Neutrophilic leukocytosis is an abnormally high number of neutrophils (a type of white blood cell) in the blood.

Neutrophils help the body fight infections and heal injuries. Neutrophils may increase in response to a number of conditions or disorders. 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 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. Myelocytic leukemias can lead to an increased number of immature or mature neutrophils in the blood.

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 bloodstream. Immature neutrophils in the bloodstream may indicate the presence of a disorder in the bone marrow, such as leukemia. When immature neutrophils are found in the bloodstream, doctors usually take a sample of bone marrow (bone marrow examination—see Symptoms and Diagnosis of Blood Disorders: Bone Marrow Examination).

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.

Last full review/revision January 2013 by Mary Territo, MD