Glucose is kept within a range we called "normal" in the blood. Fasting (for 8 hours) normal values of blood glucose are 90 - 130 mg/dl. Hyperglycemia is defined as fasting blood glucose (for 8h) above 90 - 130 mg/dl. Postprandial blood glucose is defined: above 180 mg/dl. A history and physical examination should be performed with a focus on trying to identify a precipitating cause of the hyperglycemia.
Patients with mild hyperglycemia may be asymptomatic. Once the blood glucose rises about 190 mg/dl, patients may present with polyuria (increased urinary volume), plydipsia (increased fluid intake), polyphagia (increased PO intake) and weight loss. It is also important to consider an illness as a precipitating factor (infection, myocardial infarction).