Sensitivity of immunofixation electrophoresis for detecting IgM paraproteins in serum.

Detection and characterization of small IgM paraproteins by immunoelectrophoresis are often difficult because of (a) slow diffusion of the macromolecular immunoglobulin, and (b) the obscuring of the light chain, especially kappa, by the light chains of background polyclonal immunoglobulin (the "umbrella effect"). Various methods of sample pretreatment have been tried in an attempt to overcome this problem. Using immunofixation electrophoresis (IFE), we were able to detect and characterize IgM paraproteins in serum at concentrations as low as 0.25 g/L, even when the polyclonal immunoglobulin background was increased. With routine IFE, our laboratory identified 58 patients with IgM paraproteinemia in a four-year period. In 36, the paraproteins were present at less than 5 g/L; 10 of these were detectable only by IFE.