Tests of Hemostasis and Thrombosis: Learning Objectives
Medical Importance
The screening tests of hemostasis were developed to help identify patients with hemostatic defects that could cause excessive bleeding. Screening tests are available for each of the three phases of hemostasis: coagulation (fibrin clot formation), platelet plug formation, and fibrinolysis. In general, the tests are sufficiently sensitive to be abnormal in most patients with a hemostatic defect severe enough to cause bleeding. However, they occasionally are overly sensitive and may be abnormal due to disorders that do not cause hemorrhage.
These tests are used very commonly in clinical medicine, both to help predict hemorrhage at surgery and to identify specific defects in patients with a history of excessive bruising or bleeding. Based on this information, appropriate therapy can be administered, such as fresh frozen plasma or clotting factor concentrates for coagulation defects, platelet transfusions or von Willebrand factor concentrates for platelet function defects, or antifibrinolytic agents for systemic fibrinolysis.
Objectives
- Given values for the PT/INR, PTT, TT (thrombin time), fibrinogen concentration, and platelet count, be able to construct an appropriate differential diagnosis of possible disorders giving rise to these abnormalities.
- Given values for various clotting factor concentrations, be able to predict which screening tests of coagulation will be abnormal.
- Be able to explain how a 1:1 mixing study can distinguish a clotting factor deficiency from an inhibitor of coagulation.
- Be able to explain the utility and derivation of the INR.
- Be able to compare and contrast three tests of platelet function - bleeding time, PFA-100, and platelet aggregation studies.
- e able to diagram the formation of the D-dimer and explain its utility in diagnosis venous thromboembolic disease.