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Monitoring serum levels of lacosamide, other than to establish individualized reference ranges may be helpful in several settings, including patients with liver and/or kidney failure or settings that may result in altered pharmacokinetic characteristics and to assess patients' compliance with therapy. In this study, the EurekaOne liquid chromatography-mass spectrometry (LC-MS/MS) method (in use method) and the ARK immunoassay method (new method) for lacosamide monitoring were compared.Lacosamide concentrations were determined in 39 patient samples using (1) antiepileptic drug LC-MS/MS kit by EurekaOne on a Thermo Fisher Scientific TSQuantum Access Max system and (2) the lacosamide immunoassay by ARK Diagnostic Inc. (research use only kit), on a Abbott Architect System.Measured total imprecision of the new method is 6.29% at 6.59 μmol/L, 8.82% at 30.20 μmol/L, and 6.45% at 64.51 μmol/L. Passing–Bablok regression analysis showed a nonsignificant intercept of -0.03015 [95% confidence interval (CI), −1.2243 to 0.8593] and a slope of 1.05 (95% CI, 0.9973–1.1166), showing that the method does not deviate from linearity and absence of proportional systematic error. Bland–Altman analysis showed a systematic bias of -3.296% (95% CI, −5.81 to −0.78) with 95% of the LC-MS/MS—ARK mean % of differences ranging from −18.5 to 11.9. Despite this bias, data of the combined imprecision of the 2 methods show that the new method is still acceptable within the maximum allowable error of 15%.The performance of the new ARK method on the Architect system is acceptable and may be used routinely to measure serum lacosamide concentration in the clinic although the nature of the bias has to be carefully addressed.