Warfarin Re-initiation Gone Awry: A Case of Inadvertent Overdose Mandating Critical INR Management.

Warfarin is a commonly used anticoagulant for the primary or secondary prevention of thrombotic events.1 Tremendous inter-individual variability in warfarin dosing is apparent, necessitating frequent monitoring via the international normalized ratio (INR). The most feared complication of warfarin therapy is major bleeding, and the risk of this problem increases when the INR exceeds 4.0.2 Critical INR values (defined in the Edmonton region of Alberta Health Services as an INR > 5.0; the definition may vary by jurisdiction) are reported urgently to clinicians to enable prompt patient management. For ambulatory patients who are not bleeding, strategies to manage critical INRs include withholding warfarin therapy, with the option of administering oral vitamin K1, but guidelines vary and have changed over time (Table 1).1,3,4 It is common for patients who are taking warfarin to have their therapy interrupted, for example, when they undergo invasive procedures. For patients at higher risk of thrombosis, low-molecular-weight heparin (LMWH) is commonly prescribed during both the washout period for warfarin (before the procedure) and the build-up period for warfarin (after the procedure).5 The duration of postprocedure LMWH depends on the time it takes for the patient’s INR to reach a therapeutic level. Clinicians typically restart warfarin at the patient’s prior maintenance dose,6,7 although in a previous study, we found that it took a median of 20.5 days at the prior maintenance dose to reach a therapeutic INR8; prolonged LMWH therapy is needed during this time. Reported here is a case of warfarin re-initiation, in which the clinic’s usual strategy, based on prior maintenance dosing, resulted in inadvertent warfarin overdose requiring management of a critical INR. For both of these aspects of warfarin management (re-initiation of warfarin and management of critical INR), the literature is reviewed and applied to this case, with emphasis on practical management issues.