(See also Overview of Plasma Cell Disorders.)
The incidence of monoclonal gammopathy of undetermined significance (MGUS) increases with age, from 1% of people aged 25 years to > 5% of people > 70 years. MGUS may occur in association with other disorders (see table Classification of Plasma Cell Disorders), in which case M-proteins (monoclonal immunoglobulin proteins, which may consist of both heavy and light chains or of only a light chain) may be antibodies produced in large amounts in response to protracted antigenic stimuli.
MGUS usually is asymptomatic, but peripheral neuropathy can occur, and patients are at higher risk of enhanced bone loss and fractures. Although most cases are initially benign, up to 25% (1%/year) progress to myeloma or a related B-cell disorder, such as macroglobulinemia, amyloidosis or lymphoma.
Diagnosis of MGUS is usually suspected when M-protein is incidentally detected in blood or urine during a routine examination. On laboratory evaluation, M-protein is present in low levels in serum (< 3 g/dL) or urine (< 200 mg/24 hours). MGUS is differentiated from malignant plasma cell disorders because M-protein levels are lower and lytic bone lesions, anemia, and renal dysfunction are absent. Because of fracture risk, baseline evaluation with a skeletal survey (ie, plain x-rays of skull, long bones, spine, pelvis, and ribs) and bone densitometry should be done. Bone marrow shows only mild plasmacytosis (< 10% of nucleated cells).
No antineoplastic treatment is recommended. However, recent studies suggest that MGUS patients with associated bone loss (osteopenia or osteoporosis) may benefit from treatment with intravenous bisphosphonates but less frequently than the monthly treatments usually required to treat patients with multiple myeloma. Every 6 to 12 months, patients should undergo clinical examination and serum and urine protein electrophoresis to evaluate for disease progression.