A normal fasting blood sugar level following eight hours without food is typically between 70 and 99 milligrams per deciliter, per Virginia Mason. Diabetes can be diagnosed when two or more consecutive fasting blood sugar tests result in levels over 126 milligrams per deciliter.
Symptoms of fatigue, unplanned weight loss, excessive urination or thirst can be indicators of diabetes; however, some diabetics are diagnosed without having experienced any symptoms, notes Virginia Mason. Pre-diabetes can be diagnosed if the fasting blood sugar is between the levels of 100 and 125 milligrams per deciliter or if the A1C level is between 5.7 and 6.4 percent. Once a person receives a diagnosis of pre-diabetes, it is advisable to work with a doctor to embark on a diet and exercise regime to delay or prevent diabetes.
The oral glucose-tolerance test is similar to the fasting blood-sugar test and also used to screen for diabetes, explains Mayo Clinic. For this test, a patient is asked to fast overnight. He is then asked to drink a sugary liquid while in the doctor's office. The patient's blood sugar levels are measured regularly over the next several hours. After 2 hours, the blood sugar level should be below 140 milligrams per deciliter in a healthy patient. A level above 200 milligrams per deciliter indicates that a patient is diabetic. A doctor may follow up this test with a similar test using a more concentrated solution of glucose if the results are not conclusive.
Anyone with a body mass index above 25, and everyone over the age of 45, is advised to undergo diabetes screening tests at least every 3 years.