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In the clinical diagnosis of monocytosis or monocytopenia, the blood monocyte pool is considered as a single entity. The monocyte participates in virtually all inflammatory and immune reactions, and, thus, its concentration in the blood may be increased in many such conditions, including autoimmune diseases, gastrointestinal disorders, sarcoidosis, and certain viral and bacterial infections. Monocytosis, a consistent increase in the blood absolute monocyte count to more than 0.8 × 109/L (800/μL) in an adult, may occur in some patients with cancer and several unrelated conditions, such as postsplenectomy states, inflammatory bowel disease, and some chronic infections (eg, bacterial endocarditis, tuberculosis, and brucellosis). The inconsistency and unpredictability in the blood monocyte concentration among patients with the same disease is a function of its relatively small blood pool size, the damping effect of a large tissue pool, its relatively short circulation time, and the complexity of effectors in the relevant cytokine network that can influence the monocyte response. The most striking increase in blood monocyte concentration occurs with hematopoietic malignancies, especially clonal monocytosis, and monocytic or myelomonocytic leukemia. Depression, myocardial infarction, parturition, thermal injuries, and marathon competition are closely associated with monocytosis. Monocytopenia is notable in patients with aplastic anemia or hairy cell leukemia as a feature of pancytopenia. Although other cytopenias accompany the monocytopenia, the latter contributes significantly to the predisposition to infection, and in hairy cell leukemia it is an aid to diagnosis because of its constancy. The MonoMAC syndrome, the result of inherited (autosomal recessive) or acquired GATA2 mutations, is associated with extreme monocytopenia (amonocytosis). Table 70–1 is a comprehensive list of causes of monocytosis and monocytopenia.

Acronyms and Abbreviations

CD, cluster of differentiation; G-CSF, granulocyte colony-stimulating factor; GM-CSF, granulocyte-monocyte colony-stimulating factor; IL, interleukin; LPS, lipopolysaccharide; M-CSF, monocyte colony-stimulating factor; MDS, myelodysplastic syndrome; MonoMAC syndrome, monocytopenia and Mycobacterium avium complex infection syndrome; NK, natural killer.

TABLE 70–1.Disorders Associated with Monocytosis

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