury (AKI) risk - Hemodynamic stabilization post-anesthesia - Return to baseline renal perfusion - Restoration of oral intake and hydration status ### Clinical Pearl: Metformin-associated lactic acidosis (MALA) risk is significantly elevated in CKD stage 3b (eGFR 30–44) due to impaired renal clearance. Perioperative stress, volume shifts, and potential contrast exposure compound this risk. Current guidelines (ADA 2023, ESA 2023) recommend discontinuation in this cohort. ### High-Yield Mnemonic: **"CKD 3b → STOP metformin; resume when eGFR ≥45 + stable + oral intake"** | CKD Stage | eGFR (mL/min/1.73m²) | Metformin Action | |-----------|----------------------|------------------| | 1–2 | ≥60 | Continue | | 3a | 45–59 | Continue (caution) | | **3b** | **30–44** | **DISCONTINUE** | | 4–5 | <30 | Contraindicated |