Laboratory testing of hemostasis

Abstract

Hemostasis is the result of a complex interaction between vessels, platelets, and coagulation proteins that stops bleeding (primary hemostasis, coagulation) while maintaining blood flow in the vessel (fibrinolysis). For many years, the initiation of coagulation was divided into two distinct pathways: intrinsic and extrinsic. A cell-based model that focuses on the sequential steps of thrombin generation has now replaced the cascade model. Nevertheless, in order to best understand the principles of routinely used coagulation assays which do still refer to the division of the coagulation cascade – namely, activated partial thromboplastin time (aPTT) and prothrombin time (PT) – this model remains clinically useful. Hemostasis laboratories can perform large numbers of assays and obtain accurate and comprehensive diagnoses of hemostatic abnormalities. Some assays are also specifically designed to monitor anticoagulant treatment. This chapter discusses the methods and limitations of routinely performed assays (aPTT, PT, fibrinogen, platelet count). It also reviews other specialized tests that look at coagulation or primary hemostasis and that are useful to anesthesiologists and intensive care specialists. Routine tests (aPTT and PT) have low positive predictive values for bleeding risk in the general population since normal results cannot rule out the possibility of a hemostatic disease and any subsequent perioperative hemorrhage.