Treatment with coumarin derivatives is highly
individualised due to high intra- and inter-individual
variation in dose response and risks of severe bleeding
or thromboembolic complications. Treatment focuses on
reaching and maintaining a stable target international
normalised ratio (INR). However, unexpected INRs that are not explained by noncompliance or vitamin K intake may occur. Here we describe seven cases of unexpected INRs, and provide clues that clarify the underlying mechanism.