The spectrum of IgM monoclonal gammopathy in 430 cases.

IgM monoclonal gammopathy consists of a broad spectrum of diseases, ranging from apparently benign to malignant conditions. In a long-term follow-up study of 430 patients in whom a monoclonal IgM serum protein had been identified, 242 (56%) had monoclonal gammopathy of undetermined significance, 71 (17%) had Waldenström's macroglobulinemia, 28 (7%) had lymphoma, 21 (5%) had chronic lymphocytic leukemia, 6 (1%) had primary amyloidosis, and 62 (14%) had other malignant lymphoproliferative diseases. More than two-thirds of the patients died, and the most common cause of death was a lymphoid malignant process. Almost a fifth of the patients with an apparently benign monoclonal gammopathy subsequently had a lymphoid malignant lesion (in one patient, more than 20 years after the detection of the serum M protein). The median duration of time from the recognition of the M protein until the development of a malignant lymphoid disease ranged from 4 to 9 years. An increased number of lymphocytes or plasma cells on bone marrow examination was not a reliable indicator of the likelihood of such an outcome. Thus, follow-up of these patients should be conducted indefinitely.