Neutrophils are the most abundant type of white blood cell, or leukocyte, acting as the immune system’s first line of defense. Their primary role is to combat infections, especially those caused by bacteria. As first responders to injury or infection, they neutralize invading pathogens and begin the tissue repair process.
The Neutrophil Percentage on a Blood Test
A Complete Blood Count (CBC) with a differential is a common blood test that provides detailed information about your blood cells. Part of this test measures the neutrophil percentage, which is the proportion of neutrophils relative to all other white blood cells in the sample. This figure offers a snapshot of your immune system’s activity at a specific moment in time.
For healthy adults, neutrophils make up 40% to 60% of all white blood cells, though this range can vary by laboratory. Reference ranges also differ for children due to their developing immune systems. Since levels can fluctuate, a single reading may not provide a complete picture.
The neutrophil percentage is determined by an automated analyzer or a manual review of a blood smear. The automated method provides a rapid and precise measurement by counting thousands of cells. A manual count, where a technician visually identifies cells under a microscope, may be used to investigate abnormal findings.
This percentage is a relative value, meaning it can be influenced by changes in other white blood cell populations. For example, if another type of white blood cell increases, the neutrophil percentage might decrease even if the actual number of neutrophils remains stable. This is why healthcare providers consider this percentage alongside other values.
Causes of a High Neutrophil Percentage
An elevated neutrophil percentage, known as neutrophilia, is most frequently caused by an acute bacterial infection, inflammation, stress, or certain medications. The most common reasons include:
- Bacterial infections: When bacteria enter the body, the bone marrow increases the production and release of neutrophils to fight the invaders. Common infections that trigger this response include pneumonia, urinary tract infections, and skin infections.
- Inflammation: Non-infectious inflammation from physical injuries like burns or surgery can also result in a high count. Chronic inflammatory conditions, such as rheumatoid arthritis or inflammatory bowel disease, can cause persistently high levels.
- Stress: Physical or emotional stress from intense exercise, a seizure, or significant distress can trigger the release of neutrophils held in reserve. This response is short-lived, lasting about 20 to 30 minutes after the stimulus ends.
- Medications and other conditions: Certain medications, such as corticosteroids like prednisone, can increase neutrophil counts. Less commonly, a persistently high percentage without infection or inflammation may indicate a condition like leukemia.
Causes of a Low Neutrophil Percentage
A low neutrophil percentage, or neutropenia, indicates a reduced ability to fight off infections. Common causes include:
- Viral infections: Viruses such as influenza, Epstein-Barr, and hepatitis can temporarily suppress the bone marrow’s production of neutrophils. Levels return to normal after the infection resolves.
- Cancer treatments: Chemotherapy and radiation therapy target rapidly dividing cells, which includes cancer cells and the healthy neutrophil-producing cells in the bone marrow. This creates a risk for serious infections during care.
- Autoimmune disorders: In conditions like lupus or rheumatoid arthritis, the immune system may produce antibodies that attack and destroy neutrophils. This increases susceptibility to bacterial infections.
- Nutritional deficiencies and bone marrow disorders: A lack of vitamin B12, folate, or copper can impair neutrophil production. A low count can also be a sign of a disorder like aplastic anemia, where the bone marrow fails to produce enough blood cells.
Why the Absolute Neutrophil Count Matters
While the neutrophil percentage is a relative measure, healthcare professionals focus on the Absolute Neutrophil Count (ANC) for a more accurate assessment. The ANC is the actual number of neutrophils in a specific volume of blood, measured in cells per microliter. It is calculated by multiplying the total white blood cell (WBC) count by the neutrophil percentage.
The distinction between the percentage and the absolute count is important. A person might have a normal neutrophil percentage, but if their total WBC count is very low, their absolute number of neutrophils could be low. For example, a 50% neutrophil percentage seems normal, but if the total WBC count is only 1,000 cells/µL, the ANC would be just 500 cells/µL, indicating severe neutropenia and a high risk of infection.
Physicians rely on the ANC to make clinical decisions, especially for managing infection risk. An ANC below 1,000 cells/µL is considered neutropenia, and an ANC below 500 cells/µL is classified as severe neutropenia, which increases infection risk. This measurement helps determine if a patient on chemotherapy needs to delay treatment or receive medications to boost white blood cell production.
Ultimately, the ANC gives a clearer picture of the body’s capacity to fight bacterial infections than the percentage alone. It contextualizes the neutrophil level within the broader landscape of the total white blood cell count. By focusing on the absolute number, clinicians can better gauge a patient’s immune status and take appropriate preventative or therapeutic actions.