Demonstrating instrument-reagent flexibility: a carbamazepine enzyme immunoassay reagent system

Versatility of immunoassay reagents is beneficial to laboratories seeking cost-effective combinations of tests and automated instrumentation. In such cases, both immunoassay analytical performance and instrument independence must be assessed. Considering this, we determined the compatibility of a new carbamazepine EMIT 2000 reagent system with two fully automated but different kinetic rate analyzers (Hitachi 704 and Cobas MIRA), comparing results to a reagent-dedicated fluorescent polarization automated device (TDx) as reference. In order to more stringently assess reagent antibody specificity, we tested recovery of purified carbamazepine spiked into sera pooled from different hospital groups (normal, renal failure, hepatic failure, term pregnancy, cord blood). Cross-reactivity was additionally tested using patient sera containing various amounts of tricyclic antidepressants, compounds structurally but not functionally related to carbamazepine. Despite distinct operational differences between analyzers, precision (< 5.5% CV) and accuracy (> 95% recovery) compared well to the TDx method. However, when carbamazepine was spiked into sera from patients with hepatic failure or at term pregnancy, all three methods measured a negative bias in recovery of 16-20%. No significant cross-reactivity was observed at normal therapeutic concentration of certain tricyclic compounds, though measurable cross-reactivity was detected when present at toxic serum concentrations. We conclude that the EMIT carbamazepine immunoassay is adaptable to the different kinetic rate analyzers studied. Analytical specificity should, furthermore, be assessed in the context of interferences likely to be clinically encountered.