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Early in the course of diabetes, when hemoglobin A1C (HbA1C) level is low or near normal, which of the following contributes most to the overall HbA1C value?

1. Fasting hyperglycemia
2. Postprandial hyperglycemia
3. Both A and B contribute equally
4. No relation to either A or B

Relative Changes in Fasting and Postprandial Plasma Glucose

Is it fasting or postprandial hyperglycemia that is important? Hemoglobin A1C measures total exposure to hyperglycemia over about a 3-month period of time. Both fasting and postprandial hyperglycemia contribute to this. We have no evidence that there's anything more toxic for postprandial hyperglycemia vs fasting hyperglycemia. The relative contributions depend on the relative degree of glycemic control. When your HbA1C is very high, when you have a fasting glucose level over 200 mg/dL, most of the HbA1C will be due to fasting hyperglycemia. However, earlier in the stage of diabetes, when HbA1C levels are lower, it's going to be the postprandial values that contribute most to HbA1C.

This shows the relationship of fasting and postprandial hyperglycemia to overall HbA1C over the range from less than 5% to 7.5%. Over this range, which covers the goals recommended by various groups, there is a very small increase in fasting plasma glucose levels, and a greater increase in postprandial levels. People who have HbA1C levels of 5.5% to 6% may have normal fasting plasma glucose levels, but may have a greater increase in postprandial hyperglycemia levels, and therefore these values contribute to high HbA1C.

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