The gray top tube is used for determining levels of which substances?

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Gray top tubes contain sodium fluoride and potassium oxalate. Sodium fluoride acts as a glycolysis inhibitor, preventing red blood cells from consuming glucose after collection, so the measured glucose reflects the level at draw time. Potassium oxalate serves as an anticoagulant to keep the blood from clotting, allowing plasma to be used for testing. This combination makes gray tubes ideal when testing for glucose and, in many settings, for alcohol (ethanol) levels as well. So the substances best determined with this tube are glucose and alcohol.