Tube Colors, Additives and Test Matching

Specimen Types and Additive Fundamentals

In routine specimen collection, a phlebotomist must understand the distinct physiological products generated by different collection tubes. Whole blood consists of formed cellular elements suspended in liquid plasma. When a collection tube contains an anticoagulant, the blood is prevented from clotting, meaning the cellular components remain freely suspended in the liquid portion. Plasma is the liquid portion of unclotted, anticoagulated blood that retains all clotting factors, including the protein fibrinogen.

In contrast, when blood is collected into a tube without an anticoagulant, the natural coagulation cascade takes place. During clotting, fibrin forms a mesh that traps blood cells; some clotting factors are consumed, but others remain in the serum. Serum is the liquid portion of clotted blood that lacks fibrinogen because this protein was consumed during clot formation. To assist or accelerate this natural pathway, certain tubes incorporate clot activators, which promote coagulation through contact activation, as with silica, or by directly supplying thrombin.

Tube stopper colors generally indicate an additive or intended use, but formulations can vary between manufacturers; therefore, the phlebotomist must always verify the tube label before use. Additives found in vacuum tubes fall into four primary categories: anticoagulants, clot activators, antiglycolytic agents, and separator gels. Anticoagulants prevent blood from clotting by chelating calcium or by inhibiting the enzyme thrombin, while separator gels provide physical barriers between liquid and cellular components during processing.

Whenever a tube contains an additive, proper physical handling is essential. Tubes containing additives must be gently inverted immediately after collection to ensure adequate mixing of the blood with the chemical agents. The phlebotomist must remember that gentle inversion is distinct from agitation: vigorous shaking can cause mechanical hemolysis, destroying red cells and altering laboratory measurements. If a specimen is damaged by shaking, the phlebotomist must follow laboratory policy for reporting and handling an affected specimen.