Merck Manual

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Neutrophilic Leukocytosis


Mary Territo

, MD, David Geffen School of Medicine at UCLA

Last full review/revision Jan 2020| Content last modified Jan 2020
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Neutrophilic leukocytosis is an abnormally high number of neutrophils (a type of white blood cell) in the blood.

Neutrophils are a type of white blood cell that help the body fight infections and heal injuries. Neutrophils may increase in response to a number of conditions or disorders, including

  • Infections

  • Injuries

  • Inflammatory disorders

  • Certain drugs

  • Certain leukemias

The most common cause of an increased number of neutrophils is

  • The normal response of the body to an infection

In many instances, the increased number of neutrophils is a necessary reaction by the body, as it tries to heal or ward off an invading microorganism or foreign substance. Infections by bacteria, viruses, fungi, and parasites may all increase the number of neutrophils in the blood.

The number of neutrophils may rise in people who have an injury, such as a hip fracture or burn. Inflammatory disorders, including autoimmune disorders such as rheumatoid arthritis, can cause an increase in the number and activity of neutrophils. Some drugs, such as corticosteroids, also lead to an increased number of neutrophils in the blood. Myeloid leukemias can lead to an increased number of immature or mature neutrophils in the blood.

A high number of neutrophils may not cause symptoms. However, people often have symptoms of the disorder that is causing the increased number of neutrophils.

Hyperviscosity syndrome

Very rarely, in people with leukemia, extremely high levels of immature neutrophils (more than 100,000 cells per microliter of blood [100 × 109 per liter]) can cause the blood to become too thick and cause breathing problems, stroke, and death. This condition is a medical emergency and requires hospitalization so fluids can be given by vein and drugs to reduce the white blood cell count (hydroxyurea and chemotherapy drugs) can be given. Sometimes, a type of blood-filtering treatment (leukapheresis) is used to remove the white blood cells from the blood.


  • Blood tests

Doctors often do a blood test called a complete blood count. This test may be done for many different symptoms, including signs of infection (such as fever, cough, or abdominal pain), or signs of chronic illness (such as weight loss or fatigue).

If doctors discover an increased number of neutrophils and there is no clear reason, such as an obvious infection, a blood sample is viewed under a microscope to determine if immature neutrophils (myeloblasts) are leaving the bone marrow and entering the blood. Immature neutrophils in the blood may indicate the presence of a disorder in the bone marrow, such as leukemia.

When immature neutrophils are found in the blood, doctors usually take a sample of bone marrow (bone marrow examination).


  • Treatment of the underlying disorder

An increased number of mature neutrophils in the blood is not usually a problem in itself. Therefore, doctors focus on treating the condition or disorder that caused the number of neutrophils to increase.

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