Detection of a monoclonal B-cell population in the peripheral blood with

• overall kappa:lambda ratio > 3:1 or < 0.3:1, or

• greater than 25% of B cells lacking or expressing low level surface immunoglobulin or

• a disease-specific immunophenotype.

Repeat assessment should demonstrate that the monoclonal B-cell population is stable over a 3-month period.

Exclusion criteria

• lymphadenopathy and organomegaly, or

• associated autoimmune/infectious disease, or

• B-lymphocyte count >5 × 10⁹/L, or

• any other feature diagnostic of a B-lymphoproliferative disorder. However, a paraprotein may be present or associated with MBL and should be evaluated independently.

Sub-classification:

• CD5⁺23⁺: this is the major subcategory and corresponds to a CLL immunophenotype (Cheson et al, 1996).

• CD5⁺23⁻: correlate moderate level of CD20 and CD79b expression with atypical CLL.

• CD5⁻: corresponds to non-CLL lymphoproliferative disorder