Diabetes control is best monitored by -

Measurement of glycated hemoglobin (HbA1c) is the standard method for assessing long-term glycemic control. When plasma glucose is consistently elevated, there is an increase in nonenzymatic glycation of hemoglobin. This alteration reflects the glycemic history over the previous 2-3 months, because erythrocytes have an average life span of 120 days (glycemic level in the preceding month contributes about 50% to the HbA1c value). Measurement of HbA1c at the "point of care" allows for more rapid feedback and may therefore assist in adjustment of therapy. A rise of 1% in HbA1c corresponds to an approx average increase of 2 mmol/L (36 mg/dL) in blood glucose. Reference : page 2410 Harrison's Principles of Internal Medicine 19th edition