Steroids frequently cause leukocytosis, the medical term for an elevated white blood cell (WBC) count. This is a common and predictable side effect of anti-inflammatory glucocorticoids, such as prednisone and dexamethasone. While a high WBC count often signals an infection, the increase caused by these medications is a harmless physiological response, not a sign of disease. Understanding this distinction is important for interpreting routine blood tests during steroid treatment.
The Mechanism Behind Steroid-Induced Leukocytosis
Glucocorticoids elevate the white blood cell count primarily by redistributing existing cells rather than dramatically increasing their production. The most significant factor is demargination, which accounts for over half of the observed increase in circulating white blood cells. This process involves the detachment of neutrophils, a type of white blood cell, from the inner walls of blood vessels.
Normally, neutrophils adhere to the endothelial lining of the vessels, forming the marginal pool, and are not counted in a standard blood test. Steroids interfere with adhesion molecules, such as L-selectin, preventing the cells from sticking to the vessel walls. When the neutrophils detach, they spill into the general circulation, immediately raising the total measured WBC count.
Another major mechanism is the decreased clearance of these cells from the bloodstream. Glucocorticoids inhibit transmigration, the process by which neutrophils migrate out of the blood vessels and into tissues. By slowing this movement, more neutrophils remain circulating for a longer period, contributing to the elevated count.
The medications also delay the programmed death of neutrophils, known as apoptosis, extending their lifespan in the circulation. Steroids can stimulate the bone marrow to release a small number of new neutrophils, but the primary drivers remain demargination and delayed tissue entry. The total white blood cell count can begin to rise within a few hours of the first dose.
Distinguishing Steroid Effects from Infection
Distinguishing a steroid effect from a true infection requires looking at the specific types of white blood cells affected and the patient’s clinical presentation. Steroid-induced leukocytosis predominantly involves a dramatic rise in neutrophils, known as neutrophilia. Simultaneously, steroids often cause a decrease in other white blood cells, such as lymphocytes (lymphopenia) and eosinophils (eosinopenia).
In contrast, a bacterial infection typically causes a rise in neutrophils and triggers the release of immature neutrophils, or “band” cells, from the bone marrow. This imbalance is known as a “left shift” in the blood differential, a finding rare in steroid-caused leukocytosis. Infection-related leukocytosis may also show cellular changes, such as toxic granulation, which is absent with steroid use.
A person with steroid-induced leukocytosis is asymptomatic and does not exhibit signs of illness related to the elevated count. They should not develop a new fever, localized pain, or other systemic symptoms that accompany a bacterial infection. A high white blood cell count alongside clinical symptoms like fever, chills, or worsening localized pain strongly suggests a genuine infectious process.
Healthcare providers use the complete blood count with differential, which breaks down the percentages of each white blood cell type, to make this differentiation. The combination of high neutrophils, low lymphocytes, and the absence of a left shift suggests the elevation is an expected response to the medication. This analysis prevents misinterpreting the steroid effect as an infection requiring unnecessary antibiotics.
Monitoring and Consulting a Healthcare Provider
Steroid-induced leukocytosis is considered a benign finding that should not cause alarm when it is an isolated lab result. The white blood cell count typically begins to increase within hours of starting the medication. It may reach its maximum level within the first two weeks of continuous treatment and remain elevated for the entire duration of the therapy.
The leukocytosis is temporary, and the white blood cell count should return to the baseline level within one to two days after the steroid medication is stopped. If the leukocytosis is accompanied by new signs of illness, such as a fever, chills, or a sudden change in condition, contact a healthcare provider immediately. Steroids can mask the symptoms of a true underlying infection.
A medical consultation is warranted if the white blood cell count remains abnormally high weeks after discontinuing the steroid treatment. The healthcare provider will assess the patient’s symptoms and blood work to rule out other possible causes for the continued elevation.