Establishing Absolute Lymphocyte Count Thresholds for Further Review of Peripheral Blood Lymphocytosis to Judiciously Screen for Monoclonal B-Cell Populations in Older Adults

OBJECTIVES: Lymphocytosis may represent either a lymphoproliferative disorder (LPD) or a reactive process. The absolute lymphocyte count (ALC) threshold for further evaluation of lymphocytosis is not well established. METHODS: We prospectively performed flow cytometry on blood samples from patients 50 years or older with ALCs of 4.0 × 109 cells/L or greater without a history of an LPD. RESULTS: Monoclonal B-cell populations were found in 34 (19.1%) of 178 cases, with incidence increasing with age. In patients younger than 75 years, no monoclonal B-cell population was identified in patients with ALCs less than 4.4 × 109 cells/L, while such clones were found below and above this threshold in patients 75 years and older. CONCLUSIONS: These findings support a threshold for smear review and flow cytometry no lower than 4.4 × 109 cells/L in patients younger than 75 years and a threshold as low as 4.0 × 109 cells/L in patients 75 years and older.