This assay is useful in monitoring whole blood Tarcolimus concentration during therapy particularly in individuals co-administered CYP3A4 substrates, inhibitors or inducers. It helps to evaluate patient compliance and adjust dose while minimizing toxicity.Since 90% of tacrolimus is in the cellular components of blood, especially erythrocytes, whole blood is the preferred specimen for analysis of trough concentrations.Optimal trough blood concentrations are generally between 5.0 and 15.0 ng/mL. Higher levels are often sought immediately after transplant, but as organ function stabilizes at about 4 weeks from transplant, doses are generally reduced in stable patients for most solid organ transplants. Trough concentrations should be maintained below 20 ng/mL.