What is coagulation?
Coagulation is the series of events that result in the formation of a clot. In the body, coagulation occurs after any injury to a blood vessel or tissue, in order to stop bleeding.
Coagulation involves the interaction of cells lining the injured blood vessel (endothelial cells), specialized blood cells called platelets that form a plug in the region of the damaged blood vessel, and circulating coagulation factors.
What are coagulation factors?
Coagulation factors are substances that are made in the liver and circulate in the blood stream. They become 'activated' when blood vessel or tissue injury occurs, and in co-operation with platelets, produce a clot at the site of injury.
There are thirteen different coagulation factors. These factors are activated in a specific sequence, following two different pathways or sequences (the intrinsic and extrinsic pathways), and a final common pathway.
In the body, there is interaction between the intrinsic and extrinsic coagulation pathways. In the laboratory, the activity of the coagulation factors comprising these pathways is measured separately.
When are coagulation tests performed?
Coagulation tests may be undertaken for a variety of reasons. Some breeds of dogs are known to have a higher incidence of clotting factor deficiencies and with these breeds, coagulation tests may be used for 'screening purposes' prior to diagnostic or surgical procedures.
"Some breeds of dogs are known to have a higher incidence of clotting factor deficiencies..."
If an animal experiences episodes of bleeding for an unknown reason, evaluation of clotting function should be undertaken. Severe liver disease may lead to problems with blood clotting because the liver is the site of production of coagulation factors.
Are there any screening tests that can be used in the veterinary clinic to evaluate coagulation?
Yes. The most commonly performed test is the evaluation of platelet numbers. Platelets are blood components that are involved in the formation of platelet 'plugs' at the site of blood vessel injury. Platelets not only provide the initial 'patch' at injured sites, they also form a framework that allows coagulation factors to accumulate at the injured site and subsequently form a more permanent fibrin clot.
Platelet numbers can be determined readily, and only a single blood sample is needed. Platelets are usually evaluated as part of a routine complete blood count (CBC) that also evaluates the red blood cell and white blood cell components of blood. A marked decrease in total platelet numbers (termed thrombocytopenia) may result in episodes of bleeding. There are many causes of thrombocytopenia, including immune-mediated disease, bone marrow disorders, and infectious causes, among others.
"...thrombocytopenia may result in episodes of bleeding."
While changes in the red and white blood cells on a CBC do not provide specific information about coagulation, they may alert your veterinarian to investigate further. A reduction in red cell numbers (anemia) may occur as a consequence of coagulation factor or platelet deficiency, while changes in the white blood cell numbers or types may indicate an underlying inflammatory condition that may be responsible for the thrombocytopenia.
Because clotting factors are made in the liver, and because many diseases can result in thrombocytopenia, a serum biochemistry panel that evaluates liver function may be helpful.
The buccal mucosal bleeding time (BMBT) is a test which can be performed in the clinic and which evaluates the ability of platelets to form a platelet plug. A spring-loaded cassette is used to produce a small, precise cut on the inside of the upper lip, and the length of time required for bleeding to stop is measured. This time may be lengthened by a decreased platelet count or by decreased platelet function. The BMBT is usually reserved for patients that have normal platelet numbers but have questionable platelet function.
The ACT (activated clotting time) test can be performed in the veterinary clinic. To perform this test a blood sample is added to a special ACT tube containing a substance (usually diatomaceous earth) that activates the coagulation factors in both the intrinsic and common pathways. A severe decrease (less than 5% of normal activity) in these clotting factors or a severe decrease in platelet numbers will lead to an increase in the length of time it takes the blood to clot after it has been added to the ACT tube. The ACT is not very sensitive to milder decreases in clotting factors.
What specific tests are used to measure the activity of coagulation factors?
"More specific coagulation tests are sent to a veterinary or hematological (blood) reference laboratory..."
These more specific coagulation tests are sent to a veterinary or hematological (blood) reference laboratory and require a single blood sample.
The PTT (activated partial thromboplastin time) measures the functional activity of the coagulation factors involved in the intrinsic and common coagulation pathways. The PTT measures the time that it takes for a fibrin clot to form after calcium and an activator are added to a blood sample.
The results of the PTT will be increased (that is, the time required for a clot to form will increase) when there is at least a 70% decrease in the activity of a single factor in either of the intrinsic or common coagulation pathways. Smaller decreases in multiple factors may also result in an increase in the PTT. Anti-coagulant therapy such as heparin will increase the PTT.
