A High Number of Leukocytes

When blood tests reveal you have a higher than normal number of white blood cells (WBCs), or leukocytes, this is known medically as leukocytosis. White blood cell counts usually fall between 4,000 and 11,000 cells per microliter. There are different types of white blood cells: neutrophils, lymphocytes, monocytes, eosinophils, basophils and even nondescript immature white blood cells known as blasts. An elevation of any one of these can cause leukocytosis, which is a fairly common laboratory finding. Additionally, WBC counts may be high for any number of reasons. Leukocytosis is a fairly common laboratory finding.

A blood test can tell you if you white blood cell count is normal. Credit: Jovanmandic/iStock/Getty Images

Causes of Leukocytosis

Elevated WBC counts may accompany different clinical scenarios that range from infection, inflammation, allergic reaction, malignancy, hereditary disorders, drugs, and other miscellaneous causes -- in addition to several normal bodily responses such as stress and exercise. Unless the cause is immediately obvious, your doctor will need more information to make sense of the finding, including potentially which WBC types are over-represented, as well as your entire history, including any presenting symptoms and physical findings.


Infection is the most common cause of elevated neutrophil counts. Most bacterial infections cause what is known as "neutrophilia with bandemia," where neutrophilia reflects the high count and bandemia refers to an excess of band cells, or immature white blood cells. However, inflammation without an infection – for instance, in rheumatoid arthritis or inflammatory bowel disease – can also produce neutrophilia, as can certain medications such as heparin, some seizure medications, steroids and others. Certain malignancies such as Hodgkin's lymphoma may produce neutrophilia as well.


Leukocytosis may also be due to high numbers of white blood cells known as eosinophils, or eosinophilia. Often eosinophilia can be linked to an allergic process or drug reaction. This includes asthma, hay fever, hives, atopic dermatitis and eczema, allergy to drugs, and other allergic conditions, as well as parasitic infections. Other illnesses and some skin conditions may also be associated with high WBC counts do to eosinophilia.

Monocytes and Basophils

High counts of monocytes are associated with certain infections such as tuberculosis, syphilis, infectious mononucleosis (mono), autoimmune diseases such as lupus, and certain malignancies. The blood cancers chronic myelomonocytic leukemia, monocytic leukemia, Hodgkin disease and other blood proliferative diseases may produce monocytosis. A high basophil count leading to leukocytosis is rare, but it can occur in chronic myelogenous leukemia.


Leukocytosis greater than 30,000 indicates a bone marrow abnormality and requires immediate medical attention. Levels higher than 100,000--a medical emergency requiring an immediate hematology consult--can cause brain infarction, lack of blood to the brain or hemorrhage.

Reviewed by: Tom Iarocci, M.D.

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