When evaluating a child with a potential bleeding disorder, it is extremely crucial to ascertain whether the patient's symptoms are appropriate to a bleeding disorder and whether further investigation of an underlying bleeding disorder is required. To understand approach to a bleeding child, it is essential to know the steps in hemostasis.

Physiology of Hemostasis

Hemostasis is a delicate balance between clot formation, anticoagulation and fibrinolysis and it occurs in the following phases:

• Vascular response
• Platelet adhesion
• Platelet aggregation
• Clot formation
• Clot stabilization
• Limitation of clot by anticoagulation
• Fibrinolysis to reestablish vascular patency
  

The primary phase of hemostasis is the production of the platelet plug. This process begins immediately after vascular injury as subendothelial tissues are exposed to circulating blood. Platelet binding to subendothelial tissue i.e. adhesion is followed by platelet activation with release of platelet contents, leading to platelet aggregation. The platelet plug not only offers the first line of defense against hemorrhage but also provides the phospholipid surface required for coagulant protein's interactions necessary for the formation of a fibrin clot.

The secondary phase of hemostasis results in the formation of a fibrin clot. The coagulation proteins circulate in inactive precursor forms; however, with the initiation of coagulation, these are converted to active forms. These activated proteins then further activate other factors in chain reaction (Appendix 1), ultimately leading to the formation of the blood clot. Although the clotting cascade model has undergone significant modification since its introduction in 1964, it is still very useful for the interpretation of abnormal coagulation test results such as the prothrombin time (PT) and activated partial thromboplastin time (aPTT).

The interactions of activated platelets and the clotting cascade give rise to hemostatic response that is potentially explosive and unchecked, may lead to thrombosis and tissue damage. Fortunately, coagulation is modulated by a number of mechanisms including the removal of activated factors through the reticuloendothelial system and the control of activated procoagulants by natural antithrombotic pathways (antithrombin III, protein C, protein S). In addition to these limiting reactions, there exists the fibrinolytic pathway, which restores vessel patency following hemostasis.