Chronic Myeloid Leukemia (CML): Understanding Your Pathology Report

Section Editor: David Li MD
June 24, 2026

Chronic myeloid leukemia (CML) is a type of blood cancer called a myeloproliferative neoplasm. It is defined by an abnormal fusion gene called BCR::ABL1, which causes the bone marrow to produce too many white blood cells, especially neutrophils and the cells that develop into them. Most people are diagnosed at an early, slow-growing stage called the chronic phase.

This article will help you understand the findings in your pathology report for chronic myeloid leukemia, what each term means, and why it matters for your care.

Where is chronic myeloid leukemia found?

In chronic myeloid leukemia (CML), the disease’s location depends on the phase. In the chronic phase, the abnormal cells are found mainly in the blood and bone marrow, and the spleen and liver often enlarge as they trap excess blood cells. In the blast phase, which is a more advanced stage, immature cancer cells called blasts can spread outside the bone marrow to areas such as the lymph nodes, skin, soft tissues, liver, and spleen.

What are the symptoms of chronic myeloid leukemia?

Many people with chronic myeloid leukemia (CML) have no symptoms, and the diagnosis is made after a routine blood test shows a high white blood cell count. When symptoms do occur, they often develop gradually and may include fatigue, weakness, weight loss, night sweats, anemia, and a feeling of fullness in the upper-left abdomen due to an enlarged spleen.

If untreated, chronic myeloid leukemia tends to advance over time. Symptoms may worsen and can include fever, increasing fatigue, worsening anemia, low platelet counts, very high white blood cell counts, and progressive enlargement of the spleen.

How common is chronic myeloid leukemia?

Chronic myeloid leukemia (CML) affects about 1 to 2 people per 100,000 each year worldwide. It can occur at any age but is more common in older adults. Because modern treatments are very effective, many people now live long lives with this disease, and the number of people living with chronic myeloid leukemia continues to increase.

What causes chronic myeloid leukemia?

For most people with chronic myeloid leukemia (CML), the cause is unknown. High-dose radiation exposure has been linked to an increased risk. Unlike some other blood cancers, chronic myeloid leukemia is rarely inherited.

What is the BCR::ABL1 fusion gene, and why is it important?

Chronic myeloid leukemia (CML) is caused by a swap of genetic material between chromosomes 9 and 22. This change creates the BCR::ABL1 fusion gene and a shortened chromosome 22 called the Philadelphia chromosome. The BCR::ABL1 fusion gene produces an abnormal protein that sends constant growth signals to blood-forming cells.

This discovery led to the development of targeted medicines called tyrosine kinase inhibitors, which block the abnormal signal. These drugs are highly effective, especially when treatment begins in the chronic phase. Because BCR::ABL1 is the cause of the disease, the target of treatment, and the measurement used to follow treatment over time, there is a dedicated article that explains it in more detail: BCR::ABL1 (Philadelphia Chromosome) in Chronic Myeloid Leukemia.

What are the disease phases of chronic myeloid leukemia?

Chronic myeloid leukemia (CML) is now considered a two-phase disease.

Chronic phase — The earliest stage and the one in which most people are diagnosed. The disease grows slowly, and treatment is usually very effective.
Blast phase — An advanced stage in which immature cells called blasts make up 20% or more of the cells in the blood or bone marrow. The blast phase behaves like an acute leukemia and is much harder to treat.

You may also see the term chronic phase with high-risk features. This refers to chronic-phase disease with findings associated with a higher risk of progression or treatment resistance, such as rising blast counts, very high basophil counts, or additional chromosomal changes.

How is the diagnosis made?

The diagnosis of chronic myeloid leukemia (CML) is made using blood tests, genetic testing, and often a bone marrow biopsy. A complete blood count usually shows a very high white blood cell count, composed mainly of neutrophils and their precursors at different stages of development, with myelocytes and mature neutrophils especially increased. Basophils and eosinophils, other types of white blood cells, are commonly increased as well. Blasts are usually very low in the chronic phase (typically less than 2 percent), platelet counts may be normal or high, and anemia is common. On a blood smear, white blood cells are seen at various stages of maturation and should appear normally formed, without abnormal shapes that would suggest a different bone marrow disorder.

