SummaryInitial clinical trials of FK did not incorporate available FK levels, and difficulties were quickly experienced particularly with neurotoxicity and nephrotoxicity. The introduction of routine assay allowed broad parameters to be identified, which assisted in evaluating effective therapeutic parameters. Levels 20 ng/ml were frequently associated with toxicity and the initial therapeutic range between 10ndash;25 ng/ml was probably excessive. Reliable effective assay 5 ng/ml using the Abbott IMxis not available, and many patients will have excellent hepatic or renal function with what are currently undetectable levels of FK. However, IncStar have an ELISA assay with a sensitivity of 0.5 mg/ml. Clinical practice does not, at this time, dictate elevation of FK, although careful monitoring continues. Education of oral administration from 0.15 mg/kg to 0.1 mg/kg in combination therapy with steroids and 0.05 mg/kg with azathioprine and steroids has led to revision of therapeutic parameters, e.g., 5ndash;15 ng/ml is now widely used. Therapeutic drug monitoring is important to avoid unnecessary toxicity, but the lower limit has not been fully defined. Clearly, many patients with 5 ng/ml have excellent hepatic function.
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