Monoclonal gammopathy of undetermined significance (MGUS)

MGUS patients have M proteins (IgG kappa or lambda; or IgA kappa or lambda) in the serum but without features of multiple myeloma, macroglobulinemia, amyloidosis, or lymphoma (1).

• they have fewer than 10% of plasma cells in the bone marrow
• patients are asymptomatic and should avoid treating them (1)

In smouldering myeloma, patients have similar characteristics but may have more than 10% of marrow plasma cells (1)

MGSU can be seen in 2% of people older than 50 years (2) and around 1% to 2% of MGUS patients per year will progress to myeloma (most commonly), amyloidosis, lymphoma, or chronic lymphocytic leukemia.

• they must be followed carefully because patients who develop these diseases require treatment
• almost all patients with multiple myeloma are preceded by a gradually rising level of MGUS
• the following risk factors suggest disease progression
  • an abnormal serum-free light chain ratio
  • non-IgG class MGUS.
  • a high serum M protein level (≥15 g/L) (1)

The following criteria are used for the diagnosis of MGUS (all three are required):

• serum monoclonal protein low *
• monoclonal bone marrow plasma cells of 10%
• no evidence of end-organ damage attributable to the clonal plasma cell disorder:
  • normal serum calcium, haemoglobin level and serum creatinine
  • no bone lesions on full skeletal X-ray survey and/or other imaging if performed
  • no clinical or laboratory features of amyloidosis or light chain deposition disease (1)

* low is defined as serum M protein of 3.0 g per 100 ml (3).

Reference:

• (1) National Cancer Institute 2011. Plasma Cell Neoplasms (Including Multiple Myeloma) Treatment
• (2) Nau KC, Lewis WD. Multiple myeloma: diagnosis and treatment. Am Fam Physician. 2008;78(7):853-9.
• (3) Palumbo A et al. International Myeloma Working Group guidelines for the management of multiple myeloma patients ineligible for standard high-dose chemotherapy with autologous stem cell transplantation. Leukemia. 2009;23(10):1716-30.