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.
- Be able to diagram the formation of the D-dimer and explain its utility in diagnosis venous thromboembolic disease.
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