Definitive liquid-chromatographic demonstration that N-ethylglycine is the metabolite of lidocaine that interferes in the Kodak sarcosine oxidase-coupled method for creatinine.

Patients receiving lidocaine may show false increases of serum creatinine as assayed by the single-slide method on the Kodak Ektachem 700. Bissell et al. (Clin Chem 1987;33:951) suggested that this interference was due to oxidation of N-ethylglycine (NEG), a previously uncharacterized metabolite of lidocaine, by the sarcosine oxidase preparation used in the Ektachem creatinine slide. To investigate this possibility, we synthesized NEG, added it to drug-free human serum, and analyzed the NEG-supplemented sera for creatinine with the Ektachem 700. We found the following linear relationships between creatinine bias (y, mg/L) and NEG concentration (x, mg/L) for first (I), third (III), and fourth (IV) generation slides: I: y = 1.70x - 0.8 mg/L (n = 13, r = 1.0) III: y = 0.39x - 0.3 mg/L (n = 3, r = 1.0) IV: y = 0.79x - 1.8 mg/L (n = 13, r = 1.0) Using HPLC, we directly demonstrated the presence of NEG in sera of patients receiving lidocaine and quantified NEG concentrations in sera from four of these patients. The increasing artifactual bias in creatinine with increasing NEG concentration unequivocally confirmed that NEG is responsible for the lidocaine-associated interference in the Kodak Ektachem single-slide creatinine method.