monoclonal gammopathy of undetermined significance (MGUS)

Last reviewed 08/2023

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).