Normal blood sugar levels for nondiabetic adults are between 70 and 100 milligrams per deciliter before breakfast, also called fasting blood sugar, and below 140 milligrams per deciliter two hours after eating any meal, according to Virginia Mason Medical Center. Diabetes is often diagnosed with an oral glucose tolerance test.
People with prediabetes have a fasting blood sugar level of 100 to 125 milligrams per deciliter and a two-hour blood sugar level between 140 and 199 milligrams per deciliter. A blood test called A1c reveals a 3-month average blood sugar level. An A1c between 5.7 and 6.4 percent indicates prediabetes, as noted by Virginia Mason Medical Center.