While these findings suggest a significant biologic overlap between CLL and CLL-like MBL, it is important to emphasize that all these studies were performed on MBL cases with a high number of monoclonal cells to facilitate these analyses. In routine clinical practice the vast majority (~85%) of MBL cases are identified following investigation of lymphocytosis and have an abnormal B-cell count above 1900 cell/μl. In contrast, ~85% of MBL detected on population screening studies have a B-cell count below 500/μl, with 40% having fewer than 50 CLL phenotype cells/μl. Since several studies suggest that the number of B cells is a strong predictor of the clinical outcome of MBL patients,(18–20) it is reasonable to postulate that, from a biological point of view, MBL detected in clinical practice (“clinical MBL”) may be more similar to frank CLL than MBL detected on population screening (“population-screening MBL”).
