From the Guidelines
HbA1c should be tested approximately 3 months after starting metformin therapy. This recommendation is based on the most recent and highest quality study, which suggests that A1C testing should be performed routinely in all patients with diabetes at initial assessment and as part of continuing care, with measurements taken approximately every 3 months to determine whether a patient’s glycemic targets have been reached and maintained 1. The American Diabetes Association recommends performing the A1C test at least two times a year in patients who are meeting treatment goals and quarterly in patients whose therapy has changed or who are not meeting glycemic goals 1.
Key Considerations
- Metformin works by reducing hepatic glucose production, increasing peripheral glucose uptake, and improving insulin sensitivity, but these changes take time to significantly impact overall glycemic control.
- Earlier testing (before 3 months) may not reflect the medication's full benefit, while waiting longer than 3-4 months could delay necessary treatment adjustments if the response is inadequate.
- After this initial 3-month assessment, regular HbA1c monitoring should continue every 3-6 months depending on how well diabetes is controlled and whether medication adjustments are needed.
- If the patient experiences significant symptoms of hyperglycemia or hypoglycemia before the 3-month mark, earlier blood glucose monitoring (not necessarily HbA1c) may be warranted to ensure safety and guide potential dose adjustments.
Clinical Judgment
The frequency of A1C testing should depend on the clinical situation, the treatment regimen, and the clinician’s judgment 1. Point-of-care testing for A1C provides the opportunity for more timely treatment changes, allowing for adjustments to be made as needed to achieve optimal glycemic control 1.
From the Research
HbA1c Testing After Starting Metformin
- The studies provided do not specify the exact timing for HbA1c testing after starting metformin 2, 3, 4, 5, 6.
- However, the studies suggest that HbA1c levels should be monitored regularly to assess the effectiveness of metformin therapy 3, 4, 5, 6.
- One study found that patients who failed metformin monotherapy and received intensified treatment had a mean post-metformin HbA1c of 8.7% (72 mmol/mol) after 90 days of treatment 4.
- Another study compared the reduction of HbA1c with SGLT2 inhibitors vs. DPP-4 inhibitors as add-ons to metformin monotherapy and found that the reduction in HbA1c was slightly greater with SGLT2 inhibitors [-0.80±0.20% from 8.03±0.35%; 44 analyses, 29 RCTs, 15 with two doses, n=9321] than with DPP-4is [-0.71±0.23% from 8.05±0.43%; 61 analyses, 59 RCTs, n=17,914; P=0.0354] 6.
Factors Affecting HbA1c Levels
- Baseline HbA1c levels can affect the reduction in HbA1c with metformin therapy 3, 4, 5, 6.
- The studies suggest that the reduction in HbA1c is greater in patients with higher baseline HbA1c levels 3, 4, 5, 6.
- Other factors such as body weight, blood pressure, and cardiovascular and renal protection should also be considered when choosing between glucose-lowering agents 6.