A bone marrow biopsy is often performed at diagnosis to confirm the phase and to provide a baseline for comparison during follow-up. In the chronic phase, the bone marrow is usually very cellular, crowded with blood-forming cells, and shows a marked increase in granulocytes (the family of cells that includes neutrophils) at all stages of development. The cells that make platelets, called megakaryocytes, are often increased in number and may appear smaller than usual. In the blast phase, the bone marrow shows a marked increase in blasts, which may form large sheets, replacing normal tissue. These blasts can resemble those of acute myeloid leukemia or acute lymphoblastic leukemia, and in some cases, the blast phase first appears outside the bone marrow, such as in the skin or lymph nodes. Genetic testing, described in the next section, confirms the diagnosis by detecting the Philadelphia chromosome or the BCR::ABL1 fusion gene.

Genetic testing and monitoring in chronic myeloid leukemia

Genetic testing is essential in chronic myeloid leukemia (CML) because the Philadelphia chromosome or the BCR::ABL1 fusion gene defines the disease. Several tests are used at diagnosis and during follow-up:

Chromosome testing (cytogenetics or karyotyping) — Looks directly for the exchange of material between chromosomes 9 and 22, and can detect additional chromosome changes that may affect risk.
FISH — Uses fluorescent probes to identify the BCR::ABL1 fusion inside cells, which is helpful when the Philadelphia chromosome is not clearly seen on routine chromosome testing.
Molecular testing (RT-PCR) — Detects and measures the amount of BCR::ABL1. Results are reported on an international scale as a percentage, called BCR::ABL1IS, which is used both at diagnosis and during follow-up to monitor treatment response.

The report may also describe the type of BCR::ABL1 transcript present. Most people have one of two common types, called e13a2 or e14a2, and these usually remain stable over time. If the disease does not respond as expected, additional testing may look for mutations in the BCR::ABL1 kinase domain, which can explain resistance to specific drugs and help guide treatment selection. The companion article on BCR::ABL1 explains how to read these numbers as they change over time.

What happens after a diagnosis of chronic myeloid leukemia?

After the diagnosis of chronic myeloid leukemia (CML) is confirmed, treatment usually begins with a tyrosine kinase inhibitor (TKI), a tablet that blocks the abnormal BCR::ABL1 protein. Several TKIs are available, including imatinib, dasatinib, nilotinib, and bosutinib, with additional options such as ponatinib or asciminib for disease that becomes resistant. The choice depends on the disease phase, the risk score, any kinase domain mutations found (for example, T315I, which responds only to certain drugs), and a person’s other health conditions.

Regular blood tests and molecular testing are used to monitor the disease response. Over time, the care team looks for improvement in the blood counts, disappearance of the Philadelphia chromosome, and a steady drop in BCR::ABL1 levels. Most people treated in the chronic phase respond very well and can live long, active lives. Some people who reach a deep and stable molecular response may eventually be candidates for carefully supervised treatment discontinuation, known as treatment-free remission. Decisions about treatment are made by the care team together with the patient, based on the response shown in these test results.

What is the prognosis for chronic myeloid leukemia?

In the modern era, the outlook for chronic myeloid leukemia (CML) is strongly influenced by how well the disease responds to tyrosine kinase inhibitor therapy. Doctors monitor the response at three levels:

Hematologic response — The blood counts return toward normal.
Cytogenetic response — The Philadelphia chromosome is no longer detected by chromosome testing.
Molecular response — The level of BCR::ABL1 measured by RT-PCR drops significantly, reported on the international scale. Many people reach a deep molecular response, in which the BCR::ABL1 level becomes extremely low. If a deep response remains stable for at least a year, some people may be able to stop treatment under careful medical supervision, and about half of them can stay off treatment long term (treatment-free remission).

Risk scores based on age, spleen size, and blood counts at diagnosis can also help predict the response to therapy. Overall, most people treated in the chronic phase do very well, with high long-term survival. The blast phase remains difficult to treat and has a less favorable outlook. Your prognosis depends on your own combination of these factors, which your care team can explain in the context of your specific report.

Questions to ask your doctor

What phase of chronic myeloid leukemia do I have?
Was the Philadelphia chromosome or BCR::ABL1 fusion found in my testing?
What do my blood smear and bone marrow biopsy results show?
What does my BCR::ABL1IS result mean?
Which tyrosine kinase inhibitor is best for me, and what are the possible side effects?
How will my response to treatment be monitored over time?
Do I have any high-risk features or kinase domain mutations that affect my treatment?
What happens if my disease does not respond or becomes resistant to treatment?
Could treatment-free remission be an option for me in the future?
Is treatment safe if I am planning a pregnancy?
Are there clinical trials that I should consider?