Stage 1: Vascular phase
Injury to a blood vessel causes it to constrict slowing the flow of blood.
Stage 2 – Platelet phase
Injury to the endothelial lining causes platelets to adhere to it. Additional platelets stick to the site finally forming a temporary platelet plug in a process called ‘aggregation’.
Vascular phase and platelet phase comprise the primary hemostasis. Bleeding time test is used to evaluate primary hemostasis.
Stage 3 – Coagulation phase
This involves a cascade of interactions of coagulation factors that converts the temporary platelet plug to a stable fibrin clot. The coagulation cascade involves an intrinsic system and extrinsic system, which ultimately come together in a common pathway.
Activated partial thromboplastin time (APTT) – test used to evaluate the intrinsic pathway. This is also used to monitor heparin therapy.
Prothrombin time (PT) – test used to evaluate the extrinsic pathway. This is also used to monitor coumadin therapy.
Stage 4 – Fibrinolysis
This is the breakdown and removal of the clot. As tissue repair starts, plasmin (an enzyme) starts breaking down the fibrin in the clot. Fibrin degradation products (FDPs) measurement is used to monitor the rate of fibrinolysis.
Ramaz Mitaishvili, MD
Los Angeles, CA
