Other Tests of Hemostatic Function
Other tests of hemostasis may be indicated in specific disease situations; they include the following.
Thrombin Time
The standard thrombin time test measures the rate of fibrinogen to fibrin conversion and can be a useful assay for the effects of FDPs, heparin, and paraproteins; for example, a prolonged thrombin time can indicate the presence of FDPs that interfere with fibrin polymerization.
This test also evaluates for the presence of hypofibrinogenemia, but the Clauss TT (described earlier) is a better assay for that purpose, as any interference effect from FDPs, heparin, and other factors is largely eliminated.Specific Factor Assays
Specific factor analyses are generally indicated for the diagnosis of hereditary factor deficiencies. Prolonged PTT values along with the signalment, history, and clinical picture may indicate that specific factor assays are needed. In DIC, specific factor analysis is unlikely to be useful.
α2-Antiplasmin
α2-Antiplasmin (α2-AP) binds plasmin to cleave and inactivate it, and then the complexes are cleared from circulation. In humans, α2-AP is decreased by severe liver disease (reduced production) and with DIC (consumption). Experimental studies in ponies suggest that α2-AP may be reduced with chronic or subclinical DIC.66 Conversely, increased α2-AP is reported in horses with colic as part of the acute phase response (see Chapter 26).
Protein C
The protein C pathway provides the second major anticoagulant mechanism that regulates hemostasis. Protein C is activated by thrombin complexed to thrombomodulin and subsequently inactivates factors Va and VIIIa via proteolysis and using the cofactor protein S. In humans, decreased protein C can occur in liver failure or DIC, increasing the risk of thrombotic disease, and decreased protein C is reported in horses with severe colic.49