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 > or = 5 seconds (sec) or increased to > or = 30 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 > or = 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.(ABSTRACT TRUNCATED AT 250 WORDS)