Coagulation Cascade

Normal Hemostasis: involves a complex sequence of interrelated reactions that lead to platelet aggregation (primary hemostasis) and activation of coagulation factors (secondary hemostasis) to produce a durable vascular seal.

  • Primary Hemostasis is an immediate but temporary response to vessel injury. Platelets and von Willebrand factors (vWF) interact to form a primary plug.

  • Secondary hemostasis (coagulation) results in a fibrin clot. Injury initiates coagulation by exposing extravascular tissue factor to blood, which initiates activation of factors VII, X, and prothrombin. The subsequent activation of other factors leads to generation of thrombin, conversion of fibrinogen to fibrin, and formation of a durable clot.

Coagulation cascade

Intrinsic pathway: measured by aPTT. Inside cut in endothelium exposes negatively charged collagen. Factor 12 is activated. Activated factor 12 activates factor 11. Activated factor 11, activates factor 9. Activated factor 9 enters the common pathway to activate factor 10 in presence of factor 5. Activated factor 10 converts prothrombin (factor 2) to thrombin (acitvated factor 2), converting fibrinogen (factor 1) to fibrin. Fibrin cross-links to stabilize in presence of activated factor 13.

Extrinsic pathway: measured by PT. Outside in plasma. Vit-K dependent. Factor 7 is activated with tissue factor. Activated factor 7 enters the common pathway as above.

History can assess bleeding severity, congenital or acquired status, and primary or secondary hemostatic defects.

  • Prolonged bleeding after dental extractions, circumcision, menstruation, labor and delivery, trauma, or surgery may suggest an underlying bleeding disorder.

  • Family history of an inherited bleeding disorder.

Physical Examination

Primary hemostasis defects are suggested by mucosal bleeding and excessive bruising.

  • Petechiae (1–2 mm) subcutaneous bleeding, do not blanch with pressure, seen typically in areas subject to increase hydrostatic pressure: lower legs and periorbital areas

  • Purpuras (3–5 mm)

  • Ecchymoses, bruises (1-2 cm)

Secondary hemostasis defects can produce hematomas, hemarthroses, or delayed bleeding after trauma or surgery.

Dxtic:

  • CBC, aPTT, PT/INR, and peripheral blood smear.

  • Primary hemostasis tests

    • Low platelet count requires review of blood smear to rule out platelet clumping artifact (due to the EDTD additive, platelet glycoprotein IIb/IIIa rcp inhibitor drugs), giant platelets.

    • Bleeding time measures time until bleeding stops after a standardized skin incision, but he does not quantify the peri-operative risk of bleeding. Factors that can prolong the BT: thrombocytopenia, vWD, abnormal capillary or skin integrity, antiplatelet therapy, uremia, liver failure, anemia, and poor technique.

    • PFA-100 instrument assesses vWF-dependent platelet activation in flowing citrated whole blood. Most patients where vWD and qualitative platelet disorders have prolonged closure times. Anemia and thrombocytopenia (Plt <100K) can cause prolonged closure times.

    • In vitro platelet aggregation studies measure platelet secretion and aggregation in response to platelet agonists (e.g., ADP, collagen, arachidonic acid, epinephrine), and they assist with the diagnosis of qualitative platelet disorders.

    • vWD evaluation begins with measurement of vWF:Ag and performance or at least one vWF activity assay:

      • Ristocetin cofactor (vWF:RCo): measures vWF-mediated agglutination of control platelets in the presence of ristocetin.

      • Collagen binding assay: measures vWF affinity for collagen.

      • “Functional” immunoassay: monoclonal antibody to vWF domain that binds to platelets.

    • vWF multimer analysis by agarose gel electrophoresis separates vWF multimers by size to classify vWD type 2 subtypes.

  • Secondary hemostasis tests

    • Prothrombin time: measures time to form a fibrin clot after adding thromboplastin (tissue factor and phospholipid) and calcium to citrated plasma.

      • Sensitive to deficiencies of extrinsic pathway (factor VII), common pathway (factors X, V, prothrombin), and fibrinogen.

      • Reporting a PT ratio as an INR

    • Activated partial thromboplastin times: measures the time to form a fibrin clot after activation of citrated plasma by calcium, phospholipid, and negatively charged particles.

    • Besides heparin, deficiencies and inhibitors of coagulation factors of the intrinsic pathway (e.g., HMWK, prekallikrein, factor XII, XI, IX, and VIII), common pathway (e.g., factor V, X, prothrombin), and fibrinogen prolong the aPTT.

    • Thrombin Time (TT): measures time to form of fibrin clot after addition of thrombin to citrated plasma. Quantitative and qualitative deficiencies of fibrinogen, FDP, heparin, and DTI drugs prolong the TT.

    • Fibrinogen: Addition of thrombin to dilute plasma and the measurement of clotting time determine the level fibrinogen. Conditions causing hypofibrinogenemia and the potential for bleeding include decrease hepatic synthesis, massive hemorrhage, and DIC.

    • D-dimers result from plasmin digestion of fibrin. Elevated D-dimer concentrations occur in many disease states including VTE, DIC, trauma, and malignancy.

    • Mixing studies determine if of factor deficiency or an inhibitor have prolonged the PT or the aPTT. Mixing patient plasma 1:1 with normal pooled plasma (all factor activities =100%) restores deficient factors sufficiently to normalize or nearly normalize the PT or the aPTT. Partial correction of the prolonged PT or aPTT after mixing, indicates a specific factor inhibitor, a nonspecific inhibitor (e.g., lupus anticoagulant), heparin, or a DTI anticoagulant may have caused the prolongation.

      • Factor deficiencies that caused prolonged PT and/or aPTT that correct with 50:50 mix.