Harvard Heart Letter

Ask the doctor: What's the difference between blood sugar and hemoglobin A1c?

Ask the doctor

What's the difference between blood sugar and hemoglobin A1c?

Q. In your article on blood sugar control, you kept talking about hemoglobin A1c. I measure my blood sugar all the time, but my meter doesn't have a setting for a percentage reading. Is there a simple connection between blood sugar and hemoglobin A1c?

A. I'm sorry we confused you. Blood sugar and hemoglobin A1c are connected, but they are different, too. Your blood sugar meter measures the concentration of glucose in the bloodstream at the instant you prick your finger. The reading is in milligrams of glucose per deciliter (a tenth of a liter) of blood, abbreviated as mg/dL. Blood sugar levels vary throughout the day. In people with diabetes, they can range from below 70 mg/dL to above 200.

The hemoglobin A1c test measures the percentage of red blood cells with a sugar coating. In people without diabetes, that's only 4% to 5% of red blood cells. But in people with diabetes, who have more sugar in the bloodstream, the percentage can reach 15% or higher. The hemoglobin A1c number is a way to gauge long-term glucose control, or lack of it. Blood sugar meters can't check for hemoglobin A1c; finding that requires a trip to the doctor. The American Diabetes Association recommends that people with diabetes do this twice a year.

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