The amount of glucose (sugar) in your blood changes throughout the day and night. Your levels will vary depending upon when, what and how much you have eaten, and whether or not you have exercised. The American Diabetes Association categories for normal blood sugar levels are the following, based on how your glucose levels are tested:
A fasting blood glucose test. This test is performed after you have fasted (no food or liquids other than water) for eight hours. A normal fasting blood glucose level is less than 100 mg/dl. A diagnosis of diabetes is made if your blood glucose reading is 126 mg/dl or higher after two consecutive blood tests. (In 1997, the American Diabetes Association lowered the level at which diabetes is diagnosed to 126 mg/dl from 140 mg/dl.)
A “random” blood glucose test taken at any time. A normal blood glucose range is in the low to mid 100s. A diagnosis of diabetes is made if your blood glucose reading is 200 mg/dl or higher and you have symptoms of disease such as fatigue, excessive urination, excessive thirst or unplanned weight loss.
Another test called the oral glucose tolerance test may be performed instead. For this test, you will be asked, after fasting overnight, to drink a sugar-water solution. Your blood glucose levels will then be tested over several hours. In a person without diabetes, glucose levels rise and then fall quickly after drinking the solution. In a person with diabetes, blood glucose levels rise higher than normal and do not fall as quickly. A normal blood glucose reading two hours after drinking the solution is less than 140 mg/dl, and all readings between zero to two hours are less than 200 mg/dl. Diabetes is diagnosed if your blood glucose levels are 200 mg/dl or higher.