In the blood, glucose binds irreversibly to hemoglobin molecules within red blood cells. The amount of glucose that is bound to hemoglobin is directly tied to the concentration of glucose in the blood. Since red blood cells have a lifespan of approximately 90 days, measuring the amount of glucose bound to hemoglobin can provide an assessment of average blood sugar control during the 60 to 90 days prior to the test. This is the purpose of the glycated hemoglobin tests, most commonly the hemoglobin A1c (HbA1c) measurement.

There are many methods for measuring HbA1c, and results from one method cannot be compared directly with results from another method. You must compare your measurement with the standards for the method of measurement that was used, usually the highest non-diabetic value.

Note also that different patterns of blood sugar values can result in similar HbA1c values. For example, one person may have an irregular pattern of blood sugars, with lots of highs and lows, and an average blood sugar level for (200mg/dl or 11 mmol/l). Another person may have a very regular blood sugar level of around (200mg/dl or 11 mmol/l) all the time. Though both will have similar HbA1c levels., they have very different situations that require different actions to remedy. Frequent home blood glucose monitoring will provide the detailed information about the pattern of blood sugars to help each person decide how to adjust the insulin to improve their average control.