thromboplastin

Thromboplastin, also known as tissue factor, is a cell-surface protein–lipid complex that initiates the extrinsic pathway of blood coagulation. It is expressed by subendothelial cells and released or exposed to circulating blood after vascular injury. In the presence of calcium ions, thromboplastin binds to coagulation factor VII (or VIIa) to form a complex that activates factor X to Xa, leading to thrombin generation and fibrin clot formation.

In laboratory medicine, thromboplastin refers to a reagent containing tissue factor and phospholipids used to trigger the extrinsic pathway in clotting assays. The prothrombin time (PT) test uses thromboplastin with calcium to measure how quickly a plasma sample clots, reflecting the integrity of factors VII, X, V, II, and fibrinogen. PT results are often reported as an International Normalized Ratio (INR) to standardize measurements across laboratories.

Thromboplastin reagents are derived from animal tissues (commonly brain or placenta) or from recombinant/synthetic forms of tissue factor. The source and composition of the reagent influence assay sensitivity and standardization. Modern PT/INR methods employ thromboplastin with an International Sensitivity Index (ISI) to enable cross-laboratory comparability.

Clinically, PT/INR testing is used to monitor anticoagulant therapy with warfarin and to screen for coagulation-factor deficiencies affecting the extrinsic pathway. Limitations include variability among reagents and preanalytic factors that can affect results. Thromboplastin is distinct from reagents used for the intrinsic pathway (aPTT), which use contact activators rather than tissue factor.