Fasting blood sugar tests measure a person's blood glucose levels after he has not eaten for at least eight hours, according to WebMD. It is usually the first test doctors order to check for diabetes.
Normal fasting blood sugar levels are typically between 70 and 100 milligrams per deciliter, with slight variances between laboratories, reports MedlinePlus. Levels from 100 to 125 milligrams per deciliter indicate prediabetes and an increased risk of developing Type 2 diabetes, and levels over 125 milligrams per deciliter usually indicate a person has diabetes. Other medical problems that can cause an increase in blood sugar levels include pancreatic cancer, hyperthyroidism, rare tumors, and stress related to heart attack, stroke, surgery or trauma.