Corticosteroids, specifically like prednisone or dexamethasone, commonly cause an elevated white blood cell (WBC) count in blood tests. This phenomenon is an expected pharmacological effect of the drug, not necessarily a sign of a new infection or disease. The increase in the total count, known as leukocytosis, can occur within hours of starting the medication and is directly related to the dose administered. Understanding this effect is important because it can lead to misinterpretation of blood results.
The Direct Effect on White Blood Cell Counts
The total white blood cell count is a sum of several different types of immune cells, and corticosteroids do not affect all of them equally. The overall increase in the total WBC count is predominantly driven by a significant rise in neutrophils, which are the most abundant type of white blood cell. This specific increase in neutrophils is called neutrophilia.
While neutrophils increase, other types of white blood cells typically decrease in number. Corticosteroids commonly cause a reduction in the circulating levels of lymphocytes (lymphopenia). There is also a distinct decrease in eosinophils and monocytes (eosinopenia and monocytopenia, respectively). The magnitude of the change can be substantial, with the total WBC count often increasing by approximately 4,000 cells per microliter of blood. However, there is considerable individual variability, and some people may experience counts exceeding 20,000 cells per microliter.
The Biological Mechanism of Action
The elevated white blood cell count does not mean the body is producing new cells, but rather that the distribution of existing cells is being altered. The primary mechanism responsible for this shift is a process called demargination. Neutrophils normally adhere loosely to the inner walls of blood vessels in the marginal pool. Corticosteroids cause these cells to detach from the vessel walls and move into the main flow of the circulating blood, which is the pool measured in a blood test.
Delayed clearance of neutrophils from the bloodstream is another major factor. Steroids reduce the ability of neutrophils to migrate out of the blood vessels and into the tissues where they normally perform their immune functions. Furthermore, corticosteroids delay the programmed cell death (apoptosis) of neutrophils, which extends their lifespan. A minor contributing factor is the increased release of neutrophils from the bone marrow storage pool into the blood.
Interpreting Elevated White Blood Cell Counts
Steroid-induced leukocytosis creates a challenge for medical professionals when trying to determine if a patient has a true infection. A bacterial infection typically triggers the bone marrow to rapidly release immature neutrophils, known as “bands,” into the bloodstream. This finding is called a “left shift” and is a strong indicator of an active bacterial process.
In contrast, the leukocytosis caused by corticosteroids usually consists predominantly of mature neutrophils and generally does not show a significant left shift. This difference in the cell maturity profile helps a physician distinguish between an expected drug effect and a genuine infection.
Corticosteroids also suppress the classic signs of inflammation, such as fever and swelling, which normally accompany an infection. This effect can potentially mask a serious underlying infection, even if the white blood cell count is elevated. Physicians must interpret the blood test results in the context of the patient’s overall clinical condition and symptoms.