In a retrospective laboratory-based database study, Mulligan et al.(25) identified 414 MBL patients, 322 of whom had a CLL phenotype. Based on 220 patients with adequate longitudinal information (median follow-up 4.1 years, range 1.0–8.5 years), they found 76 (34.5%) patients progressed to CLL(26) including 28 (12.7%) patients who developed an absolute lymphocyte count (ALC) of more than 20 x 109/L and 18 (8.2%) an ALC of more than 30 x 109/L, the criterion for progression used by Rawstron et al.(18) (i.e., a rate of progression of approximately 2% per year). Further, they identified 5 MBL cases with an initial B-cell count of less than 1.9 x 109/L who progressed to fulfill the current criteria for CLL: three cases progressed to have an ALC that only modestly exceeded the current CLL diagnostic criteria(26), and 2 cases had progressive lymphocytosis to 26.8 x 109/L and 52.5 x 109/L after 3.1 and 2.8 years, respectively.
