What is in an orange tube?

• A. Sodium fluoride
• B. Sodium citrate
• C. Lithium heparin
• D. EDTA

Answer: (C)

Orange-top tubes are designed for rapid serum collection. They contain a clot activator that speeds up clot formation, typically a thrombin-based agent (often with a silica clot activator). This is different from anticoagulants used to obtain plasma, which is what green-top tubes with lithium heparin do. The other options correspond to substances found in different colored tubes: sodium fluoride is a glycolysis inhibitor in gray tubes, sodium citrate is a calcium-binding anticoagulant in blue tubes, and EDTA is used in purple tubes to preserve cells. So the content of an orange tube is not lithium heparin or the others listed; it’s a clot activator such as thrombin that yields serum quickly.