Occult B-cell lymphoproliferative disorders

The term monoclonal B-cell lymphocytosis (MBL) was recently introduced to identify individuals with a population of monoclonal B cells in the absence of other features that are diagnostic of a B-cell lymphoproliferative disorder. MBL is often identified through hospital investigation of a mild lymphocytosis, and approximately 1% of such individuals develop progressive disease requiring treatment per year. However, in population studies using high-sensitivity flow cytometry, MBL may be detectable in more than 10% of adults aged over 60 years, and clinical progression is rare. The majority of MBL cases have features that are characteristic of chronic lymphocytic leukaemia, but an increasing amount of information is becoming available about MBL with the features of other B-cell lymphoproliferative disorders. In addition to flow cytometry findings, the incidental detection of an occult B-cell lymphoproliferative disorder is also occurring in a significant proportion of tissue biopsy samples. In this review, the clinical and biological relationship between MBL and B-cell lymphoproliferative disorders will be discussed, with a focus on identifying the differences between low levels of peripheral blood or bone marrow involvement with lymphoma and the monoclonal B-cell populations that commonly occurr in elderly adults.