The normal results from a fasting blood glucose test are generally below or equivalent to 100 milligrams per deciliter, states WebMD. The normal values from a postprandial blood sugar test, which measures sugar levels two hours after eating, are below 140 milligrams per deciliter for individuals 50 years old and younger, below 150 milligrams per deciliter for people age 50 to 60, and less than 160 milligrams per deciliter for individuals age 60 and above.
Blood glucose levels vary depending on the laboratory, the person's health and other factors, notes WebMD. The normal values from random tests, which measure sugar levels at any hour, generally range from 80 to 120 milligrams per deciliter before eating or after waking up, and 100 to 140 milligrams per deciliter before sleeping.
High values possibly indicate diabetes, but doctors consult the criteria set by the American Diabetes Association to conclude a Type 2 diabetes diagnosis, explains WebMD. People who suffered stroke or heart attack, experience too much stress, or take medications such as corticosteroids are prone to high blood sugar levels. Having Cushing's syndrome or excessive production of growth hormones sometimes leads to high blood glucose levels.
Low values may indicate insulinoma, which is a tumor that creates excessive insulin, according to WebMD. Liver disease, a pituitary gland tumor, hypothyroidism and Addison's disease are also possible causes of low sugar levels.