Low-Count Monoclonal B-Cell Lymphocytosis Persists after 7 Years of Follow-up and Confers a Higher Risk of Death

Monoclonal B-cell lymphocytosis (MBL) is characterized by the presence of low numbers of circulating clonal B cells that usually display highly-similar phenotypic features to chronic lymphocytic leukemia (CLL) cells. This entity is divided into high-count (MBLhi) and low-count (MBLlo), depending on the number of peripheral blood (PB) clonal B cells (higher vs. lower than 0.5x109/L, respectively). Whereas MBLhi is frequently detected in laboratory routine analyses and precedes CLL, MBLlo is mainly detected in population-based screening studies, making its biological and clinical significance elusive. Of note, prevalence of MBLlo is significantly higher than that of MBLhi and CLL, and increases with age, with a prevalence >20% among individuals older than 70y.