Leucocytosis describes a higher-than-normal number of white blood cells (leukocytes) in the bloodstream. These immune system cells are produced in the bone marrow to defend the body against threats. An elevated count is a frequent finding on blood tests and often signifies a normal response to an infection or inflammation.
What Triggers High White Blood Cell Counts?
A high white blood cell count can be triggered by physiological demands not related to illness. Strenuous physical exercise, significant emotional stress, and pregnancy can all cause a temporary increase in circulating white blood cells. This is a natural, short-term response.
The most common cause is the body’s reaction to an infection. Bacterial, viral, fungal, and parasitic infections can all elevate white blood cell counts. Inflammatory conditions are another cause, including autoimmune disorders like rheumatoid arthritis and inflammatory bowel disease, as well as physical injuries such as burns or significant trauma.
Certain medications can increase white blood cell production, including corticosteroids, lithium, and some asthma drugs. The body may also produce more white blood cells in response to tissue death (necrosis), such as after a heart attack. Smoking is another factor that can lead to a chronically elevated count.
Disorders affecting the bone marrow can also result in high counts. Sometimes, the elevation shows the bone marrow is recovering after suppression. In more serious instances, it can indicate conditions like leukemia or myeloproliferative neoplasms, which involve abnormal blood cell production. These conditions cause a more sustained and higher elevation compared to reactive causes.
Diagnosing and Categorizing Leucocytosis
Leucocytosis is identified through a complete blood count (CBC), which measures the total number of white blood cells. A count above 11,000 cells per microliter is considered leucocytosis in most non-pregnant adults, though normal ranges vary. Age-specific reference ranges are important, as a much higher count can be normal for newborns.
A CBC is often performed with a differential, which breaks down the count into the five main types of white blood cells: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Identifying the elevated cell type helps narrow down the cause.
An increase in a specific cell type gives the condition its name. Neutrophilia (high neutrophils) is associated with bacterial infections, while lymphocytosis (high lymphocytes) often indicates a viral infection. Eosinophilia (high eosinophils) is linked to allergies or parasitic infections, and monocytosis (high monocytes) can be seen in various infections and some cancers.
When is Leucocytosis a Health Concern?
Leucocytosis is a sign, not a disease, and its importance depends on the context. Factors like the absolute count, the specific cell type elevated, and whether the condition is acute or chronic help determine its seriousness.
The presence of other symptoms is a large part of the assessment. An elevated count with fever, fatigue, or unexplained weight loss prompts a more thorough investigation than an isolated finding in a healthy person.
A transient and mild leucocytosis with a clear cause, like a common bacterial infection, is not a major concern. The white blood cell count is expected to return to normal as the underlying condition resolves.
Conversely, a persistently elevated or extremely high count without an obvious cause warrants further evaluation. Such findings may suggest more significant conditions like severe infections, chronic inflammatory diseases, or hematologic malignancies.
Addressing the Underlying Causes of Leucocytosis
Treatment is directed at the root cause of the elevated white blood cell count, not the count itself. If a bacterial infection is the cause, antibiotics are prescribed. If leucocytosis is a medication side effect, a provider may adjust the dosage or switch drugs. For inflammatory conditions, medications that reduce inflammation are used to control the disease and normalize the count.
Physiological leucocytosis, from stressors like intense exercise or pregnancy, requires no treatment. The white blood cell count returns to its normal baseline on its own.
In rare situations of an extremely high count (hyperleukocytosis), the blood can become too thick and impair circulation. This is most often seen with certain types of leukemia. In these uncommon cases, a procedure called leukapheresis may be used to directly lower the number of white blood cells.