Monoclonal gammopathy of undetermined significance  (MGUS)=M-protein found in blood without other diagnostic criteria for multiple

Stage I: All of the following:

• Hemoglobin >10 g/dL.
• Normal serum calcium.
• Normal bone structure.
• Low M-protein production as shown by:
  • IgG <5.0 g/dL.
  • IgA <3.0 g/dL.
  • Urinary kappa (κ) or lambda (λ) light chains <4 g/24 hours.
  • Low myeloma cell mass (<0.6 X 10¹²) cells/m².

Stage I multiple myeloma:

• Beta-2-microglobulin <3.5.
• Albumin ≥3.5.

Stage II:  Disease neither stage I nor stage III:

• Intermediate myeloma cell mass: 0.6 to 1.2 trillion (10¹²)/m²

Stage II multiple myeloma:

• Beta-2-microglobulin <3.5 and albumin <3.5, or
• Beta-2-microglobulin 3.5 to <5.5.

Stage III means 1 or more of the following:

• Hemoglobin <8.5 g/dL.
• Serum calcium >12.0 mg/dL.
• More than 3 lytic bone lesions (>75%).
• High M-protein production as shown by:
  • IgG >7.0 g/dL.
  • IgA >5.0 g/dL.
  • Urinary kappa (κ) or lambda (λ) >12.0 g/24 hours.
  • Estimated myeloma cell mass: >1.2 trillion (10¹²)/m² (high burden).

Stage III multiple myeloma:

• Beta-2-microglobulin ≥5.5

Subclassified based upon renal function:

1. Serum creatinine <2 mg/dL.
2. Serum creatinine ≥2 mg/dL.