Diabetes blood testing. Diabetic's finger with a drop of blood to be placed on a test strip (lower left) in a glucometer. The glucometer will measure blood glucose level. Glucose, the simplest blood sugar, is one of the body's main sources of energy. Diabetics have to monitor their blood glucose level and maintain it at the correct level. Diabetes is caused by low levels of the hormone insulin. Without insulin, the body cannot metabolise glucose as an energy source. Instead, fats are used, causing the build up of ketones in the blood. This can ultimately lead to convulsions and diabetic coma. Excessive amounts of glucose can also be damaging.