Monoclonal B-cell lymphocytosis: a reappraisal of its clinical implications

Monoclonal B-cell lymphocytosis (MBL) is a recently defined medical condition that displays biological similarities to chronic lymphocytic leukemia (CLL), the most common subtype of adult leukemia in the Western world. MBL may be diagnosed in individuals with a normal lymphocyte count via a screening assay (screening MBL) or through the clinical evaluation of lymphocytosis (clinical MBL). Clinical MBL, which resembles CLL with a good prognosis, has attracted considerable interest because of its clinical and biological implications. The biological profile of clinical MBL appears indistinguishable from that of CLL for a large variety of markers. Differential diagnosis between clinical MBL and CLL is mainly based on peripheral blood B-cell counts. The 2008 International Workshop on Chronic Lymphocytic Leukemia (IWCLL) criteria changed the definition of CLL to a B-cell count ≥ 5.0 × 10(9)/L to eliminate overlap between CLL and MBL. However, this cut-off is arbitrary, and recent studies suggest that a B-cell count of 10.0-11.0 × 10(9)/L may represent the threshold that best predicts time to first treatment. After a diagnosis of clinical MBL, patients should be educated about appropriate monitoring and follow-up keeping in mind that, with time and counseling, most patients will understand that clinical MBL and CLL represent a continuum.