Optimal glucose levels tend to vary according to when the blood test is performed. For adults without diabetes, the range can be between 80 and 120 milligrams per deciliter when they first get up in the morning, relates MedicineNet. The blood glucose level 2 hours after having a meal should be lower than 140 milligrams per deciliter, suggests WebMD.
However, there are blood glucose tests that doctors use to diagnose if a person is diabetic. These tests include either a random or a fasting test. For a fasting test, a person does not eat for 8 hours prior to the test. A blood sugar level higher than 126 milligrams per deciliter may indicate that a person is diabetic. Similarly, a random test that shows levels higher than 200 milligrams per deciliter is a sign of diabetes, especially if the person is also urinating and is thirsty frequently, specifies WebMD.