Immunosuppressants are now an essential treatment in the prevention of transplant rejection. Tacrolimus a calcineurin inhibitor, works by blocking the production of interleukin 2 by T cells and is currently the cornerstone of immunosuppressive treatment, but its side effects are many and varied (hypertension, diabetes , nephrotoxicity,...). The nephrotoxicity of tacrolimus is a particularly serious problem. Therapeutic and pharmacological monitoring of tacrolimus is a key element in the therapeutic management of renal transplant patients, allowing individual adaptation of prescribing to maintain drug exposure within a predefined concentration range (therapeutic targets) and thus improve efficacy while limiting toxicity. The determination of residual concentrations, is still widely used as a guide for individualizing tacrolimus doses