REEMERGENCE OF THE INTERNATIONAL NORMALIZED RATIO FOR THE STANDARDIZATION OF PROTHROMBIN TIME

A survey of physicians demonstrated that half had knowledge of the International Normalized Ratio (INR) but none used the value for monitoring their patients because it was not available from the Coagulation Laboratory. The Laboratory then provided the INR value at a physician's request. A six month review of prothrombin time (PT) results showed that only the physicians from the Cardiology Clinic and the Hematology Clinic employed the INR for monitoring their patients. General Medical and Surgical, Vascular, and Orthopedic Clinics continued to use the PT in seconds. This dichotomy allowed the unique opportunity to compare variability of PT in patients followed by INR and those followed by PT in seconds. Inpatients on daily monitoring were used as the standard for close control. During a six month period, laboratory reports from all patients having regular PTs and/or INRs recorded were analyzed for mean level of PT maintained, variability between individual PTs in any given patient, and instances when the PT changed greater-than-or-equal-to 5 seconds (sec) or increased to greater-than-or-equal-to 20 sec. Physicians intended to keep the PTs between 16 and 19 sec (INR 2.0 to 3.0). Results showed statistically significantly lower values of PT, less variation in values of PT and a smaller fraction of patients with changes in PT of greater-than-or-equal-to 5 sec in the group followed by INR. This group was comparable to the inpatient group but significantly different from the outpatient group followed by PT in sec. These results suggest that either the INR allows more stable application of coumadin doses, or physicians using the INR educate their patients in better compliance in diet, medications, and other factors affecting coumadin or vitamin K metabolism. Use of the INR does appear to decrease the variability of the PT in patients so followed. The clinical implications are as yet to be determined.