IgG subclasses in monoclonal gammopathy of undetermined significance.

One hundred seventy-five patients with a serum monoclonal IgG who had been followed up for more than 5 years without the development of multiple myeloma, amyloidosis, or other serious diseases were designated as having MGUS. The monoclonal IgG of 83% of patients was classified as IgG1; that of 9%, IgG2; that of 6%, IgG3; and that of 2%, IgG4. The median values for hemoglobin, leukocytes, platelets, calcium, creatinine, size of the M spike, and plasma cell content in the bone marrow were not significantly different in the four subclass groups. The IgA level was reduced in 8% and the IgM in 20% of the total group. The uninvolved IgG subclass levels were reduced in 81% of the IgG1 patients and in all the IgG2, IgG3, and IgG4 patients. The patients with MGUS wer compared to 229 patients with multiple myeloma. The monoclonal IgG of the myeloma patients was classified as IgG1 (73%), IgG2 (14%), IgG3 (8%), and IgG4 (6%). The median hemoglobin values were higher in IgG3 and IgG4 groups. Renal insufficiency and size of the M protein spike were similar in the four subclass groups. Either IgA or IgM was reduced in 75% of the patients. The uninvolved IgG subclasses were reduced in 96.5% of all patients. The 5-year actuarial survival was 30% for the IgG1 group and 33% for the IgG2 group. Twenty patients with MGUS subsequently developed multiple myeloma. Analysis of the results from these patients did not show a difference from the other MGUS patients. The results suggest that the IgG subclass in MGUS does not affect either the features of the disease or the tendency for later development of multiple myeloma.