The short answer
Evaluation may include blood counts, marrow assessment when needed, chromosome testing, and quantitative BCR::ABL1 testing. Each result should answer a specific diagnostic, risk, or treatment question.
Evaluation may include blood counts, marrow assessment when needed, chromosome testing, and quantitative BCR::ABL1 testing.
Planning may depend on disease phase, BCR::ABL1 level and mutations, response milestones, side effects, adherence, other medicines, and goals.
A result can be diagnostic, prognostic, predictive, or useful for monitoring—and these are not identical roles.
Ask which results are confirmed and which remain pending.
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The full explanation.
What the blood and marrow show
CML is usually found first on a routine blood test, sometimes before any symptoms appear. A complete blood count checks your red cells, white cells, and platelets. In CML, the white blood cell count is often very high. A bone marrow aspiration and biopsy, where a needle removes a small marrow sample from the hip bone, confirms the diagnosis and checks how far the disease has progressed.
The Philadelphia chromosome
Almost everyone with CML has a specific genetic change called the Philadelphia chromosome. It happens when two chromosomes, 9 and 22, swap pieces. This creates a new, abnormal gene called BCR::ABL1. That gene tells cells to make a protein that pushes white blood cells to grow out of control. Three tests can find this change: cytogenetic analysis, which looks at chromosomes directly; FISH, which uses glowing DNA probes; and PCR, which measures how much BCR::ABL1 genetic material is in your blood. PCR is the most sensitive of the three, and your team will likely use it again and again to track your response to treatment.
The three phases
CML is classified by the percentage of immature cells, called blasts, in the blood or marrow. Chronic phase means fewer than 10% blasts. This is the most common phase at diagnosis and the one with the best outlook. Accelerated phase means 10% to 19% blasts. Blast phase, also called blast crisis, means 20% or more. Blast phase behaves like an acute leukemia and needs urgent, more intensive treatment.
How results connect to treatment
Chronic-phase CML is treated with pills called tyrosine kinase inhibitors, or TKIs. These block the abnormal protein made by BCR::ABL1. Options include imatinib, dasatinib, nilotinib, bosutinib, asciminib, and ponatinib. Your team picks a starting drug based on your phase, other health conditions, and side-effect risks. For blast-phase CML, treatment may combine a TKI with a stem cell transplant or, in some cases, palliative chemotherapy to control symptoms.
Why ongoing PCR testing matters
After you start a TKI, PCR testing continues on a regular schedule, often every three months at first. The goal is a falling, then very low, BCR::ABL1 level. Your team will describe specific response milestones for certain months of treatment. Missing a milestone does not always mean the drug has failed, but it usually leads to a closer look, including testing for new mutations that can make CML resistant to a specific TKI.
A practical note on absorption
Bariatric surgery can lower how much of an oral TKI your body absorbs, which can reduce how well it works. If you have had this surgery or are considering it, tell your care team so your drug and dose can be reviewed.
What to ask your team
- What phase is my CML in, and what does that mean for urgency?
- What is my current BCR::ABL1 level, and what is the target at each check?
- Which TKI are we starting with, and why this one for me?
- What side effects should prompt a call before my next appointment?
- If a milestone is missed, what happens next?
When to get help sooner
Between PCR checks, a few changes are worth reporting rather than saving for the next appointment. Symptoms that build quickly can be a sign the disease is moving out of chronic phase.
- Call 911 or go to an emergency department if bleeding does not stop, or you become short of breath at rest, or you have sudden severe pain in the left upper abdomen.
- Call your care team at once, day or night, if you develop a fever while your blood counts are low, or a fever comes with shaking chills and feeling suddenly unwell. With few working white cells, an infection can escalate within hours, so this counts as a medical emergency. If you cannot reach them quickly, go to an emergency department.
- Call your care team the same day if you have a fever with normal recent counts, drenching night sweats, or bruising and bleeding that are new for you.
- Call your care team within a day or two if fullness or pain below the ribs on the left side is growing, or you are losing weight without trying.
- Call your care team within a day or two if a side effect of your TKI is stopping you taking it as prescribed. Ask about a dose review rather than skipping doses on your own.
Sources
Words to know
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Common questions
How is CML usually found?
Often on a routine blood test, sometimes before any symptoms appear. A complete blood count checks your red cells, white cells and platelets, and in CML the white blood cell count is often very high. A bone marrow aspiration and biopsy, where a needle removes a small marrow sample from the hip bone, confirms the diagnosis and checks how far the disease has progressed.
What is the Philadelphia chromosome?
It is the genetic change almost everyone with CML has, created when chromosomes 9 and 22 swap pieces. That makes a new abnormal gene called BCR::ABL1, which tells cells to produce a protein that pushes white blood cells to grow out of control. Cytogenetic analysis, FISH and PCR can each find the change, and PCR is the most sensitive of the three.
What do the three phases mean?
They are set by the percentage of immature cells, called blasts, in the blood or marrow. Chronic phase means fewer than 10% blasts, and is the most common phase at diagnosis and the one with the best outlook. Accelerated phase means 10% to 19%. Blast phase, also called blast crisis, means 20% or more, behaves like an acute leukemia, and needs urgent, more intensive treatment.
Why does PCR testing carry on after treatment starts?
Because the goal is a falling, then very low, BCR::ABL1 level. Testing continues on a regular schedule, often every three months at first, and your team will describe specific response milestones for certain months of treatment. Missing a milestone does not always mean the drug has failed, but it usually leads to a closer look, including testing for new mutations that can make CML resistant to a specific TKI.
Does bariatric surgery affect my treatment?
It can. Bariatric surgery can lower how much of an oral TKI your body absorbs, which can reduce how well it works. If you have had this surgery, or are considering it, tell your care team so your drug and dose can be reviewed.
Questions to ask your doctor
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Sources last checked: 2026-08-11 what this meansLast updated: 2026-08-18Next planned review: 2027-07-22
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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 checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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