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The frequency and causes of aberrant International Normalized Ratios (INRs) in warfarin recipients and the percentage explainable by warfarin nonadherence were studied.The medical records of patients whose warfarin therapy was monitored by a telephone-based anticoagulation service in a Midwestern urban hospital between March 2000 and March 2001 were reviewed for causes of out-of-range INRs, the percentage of out-of-range INRs attributable to warfarin nonadherence, and demographic and clinical variables predictive of nonadherence.Data from 347 patients were studied. The cohort yielded 4305 INRs, of which 1002 (23%) were out of range (lower than 1.8 or higher than 3.4). Thirty-six percent of the out-of-range INRs were due to warfarin or dietary nonadherence, 9% were due to medical systems problems (including drug interactions), 18% were due to a change in clinical status, and 38% were idiopathic. Age younger than 65 years, age greater than 80 years, and living closer to the laboratory were predictive of warfarin nonadherence.Warfarin nonadherence was the most common cause of explainable aberrant INRs in patients taking warfarin. Age younger than 65 years,age greater than 80 years, and living close to the laboratory were predictive of warfarin nonadherence.