TESTS FOR BLOOD COAGULATION
1. What is the range in minutes for normal coagulation times among domestic animals by the capillary tube method?
2. Why would low platelet counts be associated with delayed coagulation times?
3.
How is dicoumarol associated with coagulation defects?4. Why would liver disease be suspect as a cause of coagulation defects?
5. How is vWF associated with coagulation defects?
6. Why does blood withdrawn from birds, in which endothelial cell damage does not occur, coagulate with difficulty?
7. In the absence of the contact activation system, why do birds not show hemorrhagic problems?
Tests for blood coagulation are used to determine the adequacy of coagulation in an animal. Several techniques are available. Blood is withdrawn and subjected to standard methods and the time interval is observed from withdrawal to coagulation. One of these is the capillary tube method, in which the blood is collected directly into a nonheparinized capillary tube. The tube is manually broken at 1-minute intervals until the blood in the broken ends remains connected by a fibrin thread. The time in minutes for this to occur is the coagulation time (see Figure 3-2 for normal values). A prolonged time interval indicates an inadequate mechanism in the body. Because platelets supply various factors to the coagulation mechanism, in addition to forming a platelet plug, estimation of their number is also helpful in assessing coagulation adequacy.
A common laboratory screening test is the one-stage prothrombin time. In this test, plasma is activated with a mixture of TF and phospholipids. Calcium is added and the time to coagulation is determined. If clotting time is prolonged, there may be abnormalities in plasma FV, FVII, FX, prothrombin activity, or fibrinogen concentration.
Coagulation Defects
Knowledge of the coagulation process is helpful in understanding coagulation defects when they occur.
Vitamin K deficiency results in hemorrhage because of inadequate formation of prothrombin and factors VII, IX, and X. Also, dicoumarol interferes with the utilization of vitamin K and hence with prothrombin production.Dicoumarol is a product of research on a hemorrhagic disease of livestock known as sweet clover poisoning. Both yellow and white sweet clover have high coumarin content susceptible to metabolism by several common molds, with resultant dimerization of coumarin when mold growth occurs. Sweet clover hay is thick-stemmed and subject to incomplete drying when harvested and stored (bales, stacks, haylofts). Mold growth is favored and dicoumarol is produced. Because of its hemorrhagic properties, dicoumarol is commercially available in rodenticides, in which it is laced with rodent edibles. In human medicine, a derivative of dicoumarol is used as a “blood thinner.” Other causes of coagulation defects are related to liver disease, platelet defects, a complex problem known as disseminated intravascular coagulation, as well as those that are inherited. The most common inherited defects identified in domestic animals are those associated with factor IX activation and formation of the tenase complex. In this category, factor VIII (antihemophilic factor) deficiency is the most widespread. Other common inherited defects are deficiencies of factor IX and vWF. In the latter, platelet aggregates are poorly anchored to the damaged endothelium and are more susceptible to dislodgement by circulating blood. This deficiency is known as von.Willebrand disease (vWD).
Species Differences
The interaction of activated platelets with damaged endothelium and coagulation proteins is a requirement among all animals for a normal hemostatic mechanism, even though platelet numbers and morphology may vary. The absence of factor XII (a component of the intrinsic mechanism) from the blood of marine mammals and reptiles prolongs the clotting time of their withdrawn blood. In birds, the entire contact activation pathway appears to be absent, whereby activation of factor IX by that pathway does not occur. This is noticeable if blood is withdrawn atraumatically whereby there is neither trauma to blood nor to endothelium. A coagulum will form, but serum is extracted with difficulty. For this reason, when chemical analysis is desired, one should use an appropriate anticoagulant and harvest plasma (assuming plasma compatibility with the analysis). The blood will clot extremely rapidly if trauma to the vessel wall occurs during collection. In this case, the TF pathway is activated to form thrombin and associated enhancements to activate the tenase complex (see Figure 3-15). This is the reason that avian species have an intact coagulation mechanism, even though they do not have a functional contact activation system.
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