From the Guidelines
Leukocytosis is most commonly caused by bacterial infections, and an elevated total band count (>1500/mm3) has the highest likelihood ratio for detecting documented bacterial infection, as shown in a study published in Clinical Infectious Diseases 1. The causes of leukocytosis can be categorized into several groups, including:
- Infections (bacterial, viral, fungal, or parasitic)
- Inflammation
- Tissue damage
- Certain medications (corticosteroids, epinephrine, lithium)
- Leukemia and other blood disorders
- Allergic reactions
- Stress
- Exercise Physiologically, leukocytosis occurs when the body increases white blood cell production in the bone marrow and releases stored cells into circulation in response to various stimuli. The type of white blood cells elevated can help determine the underlying cause, such as:
- Neutrophilia, which often indicates bacterial infection or inflammation
- Lymphocytosis, which suggests viral infections or certain leukemias
- Eosinophilia, which points to allergies or parasitic infections
- Monocytosis, which may indicate chronic infections or certain blood disorders According to a study published in 2009, an elevated WBC count (≥ 14,000 cells/mm³) or a left shift (percentage of band neutrophils or metamyelocytes, ≥ 16%; or total band neutrophil count, ≥ 1500 cells/mm³) warrants a careful assessment for bacterial infection in any patient with suspected infection, with or without fever 1. It's also important to note that the severity and duration of leukocytosis depend on the underlying cause, and treatment should be directed at addressing this primary condition rather than the elevated white blood cell count itself, as recommended by the Infectious Diseases Society of America 1. In recent years, studies have continued to support the idea that leukocytosis is a significant indicator of underlying infection or disease, and that prompt evaluation and treatment are essential to prevent complications and improve outcomes, as shown in a study published in 2023 1 and 2025 1.
From the Research
Causes of Leukocytosis
- Infections or inflammatory processes are the most common causes of leukocytosis, leading to an increase in white blood cells, predominantly polymorphonuclear leukocytes and less mature cell forms (the "left shift") 2
- Physical stress, such as seizures, anesthesia, or overexertion, and emotional stress can also elevate white blood cell counts 2
- Medications commonly associated with leukocytosis include corticosteroids, lithium, and beta agonists 2
- Increased eosinophil or basophil counts, resulting from a variety of infections, allergic reactions, and other causes, can lead to leukocytosis in some patients 2
- Primary bone marrow disorders, such as acute leukemias, chronic leukemias, and myeloproliferative disorders, can also cause leukocytosis 2, 3, 4, 5, 6
Secondary Causes of Leukocytosis
- Infections and chronic inflammatory conditions are common causes of secondary leukocytosis 3, 4, 5, 6
- Surgery, exercise, trauma, and emotional stress can cause an acute leukocytosis 4
- Certain medications, asplenia, smoking, obesity, and chronic inflammatory conditions can also cause leukocytosis 4
Malignant Causes of Leukocytosis
- Acute leukemias, chronic leukemias, and myeloproliferative disorders are primary bone marrow disorders that can cause leukocytosis 2, 3, 4, 5, 6
- Lymphoproliferative disorders, such as lymphomas and lymphoid leukemias, can also cause leukocytosis 5, 6
- Myeloid malignancies, such as myeloid leukemias and myeloproliferative neoplasms, can cause myeloid leukocytosis 5, 6