Although the amount of glucose in your blood varies depending on when and what you eat, the range should be relatively narrow. In general, your blood sugar is highest after you eat and lowest after you have not eaten for 8-10 hours. After fasting all night, most persons have blood glucose levels between 70 and 110 milligrams of glucose per deciliter of blood (mg/dL). After eating a large meal, a person's blood sugar will rise, but generally not above 140 mg/dL. People with untreated diabetes will have higher blood sugars after fasting and after eating.

To check if you have diabetes, your doctor will test your blood sugar levels. The results of these tests and other clinical findings will be used to decide if you have diabetes and what type. Doctors cannot diagnose diabetes on the basis of one single test. Instead, they will perform two or more glucose tests before confirming your diagnosis. The most common tests to measure glucose are the fasting plasma glucose test, the random blood sugar test, and the oral glucose tolerance test.  

• Fasting plasma glucose test. Most experts recommend using a fasting plasma glucose test to diagnose diabetes. Before taking this test, you cannot eat anything for 8 to 10 hours. Blood will be drawn from a vein in your arm and sent to a laboratory for testing. If your fasting blood glucose is 126 mg/dL or higher, your doctor will probably diagnose you with diabetes.
   
• Random blood sugar test. Many cases of diabetes are found during routine physical exams when blood is drawn for other tests. Since you don't necessarily fast before these physical exams, you may have just eaten and your blood sugar may be high. Even so, it shouldn't be higher than 200 mg/dL. If your random blood glucose is higher than 200 mg/dL, your doctor will probably suspect diabetes and may want to give you a fasting plasma glucose test.
   
• Oral glucose tolerance test. In this test, a person consumes a drink containing glucose dissolved in water. Blood is then drawn in timed intervals over a three-hour period. If plasma glucose levels rise more than expected, the person is diagnosed with diabetes. This test is often used to check pregnant women for gestational diabetes. It is rarely used to diagnose diabetes in other patients, because it is cumbersome and time-consuming.