Blood sugar is measured by determining the number of milligrams of glucose in a deciliter of blood (mg/dL). The amount of sugar in your blood naturally fluctuates, but within a narrow range. The normal range is between 70 and 100 mg/dL — the level most people have after fasting overnight. That's about equal to 1 teaspoon of sugar in a gallon of water.

If your blood sugar is consistently 126 mg/dL or higher after fasting overnight, you have diabetes. At one time, a diagnosis of diabetes required a fasting blood sugar level of at least 140 mg/dL. The standard was lowered in 1997 after a review by the American Diabetes Association showed that by the time blood sugar reaches that level, some people already have sustained organ damage.

If your fasting blood sugar is between 101 and 125 mg/dL, you have impaired glucose tolerance, more commonly known as borderline diabetes or prediabetes. Because it's a sign that you're at high risk of developing full-fledged diabetes, borderline diabetes must be taken seriously. See your doctor regularly and take steps to control your blood sugar.