Which color-top tube is used for glucose testing with a glycolysis inhibitor?

• A. Red-top
• B. Green-top
• C. Gray-top
• D. Blue-top

Answer: (C)

Preserving glucose levels in the blood sample requires stopping glycolysis after collection. The gray-top tube is designed for this purpose, containing sodium fluoride as a glycolysis inhibitor along with an anticoagulant (potassium oxalate). The fluoride blocks glycolytic enzymes, preventing cells in the blood from consuming glucose, so the glucose measurement reflects the patient’s true level at collection. Other common tubes lack this glycolysis-inhibiting ingredient, so glucose can continue to decrease after draw, making them unsuitable for accurate glucose testing.