What labs should be checked before starting metformin (biguanide oral hypoglycemic)

From the Guidelines

Before starting metformin, clinicians should check renal function with serum creatinine and estimated glomerular filtration rate (eGFR), liver function tests (AST, ALT), baseline vitamin B12 level, and hemoglobin A1c to establish baseline glycemic control, as metformin is contraindicated in patients with significantly impaired kidney function (typically eGFR <30 mL/min/1.73m²) and requires dose adjustment for moderate impairment. The following laboratory tests are essential before initiating metformin therapy:

• Renal function with serum creatinine and estimated glomerular filtration rate (eGFR) to assess the risk of lactic acidosis and determine the appropriate dose of metformin 1
• Liver function tests (AST, ALT) to evaluate the risk of metformin-induced liver injury and to assess the patient's overall liver function 1
• Baseline vitamin B12 level to monitor for potential deficiency, as metformin can decrease B12 absorption over time, potentially causing deficiency 1
• Hemoglobin A1c to establish baseline glycemic control and to monitor the effectiveness of metformin therapy 1 Additionally, checking electrolytes, particularly bicarbonate levels, helps identify patients at risk for lactic acidosis, a rare but serious side effect of metformin 1. Regular monitoring of these parameters should continue after metformin initiation, typically at 3-6 month intervals or more frequently in high-risk patients, to minimize the risk of adverse effects and to optimize glycemic control 1.

From the FDA Drug Label

Before initiating metformin hydrochloride tablets, obtain an estimated glomerular filtration rate (eGFR). Your healthcare provider should do blood tests to check how well your kidneys are working before and during your treatment with metformin hydrochloride tablets.

The labs that should be checked and verified before starting metformin include:

• Estimated Glomerular Filtration Rate (eGFR): to assess kidney function and ensure it is safe to initiate metformin therapy 2
• Blood tests: to check how well the kidneys are working before and during treatment with metformin 2 Note that the specific blood tests are not defined in the label, but eGFR is explicitly mentioned.

From the Research

Labs to be Checked Before Starting Metformin

The following labs should be checked and verified before starting metformin:

• Serum creatinine levels to assess kidney function 3, 4, 5, 6
• Estimated glomerular filtration rate (eGFR) to determine the stage of chronic kidney disease (CKD) 3, 4, 7, 5
• Vitamin B12 levels, as metformin may lead to vitamin B12 deficiency 7

Rationale for Lab Checks

These lab checks are necessary to ensure safe use of metformin, as it is contraindicated in patients with renal insufficiency due to the risk of lactic acidosis 3, 4, 5, 6. The eGFR is a more accurate measure of kidney function than serum creatinine alone 4, 5. Additionally, monitoring vitamin B12 levels is recommended due to the potential risk of deficiency associated with metformin use 7.

Specific Guidelines for Lab Results

• Patients with an eGFR below 30 mL/min/1.73 m2 should not be started on metformin 4, 7
• Patients with an eGFR between 30-59 mL/min/1.73 m2 should be closely monitored for adverse effects 4, 5
• Patients with serum creatinine levels above 1.5 mg/dL should be evaluated for CKD and monitored closely 5, 6