RESUMO
BACKGROUND: 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. METHODS: 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. RESULTS: 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 ß-migrating M-protein. The PD method was more precise than the TS method in all three estimates of imprecision. An increased CV for concentrations < 10 g/L in between-day imprecision was observed with the TS method. Interobserver imprecision was greater for M-protein concentrations < 17 g/L for both cutoff methods (14.85%, 26.42% respectively). CONCLUSIONS: Despite being less precise, the TS method provides a more accurate measurement of M-protein concentration.