Ketones are acids that the body produces when it burns fat instead of glucose for energy. Normally, the cells in the body use glucose from the blood for energy, but when carbohydrates are unavailable or unable to be used as an energy source, fat becomes the predominant body fuel instead of carbohydrates and excessive amounts of ketones are formed as a metabolic byproduct. Ketones are formed in the liver and are completely metabolized so that only negligible amounts appear in the urine in healthy individuals. However, when the body produces excess ketones, they can be detected in the urine, a condition known as ketonuria. Higher levels of ketones in the urine indicate that the body is using fat as the major source of energy.

A ketones in urine test measures the level of ketones in the urine. Normally, having some ketones in the urine is normal, but high ketone levels in urine may be a sign that the body is too acidic, a condition called ketoacidosis. This condition is most commonly seen in people with diabetes, particularly those with type 1 diabetes. A ketones in urine test is often used to help monitor people who are more likely to develop high levels of ketones, such as those with diabetes, a medical condition, diet, or exercise habits that cause the body to use fat for energy instead of glucose.

Ketones that commonly appear in the urine when fats are burned for energy are acetoacetate and beta-hydroxybutyric acid. Normally, the urine should not contain a noticeable concentration of ketones to give a positive reading. However, in conditions associated with acidosis, urinary ketones are tested to assess the severity of acidosis and to monitor treatment response. Urine ketones appear before there is any significant increase in blood ketones, making urine ketone measurement especially helpful in emergency situations.