ICSH Recommendations for Monocyte Cell Lineage Morphologic Identification, Nomenclature Harmonization, and Utilization as a Biomarker.

Monocytes are key components of the Mononuclear Phagocyte System, crucial in immune defense, inflammation, and tissue repair. Accurate identification and classification of monocyte lineage cells are essential for diagnosing both reactive and clonal hematologic disorders. However, morphological criteria and nomenclature inconsistencies have hindered reproducibility, particularly with the rise of automated and AI-driven diagnostic tools. The International Council for Standardization in Haematology (ICSH) Monocyte Working Group (MWG) was convened to establish standardized morphological definitions, harmonize nomenclature for monocytes and their precursors, and evaluate their clinical utility as biomarkers. Data were collected from global laboratories, combined with an extensive literature review and consensus feedback from the ICSH General Assembly. The MWG proposes a three-category morphological classification: blasts and blast equivalents (monoblasts and promonocytes), immature monocytes, and mature monocytes. The recommendations reaffirm the importance of morphological analysis, cytochemical staining, and flow cytometry for accurate diagnosis. Emerging automated parameters, such as monocyte distribution width (MDW), and ratios like lymphocyte/monocyte (LMR) and neutrophil/monocyte (NMR), are also recognized as valuable adjunctive biomarkers. Cytogenetic and molecular results may also impact the utilization of monocytes as a biomarker. These harmonized recommendations aim to improve diagnostic accuracy, support the development of machine learning tools, and facilitate consistent reporting across laboratories worldwide.