Ketones, which result from the metabolism of fatty acid and fat, consist mainly of three substances: acetone, beta-hydroxybutyric acid, and acetoacetic acid. The last two substances readily convert to acetone, in essence making acetone the main substance being tested. However, some testing products measure only acetoacetic acid. In healthy persons, ketones are formed in the liver and are completely metabolized so that only negligible amounts appear in the urine. However, when carbohydrate metabolism is altered, an excessive amount of ketones is formed (acidosis) because fat becomes the predominant body fuel instead of carbohydrates. When the metabolic pathways of carbohydrates are disturbed, carbon fragments from fat and protein are diverted to form abnormal amounts of ketone bodies. Increased ketones in the blood lead to electrolyte imbalance, dehydration, and, if not corrected, acidosis and eventual coma. The excess presence of ketones in the urine (ketonuria) is associated with diabetes or altered car- bohydrate metabolism. Some fad diets that are low in carbohydrates and high in fat and protein also produce ketones in the urine. Testing for urine ketones in patients with diabetes may provide the clue to early diagnosis of ketoacidosis and diabetic coma.