Time to Peak International Normalized Ratio Rise in Acute and Acute-on-Chronic Warfarin Overdoses

Guidelines exist on the management of supratherapeutic/subtherapeutic international normalized ratio (INR) values for patients on warfarin. However, there is a paucity of the literature relating to an acute overdose of warfarin. This is a retrospective cohort study for all acute and acute-on-chronic (AOC) warfarin overdoses reported to the Maryland Poison Center in patients ≥12 years between January 1st, 2000, until October 31st, 2019, managed in a health care facility. The primary outcome was to determine the time after presentation to peak INR. Secondary outcomes included risk factors associated with INR >10 and describing patient characteristics. A total of 163 overdoses were included, 68 acute and 95 AOC. In patients who did not receive reversal therapies, INR peaked at a median value of 3.8 (interquartile range 2.6-5.5) between 24 and 36 hours. The median time to phytonadione was 22.0 hours. Most patients received phytonadione (62.0%), with fewer receiving blood products (16.6%). The median warfarin dose ingested was 75 mg. The AOC group had a greater mean age (56 vs. 43 years), median INR value (2.4 vs. 1.4), and men (62.1% vs. 41.2%). Factors associated with an INR > 10 included initial INR and reported quantity ingested. Peak INR was greater in the AOC than the acute overdose group (6.1 vs. 3.4), although the bleeding rate was similar. Peak INR values after warfarin overdose occur between 24 and 36 hours after presentation. Initial INRs and reported quantity ingested may be useful to predict those needing treatment.