Monoclonal B-cell lymphocytosis (MBL) was defined by the International Familial CLL consortium in 2005 as a monoclonal B-cell lymphocyte population in the peripheral blood <5,000/uL without evidence of lymphadenopathy (i.e. SLL), an autoimmune/infectious disease or other features diagnostic of a B-cell lymphoproliferative disorder. The 2016 WHO hematopathology revision dropped the requirement of cytopenias or disease related symptoms as adequate to make the diagnosis of CLL.
The 2016 WHO classification revision also further divided MBL into “low-count” and “high-count” MBL. Low-count MBL is defined as a peripheral blood monoclonal B-cell count <500/uL. Low-count MBL cases do not need to be followed up clinically for progression. High-Count MBL (monoclonal B-cell count 500-5,000/uL) should continue to be followed clinically for progression. There is approximately a 1-2% chance per year of progression to CLL..
The cutoff value of a monoclonal B-cell lymphocyte count of 5,000/uL was chosen somewhat arbitrarily, and cases in the range of 5,000-10,000/uL may also have a relatively indolent course. These definitions may be modified in the future as more clinical and molecular data is evaluated.
MBL may have a typical CLL phenotype (e.g. CD5+, CD20+, CD23+), but it is NOT a requirement for diagnosis. Up to 12% of healthy individuals may have a small monoclonal B-cell population.
References
Swerdlow SH, Campo E, Pileri SA, Harris NL, Stein H, Siebert R, et al. The 2016 revision of the World Health Organization classification of lymphoid neoplasms. Blood. 2016;127: 2375–2390. doi:10.1182/blood-2016-01-643569
Foster AE, Nguyen TT, Al-Hammadi N, Frater JL, Hassan A, Kreisel F. Clinical Presentation, Progression, and Outcome of Patients With Clonal B-Cell Counts of Less Than 5 x 109/L, 5 to 10 x 109/L, and More Than 10 x 109/L and Chronic Lymphocytic Leukemia Immunophenotype. Am J Clin Pathol. 2014;143: 70–77. doi:10.1309/AJCPIXUB5MZK8ECI
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