Monocytosis Causes and Diagnostic Approach

Introduction to Monocytosis

Monocytosis has two major categories of causes: reactive (benign) conditions and clonal hematologic malignancies, particularly chronic myelomonocytic leukemia (CMML), which must be systematically excluded in cases of persistent elevation 1

Chronic infections, such as tuberculosis and bacterial endocarditis, are common infectious causes of monocytosis 1
Adult-onset Still's disease presents with marked leukocytosis, including monocytosis, often with white blood cell counts >15×10⁹ cells/L 2
Inflammatory bowel disease (Crohn's disease and ulcerative colitis) can cause chronic monocyte elevation 3
Chronic inflammatory conditions trigger monocyte expansion through persistent cytokine stimulation 2, 4
Atherosclerosis and coronary artery disease are associated with elevated monocyte counts, as monocytes play a pathogenic role in plaque formation 4
Tissue injury and chronic inflammation of any cause can lead to monocytosis 4, 5

CMML is the primary hematologic malignancy causing persistent monocytosis and requires specific diagnostic criteria 1
WHO 2008 criteria for CMML include persistent peripheral blood monocytosis (>1×10⁹/L), absence of Philadelphia chromosome or BCR-ABL1 fusion gene, and <20% blasts in blood and bone marrow 1
Molecular mutations commonly found in CMML include TET2, SRSF2, ASXL1, and RAS 1

Persistent monocytosis without clear reactive cause mandates bone marrow evaluation 1
Absolute monocyte count >1×10⁹/L that persists beyond 3 months requires hematologic workup 1
Essential workup includes complete blood count with differential, peripheral blood smear examination, and bone marrow aspiration and biopsy with conventional cytogenetics 1
Molecular testing for BCR-ABL1 and mutations in TET2, SRSF2, ASXL1, and RAS genes is necessary 1