Which color top tube is used for a fasting glucose test?

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Preserving glucose levels after collection is essential for an accurate fasting glucose result because cells continue to use glucose in the sample. The gray top tube contains a glycolysis inhibitor (sodium fluoride) along with an anticoagulant (potassium oxalate). Sodium fluoride slows glycolysis, keeping the glucose concentration steady until analysis, and potassium oxalate prevents clotting so the sample can be processed properly. That combination makes the gray top ideal for glucose testing, especially when a fasting sample is needed to avoid changes from recent meals. Other tubes lack the glycolysis inhibitor or are intended for different tests, so they can allow glucose to decrease or introduce additives that interfere with glucose measurement.