What is in a green tube?

• A. Lithium heparin
• B. Sodium citrate
• C. Potassium oxalate
• D. EDTA

Answer: (B)

Green-top tubes are designed for plasma determinations and use an anticoagulant to prevent clotting during collection. The anticoagulant used in these tubes is heparin, typically lithium heparin, which keeps the blood fluid so plasma can be separated and analyzed for chemistry tests. Heparin works by activating antithrombin III, which inhibits thrombin and factor Xa, stopping clot formation without heavily interfering with most routine chemistry measurements. Sodium citrate is used in blue-top tubes for coagulation studies because it binds calcium to prevent clotting; potassium oxalate with fluoride is used in some gray-top tubes; EDTA is used in lavender-top tubes for hematology. So, the green tube’s content is heparin, fitting its role for plasma chemistry testing.