This test measures the amount of ketones in the blood. Ketones or ketone bodies are byproducts of fat metabolism. They are produced when glucose is not available to the body's cells as an energy source. When fatty acids are metabolized, ketones build up in the blood, causing first ketosis, and then progressing to ketoacidosis, a form of metabolic acidosis. This condition is most frequently seen with uncontrolled type 1 diabetes and can be a medical emergency.