What High White Blood Cell Counts Mean in  Myelofibrosis

Myelofibrosis (MF) is a rare bone marrow cancer where healthy tissue is gradually replaced by scar tissue. This scar tissue impairs normal blood cell production, causing the body to rely on secondary organs like the spleen or liver to make blood cells.

If you have myelofibrosis, routine blood checks are an important part of your care. During these tests, doctors monitor white blood cell levels. An elevated white blood cell count a condition known as leukocytosis. It is defined as a white blood cell count of 25x10³/μL or higher and  is a recognized clinical feature of myelofibrosis. 

A recent nationwide study was done to better understand what a high white blood cell count means for people with myelofibrosis. Researchers presented the real-world data on how leukocytosis affects long-term myelofibrosis health outcomes at the 2026 American Society of Clinical Oncology (ASCO) meeting.  

Long-term impact of leukocytosis in myelofibrosis

To understand the long-term impact of leukocytosis, researchers evaluated electronic health records from a database of 13,237 patients with myelofibrosis. The average age of patients in the study was around 67 to 68 years.

To ensure an accurate comparison, the research team used a matching method. They paired patients with high white blood cell counts against patients with normal white blood cell counts who shared the same age, sex, and baseline health conditions. They then tracked both groups across 1, 3, and 5-year periods.

The research showed that patients with myelofibrosis and leukocytosis faced significantly higher risks of developing complications compared to those with myelofibrosis and lower white blood cell counts:

• Over 1, 3, and 5 years, patients with high white blood cell counts were roughly twice as likely to develop deep vein thrombosis (or blood clots in the legs) and pulmonary embolism (blood clots in the lungs).

• The study found a strong link between leukocytosis and disease progression. Patients with high white blood cell counts were approximately 9 times more likely to have their myelofibrosis progress to acute myeloid leukemia (AML) over the 5-year tracking period.

• High white blood cell counts were associated with a significantly higher rate of overall mortality across 1, 3, and 5 years.

The authors concluded that a high white blood cell count could work as a  strong predictor of how myelofibrosis evolves. Rather than treating elevated white blood cells as a routine lab finding, healthcare providers should view it as a key indicator for closer monitoring and tailored care management.

What to do if you have some symptoms that may point to high white blood cell counts

If you or a loved one is living with myelofibrosis, understanding your routine lab numbers empowers you to collaborate more effectively with your healthcare team. Consider these practical steps:

• Ask your hematologist about your latest complete blood count (CBC). Check if your white blood cell level meets the clinical definition for leukocytosis (25.0 × 10³/μL or higher).

• Because high white blood cell counts increase the risk of blood clots, ask your doctor if you should be monitored more closely for clot symptoms, such as sudden leg swelling, chest pain, or shortness of breath. If you develop symptoms of a blood clot, seek emergency care right away. 

• If your white blood cell counts are high, talk to your specialist about whether your appointment schedule or overall care approach should be adjusted for closer oversight.

Source: 

• Impact of leukocytosis on outcomes in patients with myelofibrosis: A propensity-matched analysis