Impact of prophylactic oseltamivir on INR in patients on stable warfarin therapy.

Influenza prophylaxis with oseltamivir is recommended for exposed high-risk patients. Patients with many comorbidities have an increased likelihood of co-administration of oseltamivir and warfarin. Evidence of a drug interaction is conflicting in the literature and is limited to a 5-day treatment course. This study evaluates the impact of prophylactic oseltamivir on international normalized ratio (INR) in patients taking warfarin. This retrospective cohort study conducted within the Veterans Health Administration included patients on warfarin who received oseltamivir for influenza prophylaxis. The primary endpoint was change in INR from baseline to day 10 of oseltamivir treatment. Secondary endpoints included change in INR based on renal function and duration of oseltamivir prophylaxis, trend in INR, and frequency of bleeding and thrombosis events. A total of 1041 patients were included and received oseltamivir for a mean of 12.9 days. The mean post-oseltamivir INR was significantly increased compared to the pre-oseltamivir INR (2.39 to 2.52; p < 0.001). Patients with a creatinine clearance of 31-60 mL/min had a significant increase in INR (2.40 to 2.59; p < 0.01). There was an increase in INR when oseltamivir was used for 7 or 8-10 days. Of included patients, 5.1% and 1.8% had a recorded thrombosis or bleeding event, respectively. There was a significant increase in INR in patients on chronic warfarin therapy and concomitant prophylactic oseltamivir, but this change may only be clinically significant for certain patient populations. The most impact on INR was within 7-10 days of oseltamivir initiation and in patients with impaired renal function.

Influence of Successful Chronic Hepatitis C Virus Treatment with Ledipasvir/Sofosbuvir on Warfarin Dosing Requirements in Four Veterans.

STUDY OBJECTIVE: To describe international normalized ratio (INR) trends and warfarin dosage adjustments required for four veterans who were receiving warfarin therapy and started treatment for hepatitis C virus (HCV) with ledipasvir/sofosbuvir with or without ribavirin. DESIGN: Case series. SETTING: Pharmacist-led anticoagulation clinic in a Veterans Affairs Health Care System. PATIENTS: Four patients aged 59-66 years who were receiving warfarin and had stable, therapeutic INRs and started ledipasvir/sofosbuvir therapy with or without ribavirin for HCV infection. MEASUREMENTS AND MAIN RESULTS: All four patients developed subtherapeutic INRs after the addition of ledipasvir/sofosbuvir with or without ribavirin. An increase in weekly warfarin dose ranging from 14-67% was required, with changes in warfarin doses starting 2-3 weeks after ledipasvir/sofosbuvir initiation. Two patients required dose reductions after HCV treatment completion, whereas the other two did not. Use of the Drug Interaction Probability Scale indicated that the interaction between warfarin and ledipasvir/sofosbuvir was doubtful (score of 1 [two patients]) or possible (score of 4 [two patients]). The mechanism of this interaction is unknown but may be related to improvements in hepatic function during HCV treatment. CONCLUSION: To our knowledge, this is the first case series describing a possible drug interaction between warfarin and ledipasvir/sofosbuvir (with or without ribavirin). Close monitoring is warranted when ledipasvir/sofosbuvir is initiated in patients receiving anticoagulation therapy with warfarin, especially those with evidence of cirrhosis prior to treatment. This is particularly important in the first month after starting treatment and the first month after completion. Failure to monitor and achieve therapeutic INR after HCV therapy completion may have the potential to result in adverse outcomes.