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By Mary Territo, MD, Emeritus Professor of Medicine, Division of Hematology and Oncology, David Geffen School of Medicine at UCLA

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Monocytopenia is a reduction in blood monocyte count to < 500/μL. Risk of certain infections is increased. It is diagnosed by CBC with differential. Treatment with hematopoietic stem cell transplantation may be needed.

Monocytopenia frequently occurs with chemotherapy-induced myelosuppression. A severe deficiency or absence of monocytes can occur in patients with mutations of the hematopoietic transcription factor gene, GATA2. Dendritic cells are decreased, and there may also be lymphocytopenia (mainly natural killer and B cells).

Despite near-absence of circulating monocytes, tissue macrophages are usually preserved. Also, immunoglobulin levels are usually normal even when circulating B cells are depressed.

Bone marrow is hypocellular and can show fibrosis and multilineage dysplasia. Karyotypic abnormalities, including monosomy 7 and trisomy 8, may be present.

Infections with Mycobacterium aviancomplex (MAC) or other nontuberculous mycobacterial infections are common (MonoMAC syndrome). Fungal infections (ie, histoplasmosis, aspergillosis) also are typical. Infections with human papillomavirus (HPV) may occur with subsequent risk of progression to secondary cancers.

There is a high risk of progression to hematologic disorders (myelodysplasia, acute myelogenous leukemia, chronic myelomonocytic leukemia, lymphomas) with a resulting poor prognosis.

Unvaccinated patients should be given HPV vaccination. Any infections are treated with appropriate antimicrobials. Allogeneic hematopoietic stem cell transplantation should be considered for symptomatic patients.