Monoclonal Gammopathy of Undetermined Significance

Aka: Monoclonal Gammopathy of Undetermined Significance, MGUS

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II. Epidemiology

  1. Ultimately responsible for 20% of Multiple Myeloma cases after progression
  2. Age
    1. Present in 2-3% of patients over age 50 years (increases to 5% in age over 70 years old)
  3. Gender
    1. More common in men
  4. Race and ethnicity
    1. More common in black patients than white patients

III. Associated Conditions: MGUS Progression to other Plasma Cell Disorders

  1. Multiple Myeloma (RR 23.9)
  2. Immunoglobulin Light Chain Amyloidosis (RR 8.8)
  3. Macroglobulinema (RR 47.6)
  4. Plasmacytoma (RR 12.7)

IV. Diagnosis: Non-IgM MGUS (IMWG 2024, requires all 3 criteria)

  1. Serum monoclonal Protein (Non-IgM) <3 g/dl AND
  2. Absent end organ injury attributed to plasma cell proliferative disorder (CRAB Factors)
    1. Hypercalcemia absent
    2. Renal Insufficiency absent
    3. Anemia absent
    4. Bone lesions absent
  3. Clonal Bone Marrow plasma cells <10%
    1. Marrow not needed if IgG M Protein <15 g/L and normal free light chain ratio (and CRAB factors negative)

V. Diagnosis: IgM MGUS (IMWG 2024, requires all 3 criteria)

  1. Serum monoclonal Protein (Non-IgM) <3 g/dl AND
  2. Bone Marrow lymphoplasmacytic infiltration <10% AND
  3. No findings that can be attributed to underlying lymphoproliferative disorder
    1. Anemia
    2. Constitutional symptoms
    3. Hyperviscosity Syndrome
    4. Lymphadenopathy
    5. Hepatosplenomegaly

VI. Diagnosis: Light Chain MGUS (IMWG 2024, requires all 6 criteria)

  1. No Immunoglobulin Heavy Chain expression on immunofixation AND
  2. Absent end-organ injury attributed to plasma cell proliferative disorder (e.g. CRAB events) AND
  3. Clonal Bone Marrow plasma cells <10% AND
  4. Urinary monoclonal Protein <500 mg/24 hours AND
  5. Abnormal free light chain ratio (<0.26 or >1.65) AND
  6. Increased level of the appropriate involved light chain
    1. Increased kappa-free light chain when free light chain ratio >1.65
    2. Increased lambda-free light chain when free light chain ratio <0.26

VII. Evaluation: Factors suggesting higher risk of Multiple Myeloma progression

  1. Criteria
    1. Elevated M Protein Level 1.5 to 3 g/dl (levels >3 g/dl consistent with Multiple Myeloma)
    2. Non-IgG MGUS
    3. Abnormal free light chain ratio
  2. Distinguishing features from Multiple Myeloma
    1. Bone Marrow plasma cells <10%
    2. Asymptomatic with no end-organ damage (see CRAB Criteria in Monoclonal Gammopathy)
  3. Interpretation
    1. Risk of Multiple Myeloma is 58% in 20 years if all 3 factors present

VIII. Evaluation: Progression Risk

  1. Low risk of progression (5% risk of progression)
    1. Criteria (No risk factor present)
      1. M-Spike <1.5 g/L
      2. IgG type
      3. Free light chain ratio normal
    2. Evaluation and Management
      1. No additional work-up (including no bone imaging)
      2. Repeat SPEP in 6 months and then every 2-3 years indefinately
  2. Intermediate risk of progression (21-37% risk of progression)
    1. Criteria (1-2 risk factors present)
      1. M-Spike >1.5 g/L OR
      2. Non-IgG type OR
      3. Free light chain ratio abnormal (involved to uninvolved >100)
    2. Evaluation and Management
      1. Hematology referral
      2. Bone Marrow Biopsy
      3. Bone imaging
      4. Repeat SPEP in 6 months and then every year indefinately
  3. High risk of progression (58% risk of progression in 20 years)
    1. Criteria (all 3 risk factors present)
      1. M-Spike >1.5 g/L AND
      2. Non-IgG type AND
      3. Free light chain ratio abnormal (involved to uninvolved >100)
    2. Evaluation and Management
      1. Hematology referral
      2. Bone Marrow Biopsy
      3. Bone imaging
      4. Repeat SPEP in 6 months and then every year indefinately

IX. Course

  1. Progresses to Multiple Myeloma in 1% of cases per year

X. References

  1. Thompson (2017) Monoclonal Gammopathy, Mayo Clinical Reviews, Rochester, MN
  2. Hughes (2026) Am Fam Physician 113(3): 244-53 [PubMed]
  3. Rajkumar (2024) Am J Hematol 99(9): 1802-24 [PubMed]

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