Evaluation of factors associated with stability of anticoagulation therapy

Eighty-two patients receiving long-term warfarin therapy provided 199.34 patient-years of data that were evaluated to determine if certain variables could identify those who might be monitored safely at less frequent intervals. Most patient demographics, social habits, medical histories, indications for anticoagulation, and concurrent therapy were not useful in discriminating between patients with stable (n = 67) and unstable (n = 15) anticoagulation. A few trends were noted, but they failed to achieve statistical significance. Patients who never achieved stability (the unstable group) tended to be younger than those who did (p 0.13). Among the stable patients, those with a diagnosis of deep vein thrombosis or pulmonary embolus, or with elevated alanine aminotransferase values, were significantly more likely to require a dosage change. The probability of requiring a dosage change at monthly visits did not correlate with the time required to become stable, but it did correlate inversely with the duration of stable anticoagulation. This probability declined from almost 16% when monitoring frequency was first extended to monthly intervals to less than 8% at 2 months of stable anticoagulation, and tended to decline further with longer periods of stable therapy. Of the 67 patients who became stable, 23 did not require a dosage change during an average of 526 days of follow-up. Among the 44 stable patients who required a dosage change, the mean time to the change was 250 days after becoming stable. Few complications occurred, almost all of them early in therapy.(ABSTRACT TRUNCATED AT 250 WORDS)