Monoclonal B-cell lymphocytosis; not the same as B-cell chronic lymphocytic leukaemia.

INTRODUCTION: Depending on the location and the extent of disease, mature B-cell disorders can be divided into 
benign monoclonal B-cell lymphocytosis (MBL), chronic 
lymphocytic leukaemia (CLL) and small lymphocytic lymphoma (SLL). Whereas SLL is characterised by its location outside the blood stream, MBL is distinguished from CLL by a monoclonal B-cell count below 5 × 109/l. Due to its low tendency to transform into CLL, correct diagnosis of MBL is essential. We hypothesised that this might not always be the case. METHODS: This study includes data on monoclonal B-lymphocyte count based on diagnostic flow cytometry from patients diagnosed in the period from 1 January 2011 to 31 December 2016 at the Department of Haematology, Aarhus University Hospital, Denmark. A total of 69 patients had less than 5 × 109/l monoclonal B-cells with a CLL-like 
immunophenotype in peripheral blood. All cases were classified based on the 2008 WHO criteria and evaluated according to the clinical diagnosis of CLL, MBL or SLL in the medical records. A total of 24 of the 69 patients were classified as MBL. RESULTS: In the study cohort, 12 (50%) patients classified as MBL were diagnosed accurately with MBL, whereas nine (38%) were diagnosed with CLL. CONCLUSIONS: The findings of this study indicate that a sizeable fraction of MBL patients are diagnosed inaccurately with CLL, even after the introduction of the MBL diagnosis. FUNDING: The Danish Cancer Society. TRIAL REGISTRATION: not relevant.