8 Monocytes.MONO

Monocytes — precursors of tissue macrophages: causes of monocytosis, when persistent values need a haematologist.

Monocytes are the largest white blood cells. They circulate in the blood for one to three days and then move into tissues, where they become macrophages and dendritic cells. These "cleaner" cells engulf bacteria, fungi and dead cells, present antigens to lymphocytes and release signalling molecules (cytokines) that regulate inflammation and repair. Monocytes are reported as a percentage of white cells and as an absolute count.

Complete blood countWhite cell differentialInflammation

Share of white cellsabout 2–8 %

Absolute count0.2–0.8 ×10⁹/L

Monocytosis> 1.0 ×10⁹/L

Life in blood1–3 days

Figure 1. Monocytes make up only a small share of circulating white cells, but most of the body's tissue macrophages derive from them.

High monocytes — monocytosis

Group	Causes
Infections	Recovery phase of acute infection, tuberculosis, infective endocarditis, brucellosis, syphilis, infectious mononucleosis, malaria
Chronic inflammation	Inflammatory bowel disease, sarcoidosis, rheumatoid arthritis, lupus
Other	Recovery of the bone marrow after chemotherapy, after splenectomy, corticosteroids, stress
Blood cancers	Chronic myelomonocytic leukaemia (persistent monocytes ≥ 1 ×10⁹/L and ≥ 10 % of white cells), acute monocytic leukaemia, some lymphomas

A moderate, temporary increase is common and usually reactive. Persistent monocytosis for more than three months, especially in older people and together with anaemia or low platelets, should be evaluated by a haematologist.

Low monocytes — monocytopenia

A low count is seen after corticosteroid treatment, chemotherapy, in aplastic anaemia and some leukaemias (hairy-cell leukaemia), and in rare inherited conditions such as GATA2 deficiency. Isolated slight decreases are usually not significant.

When to see a doctor

Consult a doctor if monocytes stay high on repeated tests, or if they are abnormal together with fever, weight loss, night sweats, an enlarged spleen or other abnormal blood counts.

References

Blumenreich MS. The white blood cell and differential count. In: Walker HK, Hall WD, Hurst JW, eds. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Boston: Butterworths; 1990. Chapter 153. ncbi.nlm.nih.gov/books/NBK261
Arber DA, Orazi A, Hasserjian R, et al. The 2016 revision to the World Health Organization classification of myeloid neoplasms and acute leukemia. Blood. 2016;127(20):2391–2405. doi:10.1182/blood-2016-03-643544
Chabot-Richards DS, George TI. Leukocytosis. Int J Lab Hematol. 2014;36(3):279–288. doi:10.1111/ijlh.12212

This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.

Published on 25 April 2024