There is a predominance of clonal plasma cells in the bone marrow with an abnormal immunophenotype (CD38+ CD56+ CD19−) mixed in with cells of a normal phenotype (CD38+ CD56− CD19+); in MGUS, on average more than 3% of the clonal plasma cells have the normal phenotype, whereas in multiple myeloma, less than 3% of the cells have the normal phenotype. Diagnosis
MGUS is a common, age-related medical condition characterized by an accumulation of bone marrow plasma cells derived from a single abnormal clone. Patients may be diagnosed with MGUS if they fulfill the following four criteria:
A monoclonal paraprotein band less than 30 g/L (< 3g/dL);
Plasma cells less than 10% on bone marrow examination;
No evidence of bone lesions, anemia, hypercalcemia, or chronic kidney disease related to the paraprotein, and
No evidence of another B-cell proliferative disorder. Differential diagnosis
Several other illnesses can present with a monoclonal gammopathy, and the monoclonal protein may be the first discovery before a formal diagnosis is made:

Multiple myeloma
Smouldering multiple myeloma
AIDS
Chronic lymphocytic leukemia
Non-Hodgkin lymphoma, particularly Splenic marginal zone lymphoma and Lymphoplasmocytic lymphoma
Hepatitis C
Connective tissue disease such as lupus
Immunosuppression following organ transplantation
Waldenström macroglobulinemia
Guillain–Barré syndrome
Tempi syndrome
POEMS

Management
The protein electrophoresis test should be repeated annually, and if there is any concern for a rise in the level of monoclonal protein, then prompt referral to a hematologist is required.
Monoclonal gammopathy of undetermined significance (MGUS) is a plasma cell dyscrasia in which plasma cells or other types of antibody-producing cells secrete a myeloma protein, i.e. an abnormal antibody, into the blood; this abnormal protein is usually found during standard laboratory blood or urine tests. MGUS resembles multiple myeloma and similar diseases, but the levels of antibodies are lower, the number of plasma cells (white blood cells that secrete antibodies) in the bone marrow is lower, and it rarely has symptoms or major problems. However, since MGUS can lead to multiple myeloma, which develops at the rate of about 1.5% a year, or other symptomatic conditions, yearly monitoring is recommended. The progression from MGUS to multiple myeloma usually involves several steps. In rare cases, it may also be related with a slowly progressive symmetric distal sensorimotor neuropathy. Signs and symptoms
People with monoclonal gammopathy generally do not experience signs or symptoms. Some people may experience a rash or nerve problems, such as numbness or tingling. MGUS is usually detected by chance when the patient has a blood test for another condition or as part of standard screening. Pathophysiology
Pathologically, the lesion in MGUS is in fact very similar to that in multiple myeloma.