Underestimation of monoclonal proteins by agarose serum protein electrophoresis

The effect of serum dilution on monoclonal protein quantitation by serum protein electrophoresis (SPE) on the agarose gel Paragon system was investigated in 388 serum samples from 106 patients with Ig G monoclonal gammopathy. It was found that the pre-electrophoretic 1:5 serum dilution recommended by the manufacturer was adequate for some but not all sera, especially those with the highest M-protein concentrations. As a result of the inadequate dilution, 232 (60 percent) of the 388 samples had M-protein concentrations that were significantly underestimated and the corresponding albumin concentrations overestimated. By Paragon SPE, the mean albumin concentration in these 232 sera was 41.8 (SD 6.7) g/L. After further dilution of these sera, the mean albumin concentration was 36.7 (SD 6.8) g/L and was, in each case, always less than that in the corresponding 1:5 diluted serum. By the bromcresol green (BCG) dye-binding method, the albumin concentration was 34.9 (SD 4.3) g/L. Similarly, the M-protein concentration for 1:5 diluted sera was 51.9 (SD 12.9) g/L vs. 59.1 (SD 16.1) g/L for the further diluted sera, with the M-protein concentration in each further diluted sample always exceeding that in the corresponding 1:5 diluted serum. Underestimation of the M-protein concentration limits its clinical utilization in evaluating the patient's response to therapy and for early detection of disease progression. A recommendation was made of a 1:10 dilution of sera that contain total protein from 91 g/L to 114 g/L and a 1:20 dilution of sera in which the protein content is in the range of 115 g/L to 152 g/L to insure accurate estimation of protein fractions by Paragon SPE.