Monoclonal B-cell lymphocytosis: Definitions and natural history

The diagnostic term monoclonal B-cell lymphocytosis (MBL) is used to characterise individuals with a circulating population of clonal B-cells, a total B-cell count of <5 × 10⁹/L, and no other features of a B-cell lymphoproliferative disorder. The clinical implications of MBL may differ depending on whether an individual with a normal lymphocyte count is identified via a screening assay (screening MBL) or identified through clinical evaluation of lymphocytosis (clinical MBL). The B-cell count used to distinguish between clinical MBL (< 5 × 10⁹/L) and CLL (≥5 × 10⁹/L) was selected largely based on tradition and technological advances and it is unknown whether the natural history of 'clinical MBL' differs from that of patients with Rai stage 0 CLL. Since, a diagnosis of 'leukemia' may lead to profound psychologic distress for patients, we believe the diagnosis of CLL should be based on an individual's risk of developing symptoms, requiring chemotherapeutic treatment and/or dying of disease. Additional studies are needed to determine whether the clinical outcome of patients with MBL differs from that of patients with Rai stage 0 CLL and to identify what B-cell threshold optimally distinguishes between these conditions.