Tacrolimus, widely used to prevent acute rejection following solid-organ transplantation, has become the cornerstone of immunosuppressive therapy after kidney transplantation. More than 70% of all renal transplant recipients receive this remarkably effective agent. 1 But tacrolimus is also highly toxic, and there is great between-patient variability in its pharmacokinetics. This, combined with a low therapeutic index, mandates routine therapeutic drug monitoring in clinical practice. 2 Typically, predose concentrations are monitored and the dose is adjusted to aim for target values that depend on immunological risk, comedication, and time since transplantation.