Urine test: presence of acetoacetate (type of ketone) – limited insight into whether the body is burning fat for fuel
Blood test: measures beta-hydroxybutyrate (BHB), a form of ketone. This is the most accurate way to test for ketones.
Breathalyzer: measures acetone in breath, acetone is a substance that BHB breaks down into, indicator of the amount of BHB present in the blood. This is the more convenient way to test for ketones.