Monoclonal gammopathy of undetermined significance (MGUS).

MGUS is characterized by a serum M-protein concentration of less than 30 milligrams (3 g/dl), fewer than 10% plasma cells in the bone marrow, no or only small amounts of M-protein (Bence Jones protein) in the urine, the absence of lytic lesions, anaemia, hypercalcaemia and renal insufficiency, and most importantly, stability of the M-protein and failure of the development of additional abnormalities. Electrophoresis on agarose, followed by immunoelectrophoresis or immunofixation for the identification of the type of M-protein, is recommended. In 1994, 971 patients at the Mayo Clinic were found with a serum M-protein. The most frequent diagnosis was MGUS, which occurred in 52% of patients. MGUS is found in approximately 3% of people older than 70 years and in at least 1% of those aged over 50. The incidence of monoclonal gammopathies increases with advancing age and is higher in African-Americans than in Caucasians. Two hundred and forty-one patients from the Mayo Clinic with a monoclonal gammopathy but no evidence of MM, macroglobulinaemia, amyloidosis, lymphoma or related disorders were followed for 24-38 years. In 62 patients (26%), multiple myeloma, macroglobulinaemia, amyloidosis or a malignant lymphoproliferative disorder developed (the actuarial rate of development of serious disease at 10 years was 16%; at 20 years, 33%; and at 25 years, 40%). Thirty patients (12%) were alive and had a stable M-protein value. In 23 patients (10%), the serum M-protein level increased to 30 milligrams (3 g/dl) or more, but they did not require therapy for myeloma or related disorders. Fifty-two per cent of patients (126) died of unrelated diseases without the development of a malignant plasma cell lymphoproliferative disorder. The actual rate of development of serious disease was the same for those with IgG, IgA and IgM M-proteins. Differentiation of MGUS from myeloma or macroglobulinaemia is difficult. The M-protein value must be measured periodically and clinical evaluation carried out to determine whether or not serious disease has developed.