9.2 Hemostasis and Coagulation

9.2 Hemostasis and Coagulation

Quick Answer: Hemostasis has two phases: primary hemostasis (platelet plug formation via GPIb-von Willebrand factor adhesion and GPIIb/IIIa-fibrinogen aggregation) and secondary hemostasis (the coagulation cascade, stabilizing the plug with fibrin). The prothrombin time (PT) screens the extrinsic and common pathways; the activated partial thromboplastin time (aPTT) screens the intrinsic and common pathways. Mixing studies distinguish a factor deficiency (corrects with normal plasma) from an inhibitor (does not fully correct, especially after incubation). These principles determine whether fresh frozen plasma, cryoprecipitate, platelets, or a specific factor concentrate is the correct component for a bleeding patient.

Primary Hemostasis

Vascular injury exposes subendothelial collagen and releases tissue factor. Circulating von Willebrand factor (vWF) binds exposed collagen and tethers platelets through the platelet glycoprotein GPIb receptor. Platelets activate, releasing granule contents (ADP, thromboxane A2, serotonin) that recruit and activate additional platelets, and fibrinogen bridges adjacent platelets through the GPIIb/IIIa receptor to form the primary platelet plug. Two classic inherited platelet disorders map directly onto this pathway: Bernard-Soulier syndrome (deficient or dysfunctional GPIb, giant platelets, thrombocytopenia, absent ristocetin-induced platelet aggregation) and Glanzmann thrombasthenia (deficient or dysfunctional GPIIb/IIIa, normal platelet count and size, absent aggregation to essentially all agonists except ristocetin).

Secondary Hemostasis: The Coagulation Cascade

The classic cascade model organizes coagulation factors into three pathways that converge on a common pathway generating thrombin and fibrin:

PT/INR is prolonged by deficiencies or inhibition of VII, X, V, II, or fibrinogen, and is the test used to monitor warfarin therapy (warfarin suppresses the vitamin K-dependent factors II, VII, IX, and X, but VII has the shortest half-life and dominates the early PT response). aPTT is prolonged by deficiencies or inhibition of XII, XI, IX, VIII, or the common pathway factors, and is used to monitor unfractionated heparin. Factor VII is the only factor measured by the PT and not the aPTT, a frequently tested distinction.

Interpreting Factor Assays and Mixing Studies

When a screening test (PT or aPTT) is prolonged, a 1:1 mixing study (patient plasma plus pooled normal plasma) helps distinguish the two major causes:

• Correction to normal on immediate mixing indicates a factor deficiency - there is enough factor activity in the normal plasma to correct the patient's deficiency.
• Incomplete correction, or correction on immediate mixing followed by re-prolongation after 1-2 hours of incubation at 37 degrees C, indicates an inhibitor (for example, a factor VIII autoantibody or a lupus anticoagulant), because the inhibitor has time to neutralize factor activity in the mixed sample.

Once a deficiency is confirmed, specific factor assays (one-stage, clot-based, reported as percent of normal activity) identify which factor is low and by how much. This distinction is central to component selection: a confirmed isolated factor VIII or IX deficiency is treated with a specific recombinant or plasma-derived concentrate rather than plasma, while multiple factor deficiencies (as in liver disease or dilutional coagulopathy) are treated with fresh frozen plasma.

Coagulation Factor Disorders and Platelet Function Disorders

Hemophilia A (factor VIII deficiency) and hemophilia B (factor IX deficiency) are X-linked recessive disorders producing an isolated prolonged aPTT with a normal PT. Von Willebrand disease (vWD), the most common inherited bleeding disorder, results from quantitative or qualitative vWF defects; laboratory findings include a normal-to-borderline platelet count, prolonged bleeding time or PFA closure time, decreased vWF antigen, and decreased ristocetin cofactor activity. Because vWF stabilizes circulating factor VIII, some vWD subtypes also show a mildly prolonged aPTT. Treatment ranges from desmopressin (DDAVP, which releases stored vWF from endothelial cells) for mild type 1 disease to vWF/factor VIII concentrate or cryoprecipitate for more severe or type 3 disease.

Disseminated intravascular coagulation (DIC) is an acquired consumptive disorder in which widespread activation of coagulation depletes platelets and coagulation factors while simultaneously activating fibrinolysis. Expected findings are thrombocytopenia, prolonged PT and aPTT, low fibrinogen, elevated D-dimer/fibrin degradation products, and schistocytes on the peripheral smear. Component support for DIC is guided by the specific deficiency pattern: cryoprecipitate for fibrinogen below approximately 100 mg/dL, fresh frozen plasma for broadly prolonged PT/aPTT with multiple factor deficiencies, and platelet concentrates for bleeding with significant thrombocytopenia.

Fibrinolysis

Clot formation is balanced by fibrinolysis, which limits clot extension and eventually dissolves it. Tissue plasminogen activator (tPA), released from endothelium, converts plasminogen bound within the clot to plasmin, which degrades fibrin into fragments including D-dimer (a cross-linked fibrin degradation product used clinically to help exclude significant clot burden and to support the diagnosis of DIC when markedly elevated). Fibrinolysis is itself regulated by inhibitors such as plasminogen activator inhibitor-1 (PAI-1) and alpha-2-antiplasmin; a relative excess of fibrinolytic activity, whether from primary hyperfibrinolysis (as in some liver disease or trauma) or secondary hyperfibrinolysis (as in DIC), can produce a bleeding picture that mimics a factor deficiency and may require antifibrinolytic therapy (for example, tranexamic acid) in addition to, or instead of, component replacement.

Why This Matters for Component Therapy

Specialist-level understanding of coagulation testing is what allows correct product selection at the bedside: PT/aPTT patterns and mixing studies triage a bleeding patient toward plasma, cryoprecipitate, platelets, or a factor concentrate before a full coagulation work-up returns, and recognizing an inhibitor pattern prevents wasted, ineffective transfusions of plasma or factor concentrate that will simply be neutralized by the inhibitor.