Aims. To determine the prevalence of international normalized ratio (INR) findings outside the normal range in hospital emergency department patients on vitamin K antagonists (VKAs). To identify factors associated with abnormal anticoagulant levels in these patients. Methods. Observational, cross-sectional, multicentric study in 4 hospital emergency departments. We included a convenience sample of patients on VKA treatment for whom INR levels were on record and who had sought emergency care for complications unrelated to anticoagulant treatment. Results. We included 376 patients with a mean (SD) age of 76.8 (10.1) years; 50.3% were women and 86.7% had atrial fibrillation. We found that 60.4% (95% CI, 55.3%-65.2%) had INRs outside the reference range. Multivariate analysis showed that changes in the patients' other long-term medications were independently associated with nontherapeutic INR results (odds ratio, 1.6; 95% CI, 1.02-2.79; P=.035). Conclusions. Over 60% of patients on VKA treatment who come to hospital emergency departments with complaints unrelated to anticoagulant therapy have INR values outside the normal range. Changes in a patient's usual medications are significantly associated with nontherapeutic INR findings.