Effect of oral diuretics on chronic warfarin therapy: a retrospective study

Background: Limited clinical documentation is suggestive of a drug interaction between warfarin and diuretics.Objective: To evaluate the effect on international normalized ratio (INR) when a daily oral diuretic is started or increased in patients on chronic stable warfarin therapy.Methods: The medical records of all active patients of two hospital-based anticoagulation clinics were retrospectively reviewed to identify patients who were started on or received a dose increase of a daily oral diuretic while on stable warfarin therapy. The primary endpoint was the mean difference between an INR recorded within 30 days prior to the diuretic initiation (pre-INR) and an INR recorded within 30 days after diuretic initiation (post-INR).Results: A total of 1254 patient charts were screened and a total of 123 patients met the study criteria. The mean difference in pre-INR and post-INR was 0.09 (95% CI -0.03 to 0.21, p = 0.12). Post-INR values were outside of the patient's therapeutic range in 39 patients (32%), but ...