Harvard Heart Letter

Heart Beat: Converting blood sugar to HbA1c

Like a crazed roller coaster, blood sugar rises and falls throughout the day, shooting up after a carbohydrate-heavy meal and falling with fasting or exercise. Blood sugar readings are an essential tool for people with diabetes. These readings are great snapshots, but they don't tell you anything about a more important metric — average daily blood sugar.

A test that can do just that is the hemoglobin A1c (HbA1c) test. It measures the percentage of hemoglobin molecules (the oxygen-carrying protein in red blood cells) that have become coated with glucose (sugar). The higher the average daily blood sugar over a three-month period, the higher the HbA1c value. People who don't have diabetes typically have an HbA1c reading of 6 or lower.

HbA1c is such a useful number that diabetes experts have adopted it for diagnosing diabetes and monitoring people with it. Unfortunately, there hasn't been an easy way to convert the blood glucose readings that many people with diabetes record at home into HbA1c values.

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