A normal blood sugar level after fasting for eight hours is 70 to 100 mg/dL, according to MedlinePlus. A fasting blood sugar test is common in screening for diabetes.
Blood sugar levels of 100 to 125 mg/dL indicate impaired fasting glucose, a type of prediabetes that increases the risk of developing Type 2 diabetes. A level of 126 mg/dL or higher indicates diabetes. Higher-than-normal levels can also result from other medical conditions, including an overactive thyroid gland, pancreatic cancer, inflammation of the pancreas, stress due to trauma, and rare tumors. An abnormal test result for someone with known diabetes may mean the condition is not properly controlled.
Fasting blood-sugar levels are typically measured in the morning after a person has fasted overnight, states Mayo Clinic. This is just one of several tests that make up a complete diabetes screening procedure. 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.
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.