Ketones are metabolic end products of fatty acid metabolism. In healthy individuals, ketones are formed in the liver and are completely metabolized so that only negligible amounts appear in the urine. However, 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. A finding of ketones in the urine indicates that the body is using fat as the major source of energy.

Three ketone bodies that appear in the urine when fats are burned for energy are acetone, acetoacetic acid and beta-hydroxybutyric acid. Normally, the urine should not contain enough ketones to give a positive reading. As with tests for glucose, acetone can be tested by a dipstick or by a tablet. The results are reported as small, moderate, or large amounts of acetone. A small amount of acetone is a value under 20mg/dl; a moderate amount is a value of 30-40mg/dl, and a finding of 80mg/dl or greater is reported as a large amount.

Screening for ketonuria is done frequently for acutely ill patients, presurgical patients, and pregnant women. Any diabetic patient who has elevated levels of blood and urine glucose should be tested for urinary ketones. In addition, when diabetic treatment is being switched from insulin to oral hypoglycemic agents, the patient's urine should be monitored for ketonuria. The development of ketonuria within 24 hours after insulin withdrawal usually indicates a poor response to the oral hypoglycemic agents. Diabetic patients who use oral hypoglycemic agents should have their urine tested regularly for glucose and ketones because oral hypoglycemic agents, unlike insulin, do not control diabetes when an acute infection or other illness develops.

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; therefore, urine ketone measurement is especially helpful in emergency situations. During pregnancy, early detection of ketonuria is essential because ketoacidosis is a factor associated with intrauterine death.

Ketosis and ketonuria occur in:

In nondiabetic persons, ketonuria may occur during acute illness or severe stress. Approximately 15% of hospitalized patients may have ketonuria, even though they do not have diabetes. In a diabetic patient, ketone bodies in the urine suggest that the patient is not adequately controlled and that adjustments of medication, diet, or both should be made promptly. In the nondiabetic patient, ketonuria reflects a reduced carbohydrate metabolism and excessive fat metabolism.

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Diabetic patients switching from insulin to oral hypoglycemic agents should have urinary ketones checked to assess the effectiveness of therapy.