The PT (prothrombin time) measures the functional activity of the coagulation factors involved in the extrinsic and common pathways. The PT does this by measuring the amount of time it takes for a fibrin clot to form after calcium and a tissue-activating factor are added to a blood sample.
The results of the PT will be increased (that is, the time required for a clot to form will increase) when there is at least a 70% decrease in the activity of a single factor in either of the extrinsic or common pathways.
Because of the specific factors involved in the extrinsic coagulation pathways, an increase in PTT is often noted with conditions such as vitamin K deficiency and rodenticide (rat poison) toxicity.
"...increase in PTT is often noted with conditions such as vitamin K deficiency and rodenticide (rat poison) toxicity."
The TT (thrombin time) measures the functional activity of the common coagulation pathway, and more specifically of fibrinogen. The conversion of fibrinogen to a fibrin clot is the end point of both the intrinsic and extrinsic pathways in the body. Both pathways converge to the final common pathway that results in fibrin clot formation.
The TT measures the amount of time it takes for a fibrin clot to form after calcium and thrombin (a coagulation factor) is added to a blood sample.
Since the TT is dependent upon both the amount and the normal functional activity of fibrinogen, any condition that depletes the amount of fibrinogen or makes it unable to function properly will cause an increase in TT. Examples of such conditions include hereditary deficiencies in the amount or functional activity of fibrinogen and increased use/consumption of fibrinogen in the body because of excessive blood coagulation (clotting). Such excessive coagulation can occur because of shock, severe infections, and tumors, to name a few examples.
Von Willebrand's factor testing at a veterinary reference laboratory involves the evaluation of a blood sample for the presence of von Willebrand's factor, a protein that assists in the adherence or 'stickiness' of platelets to sites of blood vessel injury. Von Willebrand's factor also helps to stabilize one of the coagulation factors (Factor VIII) in circulation so that this factor is not lost prematurely. Von Willebrand's disease is one of the most common hereditary bleeding disorders of dogs, affecting over 50 breeds and resulting in variable deficiencies of this factor.
"Von Willebrand's disease is one of the most common hereditary bleeding disorders of dogs..."
In addition to von Willebrand's factor testing, can the individual coagulation factors be evaluated?
Yes. Specific coagulation factor analysis is not commonly done, but it is available in specialized veterinary or hematological laboratories. Specific coagulation factor analysis is typically used to evaluate the most common inherited deficiencies, namely von Willebrand's factor, factor VIII deficiency (hemophilia A) and factor IX deficiency (hemophilia B).
Which of the tests described above is used first?
"Coagulation testing is not usually done in isolation."
Coagulation testing is not usually done in isolation. Typically, a CBC and a biochemical profile are first evaluated. Then a coagulation panel consisting of a combination of PT, PTT and possibly von Willebrand's factor testing is performed. The combination of results from these coagulation tests will indicate what part of the coagulation pathway is affected. Once these results have been assessed, the necessity for additional testing can be determined.
Are there tests that evaluate the ability of the body to inhibit or counteract coagulation?
Yes. There is a delicate balance between coagulation of blood and the dissolution or breakdown of blood clots. If such a system were not in place, instead of clots forming only at the site of injury, widespread clotting could occur to the detriment of the animal.
Antithrombin III is one of the most commonly measured inhibitors of the coagulation system. While measurement of this protein requires only a single blood sample, antithrombin III determination is performed only at specialized laboratories. Antithrombin III measurement is usually undertaken if a pet is at risk of increased coagulability (increased chance of clot formation). Increased coagulation of blood may be a complicating factor of many diseases including cancer, severe inflammation, Cushing's disease, and kidney disease.
After an injured blood vessel is repaired, does the clot remain in place?
No. Following tissue repair, the fibrin clot that has formed via the coagulation pathway to stop the bleeding is no longer required, and is cleared away. This is termed fibrinolysis and the breakdown products from these clots are called fibrin degradation products (FDPs). These FDPs can be measured at reference laboratories from a blood sample that has been collected into a special tube. Fibrin degradation products only increase with conditions that result in excessive coagulation; minor day-to-day injuries will not result in increased amounts of FDPs in the blood stream. Therefore, FDPs are evaluated only with specific disease conditions when the likelihood of excessive blood coagulation is high.