A fasting blood sugar level is a blood test to measure the amount of sugar in a person's blood after he fasts overnight, explains Mayo Clinic. It is a screening test for diabetes. A normal level is less than 100 milligrams per deciliter.
A fasting blood sugar level between 100 and 125 milligrams per deciliter is indicative of prediabetes, states Mayo Clinic. A level of 126 or higher on two different blood tests indicates diabetes.
Another test that doctors use to detect diabetes is the hemoglobin A1c test, reports MedicineNet. Some doctors think this test is more reliable, because it measures average blood sugar over about 3 months. Generally, a normal hemoglobin A1c is between 4.0 and 5.9 percent. In someone with diabetes, it is usually between 7 and 9 percent.