Variation in Leukocyte Subset Concentrations Affects Calcineurin Activity Measurement: Implications for Pharmacodynamic Monitoring Strategies

peutic drug monitoring of the calcineurin (CN) inhib-itor cyclosporin A (CsA) is mandatory because of the drug’s narrow therapeutic index. Pharmacodynamic monitoring of CN inhibition therapy could provide a tool to define and maintain the therapeutic efficacy of CsA therapy. We investigated the effect of variation in cell counts of leukocyte subsets on leukocyte CN activ-ity measurement in renal transplant recipients. METHODS: We measured leukocyte CN activity, whole blood CsA concentrations, and leukocyte subset cell counts in 25 renal transplant recipients. Blood was col-lected before graft implantation andCsA therapy, 1 day before transplantation when CsA therapy was already started, and 5 days after transplantation. Monocyte