MBL or CLL: Which classification best categorizes the clinical course of patients with an absolute lymphocyte count ≥ 5 × 109 L−1 but a B-cell lymphocyte count < 5 × 109 L−1?

To eliminate overlap with monoclonal B-cell lymphocytosis (MBL), some have proposed basing the diagnosis of chronic lymphocytic leukemia (CLL) on B lymphocyte count rather than absolute lymphocyte count (ALC). Such criteria should be based, in part, on patient outcomes. We evaluated the clinical implications of the proposed re-classification in 112 consecutive, newly diagnosed, Rai stage 0 patients. The new criteria would have changed the diagnosis from CLL to MBL in 47/112 (42%) patients. There was no difference in time to treatment (TTT) between those classified as MBL and CLL under the new criteria. In contrast, CD38 predicted TTT (p = 0.02) regardless of the proposed new classification. Molecular characteristics of the leukemic clone are a better predictor of progression than an arbitrary ALC or B lymphocyte count threshold.