Monitoring Warfarin Therapy with the International Normalized Ratio

OBJECTIVE: To report a case of suboptimal warfarin monitoring. CASE SUMMARY: A patient with a history of rheumatic heart disease and a mechanical mitral valve was admitted to the local hospital complaining of left-sided weakness. At the time, she was receiving warfarin 5 mg/d. Upon admission her prothrombin time (PT) was 15 s. An initial computed tomography (CT) scan of the head was negative. On the basis of the initial findings, it was unclear whether the symptoms were caused by a cerebrovascular accident (CVA). The patient was transferred to the University Medical Center for a more thorough evaluation. The diagnosis of CVA was confirmed by a repeat CT scan seven days after the event. On the basis of the information obtained from the local hospital, it was determined that the initial PT of 15 s converted to an International Normalized Ratio (INR) of 1.5, which is below the recommended range for patients with mechanical heart valves. Prior to discharge, the warfarin dosage was increased to obtain an INR in the recommended range of 2.5-3.5. DISCUSSION: This case illustrates the problems that exist with the current system of PT reporting and the potential advantages of using the INR. Variations in the sensitivity of the thromboplastin reagents used to perform the PT may result in misinterpretation of the level of anticoagulation and errors in warfarin dosage adjustments. The potential for suboptimal anticoagulation is greatly increased in patients, such as the one reported here, who are having PTs performed by multiple laboratories. CONCLUSIONS: To maximize efficacy and minimized the risk of bleeding complications, warfarin therapy must be individualized and closely monitored. Standardization of PT monitoring through the use of the INR would significantly reduce the potential for suboptimal anticoagulation associated with the traditional system of reporting.