Unfortunately, the relationship between whole blood tacrolimus trough concentrations and the AUC is highly variable, especially peri‐operatively, making interpretation of the former very challenging 12, 16-18. Even when tacrolimus concentrations are in the therapeutic range, toxicity may occur because of high unbound tacrolimus plasma concentrations 61. The variables influencing the bound and unbound tacrolimus concentrations may considerably change during the early postoperative phase.