Fibrinolytic System and Platelet Function Tests

Fibrinolytic System

• Fibrinolysis is the process of breaking down fibrin, preventing excessive clotting.
• The primary proteins involved are plasmin, which degrades fibrin into fibrin degradation products.
• Inhibitors of fibrinolysis include:
  • Plasminogen activator inhibitor 1 (PAI-1) which rapidly inhibits tPA and TcuPA.
  • α2-Antiplasmin (α2-AP) which inactivates plasmin by forming a complex.
  • Thrombin activatable fibrinolysis inhibitor (TAFI) cleaves the C-terminal lysine residues of fibrin to prevent plasminogen activation by fibrin.

Platelet Function Tests

• Measures the time taken for blood vessels constriction and platelet plug formation.
• Used to detect defective platelet function.
• A screening test for acquired and congenital platelet defects.
• Methods:
  • Duke Method
  • Ivy Method
• Should not be performed if platelet count is below 75,000.
• Bleeding time is measured by recording the time it takes for bleeding to stop.
• Reference range for bleeding time:
  • Ivy: 1-7 minutes.
  • Duke: 1-6 minutes.
• A prolonged bleeding time indicates an abnormality in platelet function which can include:
  • Thrombasthenia
  • Von Willebrand’s disease
  • Storage pool disease
  • Bernard Soulier syndrome
  • Prolonged fibrinolytic states
• False positive results for bleeding time can be caused by:
  • Blood pressure cuff too high.
  • Incision too deep.
  • Disrupting the clot with filter paper, especially frequent blotting.
  • Touching the incision wound outside the tip of the blood during blotting.

Coagulation Tests

• Coagulation tests measure the time it takes for blood to clot, providing a measure of fibrin formation.
• These tests are used to assess the activity of clotting factors which can lead to the diagnosis of bleeding disorders.
• The following tests are used to assess coagulation:
  • Lee-White Method
  • aPTT (activated partial thromboplastin time)
  • PT (prothrombin time)
  • INR (international normalized ratio)
  • Test for Fibrinolytic Pathway

Overview of Hemostasis

• Endothelial cells are key regulators of hemostasis by balancing antithrombotic and prothrombotic activities.
• Endothelial cell activation is triggered by:
  • Trauma
  • Infectious agents
  • Hemodynamic forces
  • Plasma mediators
  • Cytokines
• Prothrombotic Properties of Endothelium:
  • Prostacyclin: produced by COX-1
  • Nitric Oxide: produced by eNOS
  • Adenosine diphosphatase: degrades ADP
• Anticoagulant Effects of Endothelium:
  • Normal endothelium shields coagulation factors from tissue factor in vessel walls
  • Thrombomodulin and Endothelial Protein C Receptor bind to thrombin and protein C.
  • Thrombin loses its ability to activate coagulation factor and platelets and instead cleaves and activates protein C.
  • The protein C/S complex inhibits coagulation cofactors (Va and VIIIa).
  • Heparin-like molecules bind and activate antithrombin III.
  • Tissue Factor Pathway Inhibitor binds and inhibits tissue factor/factor VIIa complexes.

Fibrinolytic Effects of Endothelium

• tPA (tissue plasminogen activator) is synthesized by endothelium as a counterregulatory mechanism.

Coagulation Factor Groups

• Surface-Bound Proenzymes:
  • Factor XII, Prekallikrein, and Factor XI (known as the “contact system”) autoactivate when associated with a negatively charged surface.
  • The autoactivation of Factor XII allows for the activated partial thromboplastin time (aPTT) test.
• Vitamin K-Dependent Zymogens:
  • Factor VII, Factor IX, Factor X, Factor II, and Protein C all need vitamin K for an essential carboxylation reaction.
  • This carboxylation allows for the binding of these proteins to phospholipids and cell membranes where they are activated.

Stable Fibrin Clot Formation

• Secondary hemostasis is focused on platelet aggregation and stable fibrin clot formation.
• Secondary hemostasis is where the coagulation factors, like those from vitamin K-dependent zymogens, play a role in forming the stable fibrin clot.
• Glanzmann’s thrombasthenia is an example of a disease involving secondary hemostasis.