Unbalanced protein S deficiency due to warfarin treatment as a possible cause for thrombosis.

The prothrombin time/International Normalized Ratio is an accurate parameter for following most warfarin-treated patients. However, it reflects mostly factor VII levels, and significant disproportionate alterations in other vitamin K-dependent factors may not be reflected properly. We report a rare case of a woman who developed arterial thrombosis whilst receiving therapeutic doses of warfarin, and was found to have a significant and unbalanced protein S deficiency.