Screening Tests to Detect Coagulopathies - A "Must" for Presurgical Screening
A presurgical screening consists of CBC, urinalysis, chemistry panel, and blood coagulation. Buccal bleeding time does not detect coagulation disorders associated with lack of coagulation factors. A complete screening for coagulation should include platelet evaluation and activated coagulation time (ACT), prothrombin time (PT) and activated partial thromboplastin time (APTT), and antithrombin III (AT-III) should be added as necessary.
Screening for coagulation disorders is indicated when physical findings suggest such disorders, and when surgery or other invasive procedures are planned. Physical findings suggestive of coagulopathies include bleeding, ecchymosis, petechiae, and icterus.
2. Blood Vessel Evaluation
There is not a specific test to evaluate blood vessels and endothelial cells, but routine physical examination can detect possible vasculitis lesions. Vasculitis can occur along the limb or other vessels, in the oral cavity, and in the nail bed.
3. Platelet Evaluation
The clinical signs suggestive of abnormality of platelet number and function are ecchymosis and petechiae. These clinical signs are associated with thrombocytopenia usually less than 20,000/µL. If there is no decrease in number, functional loss associated with drug administration or von Willebrand disease (vWD) can be suspected. However, vWD can be initially asymptomatic, and should always be ruled out before doing neutering surgery in young animals.
The evaluations for platelets include CBC and ACT. It should be remembered that an impedance method (Coulter type) blood cell counter can underestimate cat platelets and Cavalier platelets where large platelets can not be counted electrically. The accurate way of evaluating platelets is to examine the blood smear. If there is a decrease, follow-up examinations to detect the cause are indicated, and such evaluations include ruling out the DIC and bone marrow examination.
Activated coagulation time (ACT) is a simple screening for platelet number and function. Becton Dickinson used to produce Vacutainer test tubes named ACT tube, but it is discontinued. It can be prepared in the hospital by placing 6–10 mg siliceous earth (SiO2) in a disposable glass test tube. One to two milliliters of whole blood is placed in this tube, incubated at 37°C, and the tube is shaken every 15–30 seconds to see if it is coagulated. The normal values for dogs and cats are, less than 120 sec and less than 65 sec, respectively. The ACT is a screening for platelets and at the same time, it is a less sensitive APTT. Therefore, it is a nice screening test for presurgical cases.
Buccal bleeding time is a less sensitive evaluation of platelet number/function, and is not indicated if thrombocytopenia is already detected in CBC. In animals, it is not possible to regulate the local blood pressure constant, and it is very difficult to control the width and depth of the wound constant unless a special cutting device is used. It does not detect any abnormality in the coagulation factors, thus this test does not provide any further advantage over ACT. Bleeding time by deep cut of the nail should not be done because it is not appropriate in companion animal medicine.
4. Coagulation Factor Evaluation
The clinical signs suggestive of coagulation factor deficiency, either congenital or acquired, depends on the size of animals affected or the type of the deficient factor. The signs can be severe Intra-articular bleeding in case of large dogs, ecchymosis and petechiae in case of smaller animals or in case of a factor deficiency in which the factor is not very important in the coagulation system. Multiple factor deficiency such as DIC can cause bleeding in the body cavities or bleeding from the blood collection site.
The coagulation pathway using coagulation factors can be divided into intrinsic, extrinsic and common pathways. The intrinsic pathway can be initiated by the interaction of platelet and endothelial cells and a release of PF3, whereas the extrinsic pathway can be activated by the necrosis of any cells releasing tissue factor (factor III). Both pathways continue to the common pathway to complete the formation of the end product, fibrin. To test those pathways, two major tests that detect abnormality in the different part of the pathways are employed.
The one-step prothrombin time (PT) evaluates the extrinsic and common pathways. The factors that can be evaluated include I, II, V, VII and X.
The activated partial thromboplastin time (APTT) evaluates both intrinsic and common pathways, thus it covers all the factors except VII. Since factor III derives from all the cells and it is not the target for testing.
The beauty of these testing is that both tests are simultaneously done. Therefore, if APTT turns normal and PT is abnormal, it is documented that both intrinsic and common pathways are normal, and the abnormality can be localized in the extrinsic pathway, which is factor VII. If PT is normal and APTT is abnormal, the factors of possible deficiency can be XII, XI, IX or VIII. If both PT and APTT are prolonged, any pathway can be suspected, and suspected illnesses in the acquired diseases include DIC, rodenticide toxicity, hepatic disease, and vitamin K deficiency.
In order to perform accurate coagulation tests, venipuncture should be clean-cut and blood is collected immediately without any activation of coagulation system. A plastic syringe and a plastic tube are used along with an anticoagulant 3.8% sodium citrate at a ratio of one part sodium citrate and nine part blood. The blood is immediately centrifuged at 2,500 to 3,000 rpm for 12–15 min and plasma is collected for testing. There are several in-hospital coagulation test instruments validated for veterinary use.
Prolonged PT or APTT is defined as more than 30% increase over the normal value upper limit. If the abnormality is located in the intrinsic pathway, further efforts are made to determine the deficient factor(s). First, normal plasma is added to the test sample at 1:10 and the prolonged test is repeated. If the test turns normal, it is confirmed that it is a deficiency of some component in plasma, possibly some coagulation factor deficiency. Next, normal serum, which is deficient if factors I, V and VIII, is added to the test sample, and the prolonged test is repeated. If the original sample is deficient in those factors, addition of normal serum does not correct the prolonged testing. If corrected, on the other hand, the deficiency can be narrowed down to those three factors. Other testing involves use of special plasma with known deficiency. For example, APTT can be repeated by adding plasma samples deficient in either factor VIII or IX.
5. Other Coagulation Tests
In order to confirm DIC, FDP (fibrinogen/fibrin degradation product) and d-dimer measurement can be employed. However, these are not the tests to facilitate early detection of DIC. Most reliable tests for early detection of pre-DIC stage can be platelet number, and blood smear evaluation to detect fragmented RBC. Also, FDP can be reliable if evaluation is done with platelet number. AT-III activity (normally 120%) is decreased in DIC because of an elevated consumption. Also, in some in-hospital instruments, it is possible to measure fibrinogen level, which is consumed in DIC.
The most frequent congenital coagulation disorder is vWD. There are tests available in some laboratories to measure vWF and factor VIII:c activities. Also, a genetic test for vWD is available commercially (www.vetgen.com).