What are the preoperative instructions for a patient taking Metformin (biguanide) and Tengly (likely a brand name, assuming it's Teneligliptin, a dipeptidyl peptidase-4 inhibitor) regarding medication management before surgery?

Preoperative Medication Instructions for Day Before Surgery

Stop the Metformin the night before surgery and discontinue Tengly (teneligliptin) 3-4 days before the scheduled procedure. 1, 2, 3

Metformin Management

Hold metformin starting the night before surgery to allow adequate drug clearance and minimize the risk of metformin-associated lactic acidosis (MALA), which carries a 30-50% mortality rate when it occurs. 1, 2, 4

• The American Diabetes Association and American College of Cardiology both recommend stopping metformin the night before elective surgery. 1, 2
• Metformin has a plasma half-life of 6.2 hours, so stopping it 12-24 hours preoperatively allows for significant clearance in patients with normal renal function. 2, 4
• The FDA drug label instructs patients to inform their doctor about metformin use prior to any surgical procedure, as temporary discontinuation may be required. 5

Risk Factors That Make Metformin Discontinuation Critical

Your patient should be assessed for these risk factors that increase MALA risk: 1, 2

• Renal impairment (creatinine clearance <60 mL/min)
• Severe heart failure (left ventricular ejection fraction <30%)
• Dehydration or fasting status
• Concurrent use of ACE inhibitors, ARBs, diuretics, or NSAIDs

When to Restart Metformin

Do not restart metformin until 48 hours after surgery, and only after confirming adequate renal function (eGFR ≥60 mL/min/1.73m²). 1, 2, 4

• For major surgery, the 48-hour waiting period is mandatory even with normal baseline renal function, as surgery can cause transient renal impairment. 2, 4
• The patient must be eating and drinking normally, with no ongoing hemodynamic instability, dehydration, vasopressor requirement, acute heart failure, sepsis, or respiratory insufficiency. 2, 4

DPP-4 Inhibitor (Tengly/Teneligliptin) Management

Discontinue Tengly (teneligliptin) on the morning of surgery. 1, 3

• Current guidelines recommend holding other oral glucose-lowering agents (including DPP-4 inhibitors) on the morning of surgery. 1, 3
• Unlike SGLT2 inhibitors which require 3-4 days discontinuation, DPP-4 inhibitors can be held just on the day of surgery. 1, 3

Perioperative Glucose Management

Target blood glucose of 100-180 mg/dL in the perioperative period. 1, 3

• If the patient is on basal insulin, administer 75-80% of the usual long-acting insulin dose or 50% of NPH insulin on the morning of surgery. 1, 3
• Monitor blood glucose every 2-4 hours while the patient is NPO and dose with short- or rapid-acting insulin as needed. 1, 3
• Consider starting glucose infusion if the patient is fasting and on insulin, stopping it if blood glucose exceeds 300 mg/dL (16.5 mmol/L). 1

Common Pitfalls to Avoid

• Do not restart metformin too early: The 48-hour rule exists because surgery-induced renal impairment may not be immediately apparent on postoperative day 1. 2, 4
• Do not confuse DPP-4 inhibitors with SGLT2 inhibitors: SGLT2 inhibitors require 3-4 days discontinuation due to euglycemic DKA risk, while DPP-4 inhibitors are held only on the day of surgery. 1, 2, 3
• Do not use sliding scale insulin alone: Basal-bolus insulin coverage perioperatively improves outcomes compared to reactive correction-only insulin. 1, 3

Evidence Nuance

While some recent research suggests metformin continuation may not significantly raise lactate levels in selected patients 6, 7, 8, all major guideline societies (American Diabetes Association, American College of Cardiology, European Society of Anesthesiology) maintain conservative recommendations to discontinue metformin perioperatively due to the catastrophic consequences of MALA when it occurs. 1, 2, 4, 9