Purpose. The effects of prednisone on the International Normalized Ratio (INR) values of a patient were examined.
Summary. A 66-year-old white man with a history of multiple myeloma was treated in an ambulatory care anticoagulation clinic for deep vein thrombosis. His INR values were normal during therapy with warfarin 14 mg weekly and thalidomide 300 mg daily. His INR values began to increase after three months of starting prednisone 10 mg daily. His weekly dose of warfarin was changed over the next two years, and his dietary intake of vitamin K was increased. For every INR value that was below the therapeutic goal, the patient was not taking prednisone; every time the INR value was above the therapeutic goal, he was taking prednisone. In November 2004, the prednisone and thalidomide were stopped and only the warfarin was continued. After a few dosage increases, ending with a weekly warfarin dose of 21 mg, the patient’s INR values remained in the therapeutic range. Multiple variables must be examined when assessing INR values, as many things interact with warfarin. For example, tobacco use, alcohol consumption, and changes in vitamin K intake can affect the INR. Since this patient did not use tobacco or consume alcohol and had a fairly consistent dietary intake of vitamin K, these variables were ruled out as influencing the INR. In this case, the changes in his INR values corresponded to the addition or deletion of prednisone.
Conclusion. A patient’s INR values increased after the addition of prednisone to his warfarin regimen.
Index terms: Anticoagulants; Dosage; Drug interactions; Drugs, adverse reactions; International Normalized Ratio; Nutrition; Prednisone; Steroids, cortico-; Thalidomide; Warfarin
