Neutrophilic leukocytosis is an abnormally high number of neutrophils 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 may do blood tests, including a complete blood count, if people have symptoms, such as prolonged fever, weight loss, or fatigue. When doctors discover an increased number of neutrophils on the complete blood count, 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 biopsy—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 June 2008 by Mary Territo, MD