Since the clonal B-cells of individuals with both CLL and CLL-like MBL share an identical immunophenotype, how to best differentiate MBL and Rai stage 0 CLL continues to be an area of controversy. From a historic perspective, the 1988(29) and 1996(30) diagnostic criteria for CLL classified individuals with a clonal population of characteristic immunophenotype and an ALC >5 x 109/L as having CLL. After recognition of MBL and publication of the 2005 MBL diagnostic criteria(6) which were based on B-cell count rather than ALC, there was initially overlap between the diagnostic criteria for CLL and MBL: individuals with an ALC≥5 x 109/L who had a B-cell <5 x 109/L fulfilled both the MBL and CLL diagnostic criteria. While this initially appeared to impact a small proportion of patients, subsequent studies indicated up to 40% of individuals with newly diagnosed Rai stage 0 CLL according to the 1988 and 1996 criteria fell in this area of overlap.(20, 31)
