Corticosteroid medications are widely prescribed for their powerful anti-inflammatory and immunosuppressive effects. A common and expected laboratory finding is an elevated white blood cell (WBC) count. This phenomenon, known as steroid-induced leukocytosis, can cause concern if mistaken for a sign of infection. Understanding this rise is a normal physiological response helps correctly interpret blood test results, as it reflects immune cell redistribution rather than a new disease process.
How Steroids Elevate the White Blood Cell Count
The primary mechanism by which corticosteroids increase the measured WBC count is known as demargination. Neutrophils normally adhere to the inner walls of blood vessels in a “marginal” pool. Steroids cause these cells to detach and enter the circulating blood pool, significantly increasing the number counted in a blood sample. This redistribution accounts for the majority of the observed rise.
Corticosteroids also inhibit the ability of neutrophils to migrate out of the bloodstream and into tissues. By slowing this movement, more cells remain in circulation for a longer period, contributing to the elevated count. Furthermore, these drugs delay the natural process of programmed cell death (apoptosis) for neutrophils, extending their lifespan in the blood.
The observed leukocytosis is predominantly a rise in neutrophils. Corticosteroids can also stimulate the release of mature neutrophils from the bone marrow storage pools. This multi-faceted action—redistribution, inhibited migration, and delayed destruction—results in a higher-than-normal reading.
Typical Duration of Steroid-Induced Elevation
The length of time the white blood cell count remains elevated depends on the steroid’s half-life, the dose administered, and the duration of therapy. Following the start of a regimen, the WBC count typically begins to rise within a few hours and reaches its peak elevation within 4 to 48 hours. The magnitude of this rise is dose-dependent; higher doses lead to a more significant increase.
For individuals taking a short course of steroids, the WBC count generally begins to normalize quickly after the last dose. The count usually starts to decline within one to two days of stopping the medication. In most cases, the WBC count will return entirely to the patient’s baseline range within three to five days after the cessation of short-term treatment.
If treatment involves prolonged or chronic corticosteroid use, the WBC count will likely remain elevated for the entire duration of the therapy. The count may reach a maximal, steady-state value within the first two weeks of treatment. After discontinuing prolonged therapy, normalization can take longer than a short course, sometimes requiring a week or more to fully revert to the pre-treatment baseline. Interpretation of a WBC count must always be done in the context of the patient’s specific medication schedule.
When to Suspect an Underlying Infection
Distinguishing between a harmless steroid effect and a true infection is important because corticosteroids can mask typical signs of illness. The differential white blood cell count is the most informative tool for this distinction. Steroid-induced leukocytosis primarily causes an increase in mature neutrophils, a phenomenon known as neutrophilia.
A true bacterial infection often triggers a distinct pattern known as a “left shift,” involving a higher percentage of immature neutrophils, called band forms, in the blood. Finding more than six percent band forms, along with features like toxic granulation, is a strong indicator of an active infection and is rarely seen with steroid-induced elevation alone. Additionally, steroids often cause a decrease in other infection-fighting cells, specifically lymphocytes and eosinophils.
Patients should be mindful of clinical signs that indicate a problem beyond the expected drug effect. While a high WBC count from a steroid is usually asymptomatic, a true infection presents with systemic indicators. These warning signs include the development of a fever, new or worsening localized pain, or the presence of purulent discharge. If these symptoms appear, particularly a fever, immediate contact with a healthcare provider is warranted for a thorough evaluation.