Merck Manual

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Mary Territo

, MD, David Geffen School of Medicine at UCLA

Last full review/revision Aug 2021| Content last modified Aug 2021
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Monocytopenia is a reduction in blood monocyte count to < 500/mcL (< 0.5 × 109/L). Risk of certain infections is increased. It is diagnosed by complete blood count with differential. Treatment with hematopoietic stem cell transplantation may be needed.

Monocytes migrate into the tissues where they become macrophages, with specific characteristics depending on their tissue localization.

Monocytopenia can increase the risk of infection, and it can indicate poor prognosis in patients with acetaminophen-induced hepatic damage and thermal injuries. Peripheral blood monocytopenia does not usually indicate a decrease in tissue macrophages; in some cases it can be associated with impaired granuloma formation in response to infections.

Monocytopenia can result from

Transient monocytopenia can occur with endotoxemia, hemodialysis, or cyclic neutropenia.

Monocytopenia due to GATA2 mutation

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 Lymphocytopenia Lymphocytopenia is a total lymphocyte count of 1000/mcL ( 1 × 109/L) in adults or 3000/mcL ( 3 × 109/L) in children 2 years. Sequelae include opportunistic infections and an increased risk of... read more (mainly natural killer and B cells), or pancytopenia.

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 avium complex (MAC) or other nontuberculous mycobacterial infections are common (MonoMAC syndrome). Fungal infections (ie, histoplasmosis Histoplasmosis Histoplasmosis is a pulmonary and hematogenous disease caused by Histoplasma capsulatum; it is often chronic and usually follows an asymptomatic primary infection. Symptoms are those of pneumonia... read more Histoplasmosis , aspergillosis Aspergillosis Aspergillosis is an opportunistic infection that usually affects the lower respiratory tract and is caused by inhaling spores of the filamentous fungus Aspergillus, commonly present in the environment... read more Aspergillosis ) also are typical. Infections with human papillomavirus Human Papillomavirus (HPV) Infection Human papillomavirus (HPV) causes warts. Some types cause skin warts, and other types cause raised or flat genital warts (lesions of the skin or mucous membranes of the genitals). Infection... read more Human Papillomavirus (HPV) Infection (HPV) may occur with subsequent risk of progression to secondary cancers. There is a high risk of progression to hematologic disorders (myelodysplasia Myelodysplastic Syndrome (MDS) The myelodysplastic syndrome (MDS) is group of disorders typified by peripheral cytopenia, dysplastic hematopoietic progenitors, a hypercellular or hypocellular bone marrow, and a high risk... read more , acute myeloid leukemia Acute Myeloid Leukemia (AML) In acute myeloid leukemia (AML), malignant transformation and uncontrolled proliferation of an abnormally differentiated, long-lived myeloid progenitor cell results in high circulating numbers... read more Acute Myeloid Leukemia (AML) , chronic myelomonocytic leukemia, lymphomas Overview of Lymphoma Lymphomas are a heterogeneous group of tumors arising in the reticuloendothelial and lymphatic systems. The major types are Hodgkin lymphoma and non-Hodgkin lymphoma (see table Comparison of... read more ) with a resulting poor prognosis.

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