Precision and reliability of paraprotein determinations by high-resolution agarose gel electrophoresis

Agarose gel electrophoresis has recently replaced cellulose acetate electrophoresis as the preferred technique for monitoring paraprotein levels in patients with plasma cell dyscrasias. The authors studied the accuracy and precision of this method for paraprotein determination. Twenty-seven serum samples with paraprotein concentrations ranging from 5 to 73 g/L were aliquotted and assayed on 20 separate occasions, and the mean and standard deviation for the paraprotein concentration in each serum was established. Linear regression analysis showed that the standard deviation of paraprotein concentration (SD) increased as a function of paraprotein concentration (PC). For IgG paraproteins, the regression equation was SD = 0.041 (PC) + 1.06; R = 0.942; standard error = 0.32. For non-IgG paraproteins the equation was SD = 0.101 (PC) - 0.04; R = 0.851; standard error = 0.5. The accuracy of paraprotein determinations by the agarose gel electrophoretic technique was assessed by comparison with values obtained with the use of a previously validated enzyme-linked immunosorbent assay (ELISA) method for quantitation of IgG subclasses. Results obtained by the two methods were similar and highly correlated: (concentration by electrophoresis) = 0.921 (concentration by ELISA) + 0.46; R = 0.988; standard error = 0.34. The laser densitometric scanning procedure showed a loss of linearity above 60 g/L, indicating the need to dilute sera with very high paraprotein concentrations in order to obtain accurate results. A table is presented that should help pathologists who interpret such scans to determine whether small changes in paraprotein measurements occurring over time represent true changes in paraprotein concentration or merely reflect the analytic variability inherent in the technique.