Which additive in the gray top tube helps preserve glucose by inhibiting glycolysis?

• A. Fluoride
• B. Citrate
• C. EDTA
• D. Heparin

Answer: (A)

Glucose in a drawn blood sample can fall because red and white cells keep carrying out glycolysis after collection. To keep glucose levels accurate, gray-top tubes include a glycolysis inhibitor. The additive that does this is fluoride, specifically sodium fluoride, which blocks the enzyme enolase in the glycolytic pathway. By inhibiting enolase, glycolysis slows dramatically, so glucose isn’t broken down before analysis. The gray-top tube also contains an anticoagulant (potassium oxalate), but the glucose-preserving effect comes from fluoride. The other additives listed are anticoagulants or chelators and don’t inhibit glycolysis, so they don’t preserve glucose in the same way.