Test Details: Tacrolimus is an immunosuppressive drug that is usually administered before transplant. Tacrolimus binds to a family of proteins termed FK506 (tacrolimus). The tacrolimus test is ordered before the beginning of therapy. It is also used to measure the amount of the drug in the blood to determine whether the concentration has reached a therapeutic level and is below the toxic level. Monitoring of tacrolimus concentrations is required due to its narrow therapeutic index, in order to maintain a balance between under-immunosuppression and subsequent rejection risk with over-immunosuppression and risk of toxicities.