A ketone test checks for ketones in your blood or urine. Ketones are substances that are made when the body breaks down fat for energy. Normally, your body gets the energy it needs from glucose (sugar). If your body cannot use glucose for energy-for example, if your body doesn't make or use insulin-ketones are formed. You might also have ketones if you are not eating enough carbohydrates and your body uses fat for energy instead. Your body wants to get rid of ketones through urine.
Ketones are most common in people who have type 1 diabetes. It is an early sign that there is not enough insulin, and blood sugar may be dangerously high. This can lead to diabetic ketoacidosis, a very serious medical problem.