Clinical and diagnostic implications of monoclonal B-cell lymphocytosis

Monoclonal B-lymphocytosis (MBL) is defined as the presence of a population of monoclonal B-cells, usually with a chronic lymphocytic leukaemia (CLL) phenotype, which comprise fewer than 5000 cells per μl with no evidence of tissue involvement. Over the past few years, MBL has been clearly defined and differentiated from CLL so that individuals with MBL are no longer inappropriately labelled as suffering from leukaemia. In this review, we will describe the entity of MBL and summarise the evidence that underlies the current theory on the pathophysiology of the disorder, the relationship with CLL and the probability of developing progressive disease requiring treatment. In addition, we will evaluate the importance of further clinical investigations, in particular, the relevance of screening for MBL and undertaking bone marrow investigations according to the clinical setting and B-cell phenotype.