Prothrombin Time

Indications for performing this test: This test is used to evaluate the common and extrinsic clotting pathway, and is used in cases of suspected hemostatic disorders. The intrinsic pathway is evaluated with the APTT test.

Coagulopathies and Hepatic Disease: The clotting factors are acute phase reactant proteins that are manufactured in the liver. In cases of hepatic disease, it is common to see coagulopathies as a result. Thus, it is indicated to perform an assessment of the clotting pathways prior to invasive procedures in an animal with hepatic pathology.

The Test: This test measures the time required for a fibrin clot to form in citrated plasma after the addition of tissue thromboplastin and recalcification. Nine parts of the patient's whole blood is mixed with one part 3.8% trisodium citrate anticoagulant (in the commercial citrate tubes). The blood is separated into cells and plasma by centrifuging it within 30 min of collection. The patient's plasma is then mixed with tissue thromboplastin. Tissue thromboplastin is usually of an animal source, either rabbit brain, lung or a brain-lung combination. Prolonged clotting times with the OSPT indicate a deficiency in the extrinsic clotting factors or the common clotting factors including, VII, X, V, II and I. A normal OSPT time for a dog is 6-8 seconds, in a cat 4-8 seconds, and 8-10 seconds in a horse. There is a cageside test available for use in the dog.

Sodium Citrate tubes

Interpretation of OSPT and APTT results:

Normal OSPT and prolonged APTT = defect in the intrinsic pathway

Normal APTT and prolonged OSPT = defect in the extrinsic pathway

Prolonged APTT and OSPT = multiple defects or defect in the common pathway