Primary haemostasis and its assessment by laboratory tests.

Platelets constantly patrol the inner surface of blood vessels searching leaks to be sealed, in order to prevent blood loss. When they detect a vessel injury their action can be divided into three phases. ADHESION: The platelets adhere to the injured blood vessel wall via their receptors glycoprotein (GP) Ib and integrin alpha2bbeta3 (GPIIb/IIIa) mediated by the ligands von Willebrand factor (VWF), fibrinogen and others. AGGREGATION: Platelets stick to each other through fibrinogen bridging integrin alpha2bbeta3 (GPIIb/IIIa) on adjacent platelets. SECRETION: During activation the content of platelet granules is released by exocytosis, thus augmenting and propagating formation of a haemostatic plug or thrombus. Laboratory tests mimic one or several aspects of these three phases to obtain reliable data on a patients platelet function. In this overview assays, test principles, and pitfalls are presented.