The short answer
CML is tracked by phase, not a size-and-spread stage. Most people are diagnosed in chronic phase, which responds well to daily pill treatment. Accelerated and blast phase describe more active disease and are defined by rising blast percentages.
CML is described by phase, chronic, accelerated, or blast, rather than a size-and-spread stage.
Most people are diagnosed in chronic phase, when blasts (immature leukemia cells) make up less than 10% of cells in the blood or bone marrow.
Accelerated phase means blasts make up 10% to 20% of cells, or other specific changes are present, and treatment tends to be less effective than in chronic phase.
Blast phase means blasts make up at least 20% of cells and the disease behaves much like an acute leukemia, needing urgent, intensive treatment.
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The full explanation.
The simple version
CML is not staged the way solid tumors are staged. Instead, it is described by phase: chronic, accelerated, or blast. Phase reflects how active the disease has become, based mainly on the percentage of blasts, or immature leukemia cells, in the blood or bone marrow.
CML phase measures how active the disease is, mainly through the blast percentage, not how far it has physically spread.
Chronic phase
Most people are diagnosed here. It's defined by:
- Fewer than 10% blasts in the blood or bone marrow
- Fewer than 20% basophils (a type of white blood cell) in the blood
- No advanced chromosome abnormalities beyond the one that defines CML
Chronic phase typically causes mild or no symptoms and responds well to standard treatment with daily pills called tyrosine kinase inhibitors (TKIs).
Accelerated phase
This is an intermediate phase, reflecting disease that is becoming more active. Any one of the following places someone in accelerated phase:
- Blasts make up at least 10%, but no more than 20%, of the blood or bone marrow
- Basophils make up 20% or more of the blood
- Combined blasts and promyelocytes make up 30% or more of the blood
- Low platelet counts that are not explained by treatment
- New chromosome changes beyond the original one
People in accelerated phase may notice fever, poor appetite, and weight loss, and this phase generally responds less well to standard treatment than chronic phase.
Blast phase
This is the most advanced phase, defined by at least 20% blasts in the blood or bone marrow. Leukemia cells may also form tumors outside the bone marrow. Blast phase behaves much like an acute leukemia, progressing quickly, and needs urgent, more intensive treatment.
Risk scores used in chronic phase
For people diagnosed in chronic phase, doctors often calculate a separate risk score to help predict how well initial treatment is likely to work:
- Sokal score
- Hasford (Euro) score
- EUTOS long-term survival (ELTS) score
These combine factors such as age, spleen size, platelet count, and blast percentage into a low-, intermediate-, or high-risk category. This is a separate tool from phase — it's used specifically to help choose and monitor initial treatment, not to describe how advanced the disease is.
Why this matters day to day
Phase is checked regularly through blood tests, because CML can move between phases over time, especially if treatment stops working as well. Staying in chronic phase, with regular monitoring, is the goal of ongoing treatment for most people.
Questions to ask
- What phase is my CML in right now?
- What is my Sokal, Hasford, or ELTS score, if calculated?
- How often will my blast percentage be rechecked?
- What would signal a shift to a more active phase?
Sources
Words to know
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Common questions
Why is CML tracked by phase instead of a stage?
CML behaves differently over time in a way that's best described by how active the disease has become, measured mainly by the percentage of blasts (immature leukemia cells) in the blood or bone marrow. Phase captures this directly, which matters more for treatment decisions than a size-and-spread stage would.
What is chronic phase, and is it the same as being cured?
Chronic phase is the earliest and most common phase of CML at diagnosis, generally with mild or no symptoms and a good response to standard treatment with daily pills called tyrosine kinase inhibitors. It is not the same as being cured. Most people need ongoing treatment and monitoring, though many live long, active lives while in chronic phase on treatment.
What happens in accelerated phase?
Accelerated phase means the disease is becoming more active. It's defined by having 10% to 20% blasts in the blood, 20% or more basophils (a type of white blood cell), combined blasts and promyelocytes making up 30% or more of the blood, low platelet counts not explained by treatment, or new chromosome changes. People in accelerated phase may notice fever, poor appetite, and weight loss, and typically respond less well to standard treatment than those in chronic phase.
What is blast phase, and why is it treated differently?
Blast phase means at least 20% of the cells in the blood or bone marrow are blasts, and leukemia cells may form tumors outside the bone marrow. This phase behaves much like an acute leukemia, progressing rapidly, and needs urgent, more intensive treatment, often including combinations used in acute leukemia care.
What are Sokal, Hasford, and ELTS scores?
These are risk-scoring systems used specifically in chronic-phase CML at diagnosis. They combine factors such as age, spleen size, platelet count, and blast percentage into a score that helps predict how well a person is likely to respond to standard treatment. They are separate from phase, and your team may reference one or more of them when discussing your treatment plan.
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Last updated: 2026-08-03Next planned review: 2027-08-03
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source verified — This page was created with AI assistance and checked against the sources listed on it. Source checking is not a medical review.
Human medical review: not completed. Cancer Explained is not clinician-reviewed, and that is a deliberate design choice rather than a gap we are waiting to close. We restate published federal guidance and cite it; the authority belongs to the source, not to us. That is why every page names where its claims come from — so you can verify us instead of trusting us. Use it to understand your situation and to ask better questions of the people treating you.
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