Effect of three loading doses of warfarin on the international normalized ratio for dogs

OBJECTIVE: To evaluate effect of 3 loading doses of warfarin sodium on the international normalized ratio (INR) for dogs. ANIMALS: 18 dogs weighing between 25 and 30 kg. PROCEDURE: Dogs were randomly allocated into 3 groups and received 2, 4, or 6 mg of warfarin administered orally once a day for 2 days after surgery for bilateral iliac artery grafting. Activated partial thromboplastin (APTT) and prothrombin times (PT) were measured before and after treatment. Prothrombin time also was reported as an international normalized ratio. RESULTS: The APTT were not significantly different among groups before or after treatment. The INR and PT were significantly increased in all groups after treatment. The INR and PT of the 6-mg group were significantly greater than those of the 2-mg and 4-mg groups. None of the dogs had clinical evidence of bleeding. CONCLUSIONS AND CLINICAL RELEVANCE: A warfarin loading dose of 6 mg/d can be safely administered for 2 days to dogs weighing between 25 and 30 kg. Anticoagulation can be achieved safely in dogs in 2 days by use of warfarin. The effects of warfarin can be monitored with the INR.