Changes in serum anion gap and sodium level in monoclonal gammopathies

In a group of patients with monoclonal gammopathies a decrease in the serum anion gap was seen with increasing serum concentrations of monoclonal IgG and IgM but not monoclonal IgA. This was probably related to the fact that IgG and IgM are cationic but IgA is anionic at a physiologic pH. The serum sodium level decreased by 0.7 mmol/l for every increase of 1 g/dl in the serum level of the monoclonal immunoglobulin, likely because of the volume displacement effect of the monoclonal protein.