Introduction to Hemostasis and Thrombosis

These Web pages provide a basic overview of hemostasis and thrombosis with particular focus on the role of the clinical laboratory. Hemostasis can be defined as the physiological process that keeps blood flowing while allowing solid clot formation, or thrombosis, to prevent blood loss from sites of vascular damage.^1,2 This requires a delicate balance between a potent coagulation mechanism that rapidly forms a stable clot and a regulatory system that limits clot formation to the site of injury. As the site of damage is repaired, fibrinolysis degrades the clot and restores unrestricted blood flow.

In general, there are five clinical situations that call for the use of coagulation tests in the assessment of patients. These are:

1. Excessive bleeding: Defects in primary and secondary hemostasis are potential causes of excessive bleeding (see "Hemostasis").
2. Increased risk of thrombosis: A number of congenital and acquired conditions can lead to pathologic clot formation (see "Thrombophilia").
3. Therapeutic monitoring: More details about tests used to monitor anticoagulant therapy are provided under the following test descriptions:
   • 005199 Prothrombin Time
   • 117904 Factor Xa, Chromogenic
   • 005207 Partial Thromboplastin Time, Activated (aPTT)
   • 117101 Heparin Anti-Xa
   • 500190 Ecarin Clotting Time
   • 500465 Fondaperinux Anti Xa
4. Pathologic thrombosis: More details about tests used to evaluate thrombotic conditions are provided under the following test descriptions:
   • 116012 Disseminated Intravascular Coagulation (DIC) Profile
   • 117853 Disseminated Intravascular Coagulation (DIC) Profile, Comprehensive Plus
   • 150075 Heparin-Induced Platelet Antibody
   • 115188 D-Dimer
5. Causes of Abnormal Screening Results: More details about tests used to evaluate an extended PT or aPTT result with no history of bleeding or thrombosis. (see "Causes of Abnormal Screening Results")

References