Oral anticoagulant therapy and international normalized ratios in swine

While monitoring coagulation testing in Yucatan miniature swine being given oral anticoagulants, we noticed instances of high international normalized ratios (INR) without clinical complications in our animal model. All pigs (n = 17) weighed approximately 35.2 kg and were dosed daily with 2 to 3 mg of coumadin. Plasma samples were obtained and assayed for prothrombin time (PT), calculated INR, and activated partial thromboplastin time (APTT) at baseline, and after 7 and 14 days of coumadin therapy. Results of initial testing indicated high INR values after anticoagulation and short APTT values at baseline, which led us to consider the activity of vitamin K-dependent coagulation factors in the pig. This information was not available in literature concerning this strain of swine, and was surprising given that pigs are frequently used cardiac research models. Using the same plasma samples, we repeated the PT, INR, and APTT determinations using different reagents and a different analyzer. We also determined activities of coagulation factors II, VII, IX, and X. Large PT and INR differences were seen between the two instrument/reagent combinations, possibly due to the differences in the thromboplastins used and differences in the photo-optic versus manual clot-detection method of the instruments. Vitamin K-dependent factors in all pigs responded to coumadin by decreasing to < 30.0% activity, except for factor IX. The high INR values were not as pronounced when the second instrument/reagent combination was used, and the results seemed more in line with the animals' clinical condition. With this instrument/reagent combination, the pig can be considered a good model for research requiring oral anticoagulant medication.