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A COMPARISON OF THE CLINICAL UTILITY OF THE RADIOIMMUNOASSAY, HIGH‐PERFORMANCE LIQUID CHROMATOGRAPHY, AND TDx CYCLOSPORINE ASSAYS IN OUTPATIENT RENAL TRANSPLANT RECIPIENTS

Rapid and precise cyclosporine measurements are necessary to maximize immunosuppression and minimize toxicity. The new cyclosporine and metabolites TDx fluorescent polarization immunoassay was studied to determine its precision, sensitivity, and stability. Further, it was compared with the existing radioimmunoassay and high-performance liquid chromatography assays for clinical utility. The TDx procedure utilizes 50 microliter of serum or blood. The range of the assay is 0-1000 ng/ml for serum and 0-2000 ng/ml for whole blood. Precision studies of three control levels provided coefficients of variation of 2.1-5.8% for both assays. The sensitivities of the assays were less than 25 ng/ml for serum and less than 50 ng/ml for whole blood. In order to compare the TDx with the currently used RIA and HPLC methods, 362 simultaneous whole blood and serum samples were obtained from 122 renal transplant recipients over a 4-month period. The serum and whole blood TDx assays excluded fewer patients than either the RIA or HPLC assays with regard to sensitivity. The mean serum cyclosporine concentration for samples above the sensitivity was as follows: TDx 120 +/- 5 ng/ml, RIA 116 +/- 4 ng/ml, and HPLC 100 +/- 5 ng/ml. The mean whole-blood cyclosporine concentration for samples above the sensitivity was as follows: TDx 585 +/- 19 ng/ml, RIA 543 +/- 14 ng/ml, and HPLC 178 +/- 5 ng/ml. Serum and blood TDx levels correlated well with RIA levels, with regression coefficients of r = 0.813 and r = 0.897, respectively. Serum and blood TDx levels did not correlate strongly with HPLC levels, with regression coefficients of 0.332 and 0.781, respectively. Seventeen patients were diagnosed as having acute cyclosporine nephrotoxicity. The serum and whole-blood cyclosporine concentrations in these patients were at the upper end of the therapeutic range for all analytical methods. Five patients had acute cellular rejection; serum and whole-blood cyclosporine levels in these patients did not differ significantly from the stable patients when measured by each of the analytical methods. In conclusion, the TDx cyclosporine and metabolites assay provides reliable blood and serum concentrations that correlate well with RIA measurements in renal transplant recipients. This assay offers rapid sample turn-around times making possible same-day results for all patients without putting great demands upon the laboratory.