Excess glucose in the blood, usually caused by diabetes mellitus. Blood sugar (glucose) content is regulated by a number of hormones, but only one of them, insulin, can reduce the level if it becomes too high. It causes body cells to absorb sugar, thus lack of insulin leads to a build up of excess blood sugar, but body cells receive too little. Some of the excess sugar is transported to the kidneys and removed via the urine; this process requires large quantities of water, making dehydration one of the dangers of hyperglycaemia. Because body cells are not receiving enough sugar, they have to produce energy by burning fat, a process which functions abnormally because of the sugar shortage, producing acid waste products (ketones, including acetone). These acidify the blood and thus prevent a number of metabolic reactions; acetone is present in exhaled air, and the smell of acetone on the breath is an important sign of disturbed sugar metabolism. Acetone is also present in the urine in serious cases, and it can be detected with test papers. Unlike other body cells, brain cells cannot burn fat if they are short of sugar; this and blood acidity are the most important reasons for coma in hyperglycaemia, but low blood sugar levels (hypoglycaemia) can also cause unconciousness, usually very rapidly. Hyperglycaemia can generally be prevented by careful control of a diabetic condition. If test papers indicate acetone in the urine a doctor should be consulted immediately. If metabolic disturbance and dehydration are severe the patient should be admitted to hospital to re-establish optimal body fluid balance.