<aside> 📋

Monoclonal Gammopathy of Undetermined Significance (MGUS) is a premalignant plasma cell disorder characterized by the presence of a monoclonal immunoglobulin (M-protein) in the blood without features of multiple myeloma or other lymphoproliferative diseases.

</aside>

It is the most common monoclonal gammopathy and is usually asymptomatic.

Pathophysiology

Clonal plasma cell proliferation in bone marrow (but <10%)
Produces a monoclonal immunoglobulin (usually IgG, IgA, or light chains)
May progress to multiple myeloma (MM), Waldenström macroglobulinemia, AL amyloidosis, or other disorders
Risk of progression: ~1% per year

Epidemiology

Feature	Description
Prevalence	~3–5% of people >50 years
Sex	Slight male predominance
Race	Higher in African descent
Age	Mean age ~70 years

Diagnosis

IMWG 2014 diagnostic criteria:

Parameter	Threshold
Serum M-protein	<3 g/dL (intact IgG, IgA, or IgM)
Bone marrow clonal plasma cells	<10%
No end-organ damage	No CRAB features (hyperCalcemia, Renal failure, Anemia, Bone lesions)
No myeloma-defining event	No biomarkers of malignancy

Laboratory Findings

Test	Typical Findings
SPEP	M-spike <3 g/dL
Immunofixation	Detects monoclonal IgG (most common), IgA, or light chains
Serum free light chain assay	Normal or mildly abnormal κ/λ ratio
Bone marrow biopsy	<10% plasma cells
No Bence-Jones proteinuria	Or <500 mg/24 hr

Differentials

Disorder	Distinguishing Features
Smoldering MM	M-protein ≥3 g/dL or plasma cells ≥10%, no CRAB
Multiple Myeloma	M-protein + ≥1 CRAB features or myeloma-defining event
AL Amyloidosis	Organ involvement (nephrotic syndrome, cardiomyopathy)
Waldenström Macroglobulinemia	IgM M-protein, hyperviscosity, lymphadenopathy