What Do White Blood Cell Counts Mean in Myelofibrosis?

In myelofibrosis, your white blood cell (WBC) count can provide clues about your blood-cell production and overall health. A low count (leukopenia) may reflect advanced disease, pooling of blood cells in an enlarged spleen, or a side effect of your treatments. A high count (leukocytosis) might reflect the disease itself, medications, or sometimes an infection.

Because these numbers fluctuate for many reasons, a single test result is just a snapshot. Your hematologist will look at the trend over time, the specific types of white cells (your “differential”), your current treatments, and your physical symptoms to understand what your numbers mean.

(Note on reading your lab report: Laboratories use different units. A count of 0.5 × 10^9/L is the same as 500 cells/µL, and a count of 25 × 10^9/L is the same as 25,000 cells/µL. Always use your specific laboratory’s reference ranges and consult your doctor.)

When Your Count is Low (Leukopenia and Neutropenia)

A low total white blood cell count (leukopenia) can happen for several reasons as myelofibrosis advances. As scar tissue (fibrosis) replaces healthy marrow, your body’s ability to produce normal blood cells may drop ^[1]. An enlarged spleen may also pool or sequester the healthy white cells you do make, removing them from circulation.

Treatments play a major role. Medications called JAK inhibitors (such as ruxolitinib and fedratinib) can cause myelosuppression, meaning they lower your blood counts ^[2][3][4]. Your doctor will monitor your labs and may adjust your dose or pause treatment if your counts fall too low. Never skip or stop your medication without your prescriber’s advice.

Infection Risk and Severe Neutropenia

The most important number for assessing bacterial infection risk is your Absolute Neutrophil Count (ANC). Neutrophils are the specific white blood cells that fight bacterial infections. When your ANC drops below 0.5 × 10^9/L (500/µL), this is known as severe neutropenia ^[5].

A fever during severe neutropenia is a medical emergency. If you have a single temperature reading of 101°F (38.3°C) or a temperature of 100.4°F (38.0°C) sustained for one hour, you must contact your hematology team or seek emergency medical care immediately ^[5].

Do not wait to see if you feel worse. Clinicians generally aim to start broad-spectrum antibiotics promptly—often within an hour of your medical assessment—to prevent life-threatening complications ^[5]. Furthermore, infection can be serious even without a high fever. Seek urgent care if you experience shaking chills, a new cough, shortness of breath, confusion, or severe weakness.

When Your Count is High (Leukocytosis)

A high white blood cell count (leukocytosis) is common in myelofibrosis ^[6]. It is important to distinguish a high count caused by the disease from one caused by an infection:

• The Disease Itself: Myeloproliferative neoplasms like myelofibrosis involve the abnormal overproduction of myeloid cells. The disease also involves systemic inflammation, which can further elevate your WBC count ^[7][6].
• Medication Effects: If you are prescribed corticosteroids for any reason, they can cause a rapid, temporary spike in your white blood cell count ^[8].
• Infection: While an infection can cause a high WBC count, a high count alone does not prove you are infected ^[9]. Doctors will look for physical symptoms, check your vital signs, review your medication history, and run targeted tests or cultures to diagnose an infection ^[9][6].

While a high count is common, a WBC count climbing consistently above 25 × 10^9/L (25,000/µL) is considered a risk factor in several prognostic models ^[10][6]. It does not automatically mean your disease is accelerating, but it is one piece of the puzzle your team will evaluate alongside your red blood cells, platelets, spleen size, and symptoms.

The Importance of the “Differential” and Blasts

Your complete blood count (CBC) includes a “differential”—a breakdown of the specific types of white cells. When numbers are abnormal, your doctor may request a peripheral blood smear, which is a microscopic review of your blood cells by a specialist.

Blasts
Blasts are very immature blood-forming cells that usually stay in the bone marrow. In myelofibrosis, they can spill into the blood. Seeing 1% to 3% blasts in your peripheral blood increases clinical concern and warrants closer monitoring, though it does not by itself prove the disease is accelerating ^[10][11].

However, if blasts reach 10% to 19% in your blood or bone marrow, it indicates the disease has entered an “accelerated phase” ^[12][13]. A blast count of 20% or higher means the disease has transformed into acute myeloid leukemia (blast phase) ^[12][13]. (Note: Not every immature cell reported on an automated test is a blast; a hematologist or pathologist must confirm the diagnosis).

Basophils
An abnormally high number of basophils (basophilia) is another finding your doctor might monitor. While mild basophilia can happen for other reasons, marked or persistent basophilia is one factor a doctor may consider when evaluating if the disease is progressing ^[14][15].

When to Seek Prompt Assessment

You should contact your hematology team if you notice a shifting differential on your lab reports, or if changes in your white blood cells are accompanied by worsening anemia, uncontrolled bleeding, difficulty breathing, or severe fatigue ^[13][16]. These findings are warning signs to discuss—not a diagnosis—and your doctor will help you interpret them in the context of your overall health.