Therapeutic drug monitoring in patients with chronic renal failure: evaluation of the Abbott TDx drug assay system.

Immunoassay techniques have been widely used for therapeutic drug monitoring, but lack of antibody specificity can lead to measurement of erroneous drug concentrations due to cross-reactivity with other drugs, metabolites, or endogenous substances, particularly in patients with excretory organ compromise such as renal dysfunction. The Abbott TDx system, a popular automated immunoassay method for therapeutic drug monitoring, was used to measure apparent serum concentrations of carbamazepine, digoxin, gentamicin, lidocaine, phenobarbital, phenytoin, quinidine, valproic acid, and vancomycin in patients with renal failure who were not receiving these drugs. Endogenous substances and other concomitantly administered drugs did not lead to spuriously elevated drug levels, and a previous report of cross-reactive digoxin-like substances was not confirmed. Pooled plasma samples from the patients were spiked with digoxin or phenytoin, each at two concentrations, and the samples were assayed for the drug concentration using the TDx system. No falsely elevated values were found. This work suggests that the TDx system may be better suited for the measurement of these drugs in patients with renal failure than some other immunoassay methods.