HbA1c level in blood explains -

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. 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 and page 808 Davidson's Principles and practice of Medicine 22nd edition