Time Trend in OutpatientWarfarin Therapy Based on International Normalization Ratio

Introduction- The anticoagulation activity of warfarin is monitored by the prothrombin time (PT) using the international normalization ratio (INR). Factors such as genetic polymorphism and ethnic differences can cause an unpredictable dose response. In our study, the primary end point was time in days to therapeutic INR in the Iranian race. The secondary end point was time in days to stable dose for our patients, and the third end point was determination of stable dose related to sex and age distribution of our patients. Method- The anticoagulation clinic records of patients taking warfarin during an index period were retrospectively reviewed. INR measurements were performed on citrated venous blood samples. Under-anticoagulation was defined as any out of range INR<1.8 and overanticoagulation as INR >3.4. Result- Stable warfarin dose was achieved in only 5% of the patients by day 14, 55% by day 21, 85% by day 28, and >95% by day 35. The mean stable dose showed an inverse relation with the day 5 INR. However, about 12% of the patients required a final stable dose of < 2.5 mg. No patients suffered any hemorrhagic or thrombosis episodes during the first month of warfarin therapy. After the first month, hemorrhagic complications such as gum bleeding, hematuria, and bloody stool were seen in about 5.5%; however, hospitalization due to hemorrhagic cardiovascular accident was less than 0.7% and thrombosis events were less than 2%. We conclude that warfarin dose during the second and third weeks was highly predictive of the patients’ "stable dose", which is different from the time to reach the therapeutic INR level (Iranian Heart Journal 2008; 9 (3):37- 41).