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Why Is My Dog's Prothrombin Time Extended ?

Why Is My Cat's Prothrombin Time Extended ?

To see what normal blood and urine values are for your pet, go here

For an explanation of causes of most abnormal blood and urine tests go here

To see how tests are often grouped, go here

For an extensive list of health problems that can delay blood clotting in your pet, go here

Ron Hines DVM PhD

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Your Pet's Prothrombin Time


When your veterinarian suggests running a prothrombin time on your dog or cat ’s blood, it is because the vet has some questions regarding the ability of your pet’s blood to clot normally.

The mechanism by which blood clots is very complex. It consists of two routes or pathways (the intrinsic cascade pathway and extrinsic cascade pathway); with some steps common to both. Two tests, the Prothrombin Time (PT) and the Partial Thromboplastin Time (PTT or aPTT) help your veterinarian decide where in this complicated clot process problem(s) might exist.

Your veterinarian might also run the PT test before surgical procedures that have the potential for extensive bleeding. Other tests that help determine why your pet's blood is not clotting properly are a thrombocyte count, fibrinogen levels, partial thromboplastin time (PTT) , thrombin clot time TCT and , perhaps, a d-dimer test.

Prothrombin is a protein produced by your pet’s liver. When it is activated normally during bleeding, it changes very rapidly into another protein called thrombin. When not enough prothrombin is present, your pet’s PT time will be extended (increased). PT primarily measures the extrinsic blood-clotting pathway. It is used in conjunction with activated partial thromboplastin time (aPTT), which primarily measures the intrinsic (slower) clotting pathway. Your veterinarian will use the staff of the clinical laboratory to coach him/her through the intricate process of locating broken links in the clotting process - it is not something your ordinary clinical veterinarian is likely to understand in detail.

Reasons Your Pet’s Prothrombin Time Could Be Increased (Prolonged) :

A Vitamin K deficiency can increase clotting time. An important cause of vitamin K deficiency is consumption of certain rodent poisons.

Administration of medications to dissolve or prevent blood clots (eg Coumadin, warfarin) such as might be used to dissolve the clots that form as saddle thrombi in cats with cardiomyopathy.

Chronic liver disease can prevents normal prothrombin formation leading to extended prothrombin time.

Genetic tendency toward bleeding occasionally occur in dogs and cats (hemophilia). (Some that have been discovered are deficiencies in coagulation factor ,VII,V,II or I).

A bit less than half of pets that develop disseminated intravascular coagulation (DIC) experience abnormally long PT times.

PT times tend to be longer in newborn pets.

Sulfa-type antibiotics, such as trimethoprim/sulfa has been know increase bleeding and extend PT time.

Prothrombin tim results from one lab do not compare well to results from another. When you are monitoring a pet over time, it is best to always use the same laboratory.

Complementary Tests :

CBC/WBC and blood chemistry values (including observation for elevated AST, ALT, GGT and AP and low albumen that are all suggest liver problems - a common cause of long PT time, or allow thrombocyte count and abnormal RBC shape (poikilocytosis) as indications of (DIC)

.................... DxMe