Monoclonal B-cell lymphocytosis (MBL) is a common cause of lymphocytosis in older individuals. Although cytogenetic/molecular testing is usually reserved for patients requiring treatment, recent studies suggest that early testing may have a prognostic benefit in MBL and early-stage chronic lymphocytic leukemia (CLL). We evaluated local practices of expanded diagnostics in the MBL/CLL population to assess (1) adherence to international workshop on CLL (iwCLL) guidelines with respect to additional cytogenetic testing and (2) anticipated costs of expanded diagnostic testing.
A retrospective chart review was conducted on all patients who underwent flow cytometry testing at our center for a suspected hematologic disorder between 2016 and 2021. Patients were subdivided into CLL, high-count MBL, and low-count MBL, and cytogenetic/molecular testing numbers were calculated as well as associated costs.
Of 974 patients who underwent flow cytometry testing, 100 had CLL, 49 had high-count MBL, and 5 had low-count MBL. Cytogenetic testing was performed in 54/100 CLL, 2/49 high-count MBL, and 0/5 low-count MBL patients. Most testing occurred in symptomatic CLL patients (38/54) especially after the 2018 iwCLL guideline changes. The estimated cost of cytogenetic testing for the 56 patients tested was $27 600. Expanding testing to all patients would have incurred an additional $87 900 during the study period.
This retrospective study shows high adherence to iwCLL guidelines for the investigation of early-stage CLL/MBL, especially after the 2018 guideline changes. Future studies are needed to weigh the benefits of improved prognostication against resources/costs required by expanded cytogenetics/molecular testing in these common hematologic conditions.

© 2025 The Author(s). International Journal of Laboratory Hematology published by John Wiley & Sons Ltd.