What additive is present in PST tubes?

• A. Ammonium Heparin
• B. Sodium Heparin
• C. Lithium Heparin
• D. Heparin

Answer: (B)

PST tubes are designed to yield plasma for chemistry testing by preventing clotting and by providing a gel barrier that keeps cells away from plasma after centrifugation. The additive used is heparin, specifically the sodium salt. Heparin inhibits clotting by activating antithrombin III, which blocks thrombin and factor Xa, so blood remains liquid and plasma can be separated easily. The sodium salt is chosen because it gives broad compatibility with many assays and minimizes interference with common chemistry measurements; other heparin salts like lithium or ammonium can complicate certain tests or affect results, and a general "heparin" label can be ambiguous. The gel component forms the barrier between plasma and cells once centrifuged, preserving sample integrity for accurate results.