This term refers to the response to vascular injury that produces a platelet clot at the site of damage.1,2 Primary hemostasis serves to immediately limit bleeding through the formation of a loose platelet plug. Platelets play a key role in the rapid response to blood vessel injury by:
- Adhering to the endothelial wall at the site of injury
- Releasing potent anticoagulant compounds
- Aggregating to form a plug
- Providing a phospholipid surface for activated coagulation enzyme complexes
Defects in primary hemostasis are generally associated with mucocutaneous bleeding, characterized by epistaxis, ecchymosis, genitourinary bleeding, or gingival bleeding. A typical patient with defective primary hemostasis might experience profuse bleeding from small cuts and require the application of pressure for a prolonged period to stop the bleeding.3,4 This type of bleeding pattern is different from that typically seen in patients with defects in secondary hemostasis, where deep tissue bleeding and hemarthroses are more the norm.2
Platelet counts and platelet function tests are a useful aid in the assessment of primary hemostasis. von Willebrand factor (vWF) serves to attach platelets to the blood vessel walls and to each other during primary hemostasis. Defects in vWF concentration or activity are very common, affecting approximately 1% of the population.1 The role of the clinical laboratory in the assessment of vWF function is described in detail under the test descriptions below.
1. Brandt JT. Overview of hemostasis. In McClatchey KD, ed. Clinical Laboratory Medicine. 2nd ed. Baltimore, Md: Lippincott Williams and Wilkins; 2002:987-1009.
2. Kottke-Marchant K, Corcoran G. The laboratory diagnosis of platelet disorders. Arch Pathol Lab Med.2002; 126(2):133-146. PubMed 11825107
3. Liu MC, Kessler CM. A systemic approach to the bleeding patient. In Kitchens CS, Alving BM, Kessler CM, eds. Consultative Hemostasis and Thrombosis. Philadelphia, Pa: WB Saunders; 2002: 181-196.
4. Triplett DA. Coagulation abnormalities. In McClatchey KD, ed. Clinical Laboratory Medicine. 2nd ed. Baltimore, Md: Lippincott Williams and Wilkins; 2002:1033-1049.