Analytical variation in the measurement of serum monoclonal component by capillary electrophoresis

The amount of serum monoclonal components (MC), and moreover, their variation with time are important factors for differentiating malignancy-associated from benign gammopathies. Capillary electrophoresis (CE) of serum proteins allows better quantitative measurement in comparison to conventional gel-supported electrophoresis. We investigated the analytical variation of CE measurement of serum MC of different sizes: the variation due to integration settings and the analytical variation of the CE procedure were assessed in separate experiments. "Small" MC (in the interval 4-14% of total protein) were measured with analytical imprecision (CV%) in the interval 5-25%. The step of setting the integration limits was found to generate an important portion of such an overall variability. The imprecision was negatively, nonlinearly related to MC concentration: at the "critical" MC value of 40% of total protein, the analytical imprecision was CV approximately 2.5%. The variation computed from sets of subsequent routine MC measurements in "stable" patients with MGUS, over observation intervals from 10 to 30 months, behaved similarly to the "pure" analytical variation measured experimentally. This suggested very low, if any, biological variation of MC. Differences between subsequent MC measurements in a patient should be interpreted in light of the analytical variation.