Monocytes: High & Low Levels (& Reference Ranges)

Monocytes are a type of leukocyte (white blood cell) that are part of the immune system. They defend the body from foreign invaders and infectious agents, like viruses and bacteria. 

Monocytes are produced in the bone marrow and circulate for a limited amount of hours in the blood. They usually travel to other tissues and undergo a differentiation process to become macrophages.

Monocyte levels can be evaluated in a CBC (complete blood count) which outlines the quantity of defense cells in the body. This blood test identifies whether levels are normal, high or low. 

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High monocytes

An increased amount of monocytes (also known as monocytosis) is usually a sign of: 

  • Chronic infections, like tuberculosis
  • Ulcerative colitis 
  • Protozoan infections
  • Hodgkin‘s disease 
  • Myelomonocytic leukemia 
  • Multiple myeloma
  • Autoimmune diseases, like lupus and rheumatoid arthritis 

High monocyte levels normally do not cause symptoms and are usually only identified through a blood test. However, patients may present with symptoms that are related to the underlying cause of monocytosis. Symptoms should be assessed and treated as directed by a doctor. 

Low monocytes

Low monocyte levels (also known as monocytopenia) are normally a sign of a weakened immune system. This can happen with blood infections, chemotherapy treatment and bone marrow disease, like aplastic anemia and leukemia. 

In addition, low monocytes can occur with skin infections, corticosteroid use, and HPV infections. 

It is rare for monocyte levels to be close to 0. Patients with severely low monocytes may be investigated for monoMAC syndrome. This a genetic illness characterized by a lack of monocyte production in the bone marrow. This condition can be treated with medications to combat infections, like antibiotics, as well as a bone marrow transplant to cure this genetic disease. 

Reference ranges

Reference ranges for monocytes vary depending on each laboratory. Usually, monocytes makeup 2 to 10% of total leukocytes in a blood sample, which equates to about 200 to 1000 monocytes per microliter (µl) of blood.

In general, abnormal levels do not directly cause symptoms. Patients, instead, will notice symptoms of the the condition causing the changes to monocyte levels. Patients may only be aware of abnormal levels after performing a blood test.