Analysis of risk factors for over-anticoagulation in patients receiving long-term warfarin

Br J Haematol. 1998 Nov;103(2):422-4. doi: 10.1046/j.1365-2141.1998.00988.x.


A cohort of patients with an INR >7.0 were identified prospectively and compared with a group of patients with stable anticoagulant control. During the study 15,100 INR measurements were recorded and 31 (0.2%) were >7.0. Odds ratios of patient characteristics were calculated as an estimate of relative risk for the development of a high INR. The highest risk factor was a target INR of 3.5 (OR 7.3, 95% CI 2.6-20.2). The second highest risk factor was antibiotic therapy in the 4 weeks preceding the high INR (OR 6.2, 95% CI 1.4-27.7). Bleeding was reported more frequently in the high INR group (OR 5.4, 95% CI 2.1-13.9). Five major bleeds occurred in this group compared to none in the stable group. This analysis identifies risk factors for over-anticoagulation and hence when to intensify monitoring and when to consider pre-emptive warfarin dose reductions.

MeSH terms

  • Adult
  • Aged
  • Aged, 80 and over
  • Anti-Bacterial Agents / pharmacology
  • Anticoagulants / administration & dosage
  • Anticoagulants / adverse effects*
  • Blood Coagulation / drug effects
  • Drug Administration Schedule
  • Drug Interactions
  • Hemorrhage / chemically induced*
  • Humans
  • International Normalized Ratio
  • Middle Aged
  • Prospective Studies
  • Risk Factors
  • Warfarin / administration & dosage
  • Warfarin / adverse effects*


  • Anti-Bacterial Agents
  • Anticoagulants
  • Warfarin