Vertical cutoff methods in serum protein electrophoresis for the measurement of monoclonal protein concentrations: Which is best?

•Two methods are used to demarcate the monoclonal protein area in the electropherogram.•Clinical guidelines do not specify a cutoff method for demarcating the monoclonal protein.•The tangent skimming method is more accurate than the perpendicular drop method.•The perpendicular drop method is more precise than the tangent skimming method.•Observer variability is an important factor in overall cutoff imprecision. Monoclonal protein (M-protein) concentrations are measured by serum protein electrophoresis (SPE). Two methods are used for demarcating the M-protein area in the electropherogram: perpendicular drop (PD) and tangent skimming (TS). The aim of this study was tocompare both methods and to establish which is the most accurate and precise. We studied 24 sera containing M-protein (5–44 g/L). The systematic error (SE) was evaluated in a dilution series of 12 sera. Within-day, between-day, and interobserver variability were assessed. SPE was performed by capillary and agarose gel electrophoresis. M-protein concentrations were measured using both cutoff methods. The PD method shows a constant SE ranged 1.00–2.27 g/L, while constant SE for TS is ranged −0.30–−0.57 g/L. None of the cutoff methods or electrophoretic methods showed a proportional SE, with the exception of the TS method in capillary electrophoresis for β-migrating M-protein. The PD method was more precise than the TS method in all three estimates of imprecision. An increased CV for concentrations