Comparison of cyclosporine blood levels measured by radioimmunoassay and TDx assay using monoclonal antibodies

Whole blood steady-state trough concentrations of cyclosporine were measured by radioimmunoassay (RIA) and TDx assays employing monoclonal antibodies in 82 samples from 39 renal transplant patients. Intra- and interassay coefficients of variation for the new TDx technique averaged 3.2 and 5.4%, respectively. In the concentration range of 40 to 717 ng/ml, there was a linear correlation (r = 0.884, p less than 0.001) between both assays [slope of 0.863; y intercept (RIA values) = 15 ng/ml], indicating that the methods can be used interchangeably. Since the automated TDx assay is faster and causes less waste problems, this reliable technique offers some definite advantages for routine clinical use.