Neutrophils are white blood cells produced in the bone marrow that circulate in the bloodstream, where they account for about 50% to 70% of the total white blood cell count in healthy adults. An elevated neutrophil count, a condition known as neutrophilia, is a common finding on a standard blood test, often signaling that the immune system is actively responding to a stimulus. Since this increase is a symptom of an underlying process, not a disease itself, understanding what triggers neutrophilia is the first step in interpreting the result.
The Essential Function of Neutrophils
Neutrophils are part of the innate immune system, acting as immediate responders to tissue damage or invading microorganisms. They constantly patrol the bloodstream and are the first immune cells to migrate from the blood into affected tissue when infection or injury is detected.
Their primary mechanism for neutralizing threats is a process called phagocytosis, where the neutrophil physically engulfs and digests foreign particles, such as bacteria or cellular debris. Neutrophils also release antimicrobial compounds stored in their granules and form Neutrophil Extracellular Traps (NETs). NETs are web-like structures made of DNA and proteins that trap and kill pathogens outside the cell.
The body maintains large reserves of neutrophils in the bone marrow, ready for immediate release. Since their lifespan in circulation is relatively short, often less than a day, the bone marrow constantly produces a massive number of new cells. This rapid production and release system explains why their count rises quickly in response to physical stress or infection.
Primary Drivers of Neutrophilia
The most frequent reason for a high neutrophil count is an acute bacterial infection, such as pneumonia, appendicitis, or a urinary tract infection. In these cases, the bone marrow ramps up production and releases stored cells to combat the invading bacteria, causing a noticeable increase in the absolute neutrophil count. This elevation is a direct and appropriate biological response to the presence of a pathogen.
Neutrophilia also commonly occurs in response to significant inflammation or tissue damage, even without an active infection. Conditions like burns, major surgery, trauma, or inflammatory disorders such as gout, rheumatoid arthritis, or inflammatory bowel disease can all trigger the release of neutrophils. The body perceives tissue damage as a threat requiring immune cleanup and repair, leading to temporary elevation.
Certain medications and physical stressors can cause a temporary, “shift” neutrophilia by altering the distribution of existing cells. Corticosteroids, for instance, cause neutrophils normally stuck to blood vessel walls to enter the circulating bloodstream, thereby increasing the measured count. Intense physical exertion or emotional stress can also cause a temporary rise in neutrophil levels.
Bone Marrow Disorders
While less common, persistent and extreme neutrophilia can sometimes point toward a disorder of the bone marrow itself, known as a myeloproliferative neoplasm. These conditions, such as Chronic Myelogenous Leukemia (CML), involve the unregulated overproduction of white blood cells, including neutrophils. In these situations, the neutrophil count is often extremely high and does not resolve once acute causes are treated.
Interpreting Your Neutrophil Count
Neutrophil levels are measured as part of a routine Complete Blood Count (CBC) with a differential, a test that quantifies the different types of cells in the blood. The most meaningful measurement is the Absolute Neutrophil Count (ANC), which is the true number of neutrophils per volume of blood, typically reported as cells per microliter. The ANC is calculated by multiplying the total white blood cell count by the percentage of neutrophils and band cells (immature neutrophils).
For a healthy adult, the normal ANC range typically falls between 1,500 and 8,000 cells/μL, though laboratory reference ranges can vary slightly. Neutrophilia is defined as an ANC exceeding the upper limit of the normal range, often around 7,500 to 8,000 cells/μL. Counts persistently above 20,000 cells/μL are considered significant and warrant prompt investigation.
It is important to look at the ANC rather than just the neutrophil percentage, as the percentage alone can be misleading if the total white blood cell count is very low or very high. An increase in band cells (immature neutrophils), sometimes called a “left shift,” suggests the bone marrow is rapidly pushing out cells in response to an acute demand, such as a serious infection.
When to Seek Further Medical Attention
A single elevated neutrophil result must be considered within the context of a person’s overall health, symptoms, and medical history. Since most cases of neutrophilia are temporary responses to common, self-limiting issues, a mild elevation without accompanying symptoms may simply be monitored with a repeat test. The underlying cause must be identified and addressed, not the high count itself.
A healthcare provider begins the diagnostic process by taking a thorough history and performing a physical examination to look for signs of infection or inflammation. They evaluate for symptoms such as fever, unexplained weight loss, night sweats, or localized pain, which could point toward a more serious cause. If the cause is not immediately clear, or if the elevation is severe or persistent, further testing may be recommended.
Follow-up tests might include blood cultures to detect bacteria, imaging studies (like X-rays or CT scans) to look for abscesses or deep inflammation, or a blood smear examination. Urgent investigation is warranted for extreme elevations (ANC over 20,000 cells/μL) or for any persistent elevation lacking a clear explanation. These severe findings may require consultation with a hematologist to rule out rare blood disorders.