Glycemic control in diabetes is best assessed by-

Glycated hemoglobin provides an accurate and objective measure of glycemic control integrated over a period of 2-3 months (life span of RBCs is 120 days). HbA1c is most sensitive to changes in glycemic control occuring in the month before measurement. The rate of formation of HbA1c is directly propoional to the ambient blood glucose concentration; a rise of 1 % in HbA1c corresponds to an approx. average increase of 2 mmol/L (36 mg/dL) in blood glucose. In most individuals, the target HbA1c should be <7%. A value of 10 % indicates very poor control. In patients achieving their glycemic goal, the ADA recommends measurement of the HbA1c at least twice per year; more frequently (every 3 months) when glycemic control is inadequate or when therapy has changed. Reference : page 2410 Harrison's Principles of Internal Medicine 19th edition and page 808 Davidson's Principles and practice of Medicine 22nd edition