Supratherapeutic International Normalized Ratio with Concomitant Warfarin and Cupric Chloride (Copper) Therapy

We report a case of supratherapeutic international normalized ratio values in a hospitalized patient started on warfarin to prevent recurrent deep vein thrombosis. Her medical history showed a failed course of apixaban and so warfarin therapy was initiated during this admission but quickly stopped after sudden, significant increases in international normalized ratio for several days after termination of therapy. A review of any drug-drug interaction did not expose any reported interactions between warfarin and her current medications at that time. However, the concurrent administration of cupric chloride did raise the possibility of an undocumented interaction between these two entities. The pharmacokinetic/pharmacodynamic properties of both drugs provided a study into the consequences of a combination therapy, beyond the anticipated outcome, which is normally based on previously observed results.