'Fasting' blood glucose levels -those observed eight hours prior to the last meal- above 100 mg/dL are considered high. Observed glucose levels above 140 mg/dL within two hours of a meal are also considered high. 'Hyperglycemia' is the medical term for high blood sugar and is often a symptom of diabetes.
A fasting blood sugar level from 100 to 125 mg/dL indicates prediabetes. Individuals testing 126 mg/dL or higher on multiple tests are considered diabetic, according to Mayo Clinic.
Doctors can monitor the treatment plans of diabetics through an A1C blood test, which measures blood sugar levels over the previous two or three months.