Chronic myeloid leukemia (CML) is diagnosed by finding a raised white cell count with a typical pattern on the blood film, then confirming the BCR-ABL1 fusion gene (the Philadelphia chromosome) with genetic testing. A complete blood count usually raises the suspicion, while FISH, cytogenetics, and PCR confirm it and give a baseline for monitoring. Many patients have no symptoms when it is found. Below I explain each step of the diagnosis and how the results guide treatment.
What Is Chronic Myeloid Leukemia?
CML is a hematologic cancer in which the bone marrow makes far too many granulocytes, a family of white blood cells that includes neutrophils, and their precursors. It belongs to the group of myeloproliferative neoplasms, conditions in which the marrow overproduces one or more blood cell lines.
The driver is a swap of genetic material between chromosomes 9 and 22, written t(9;22). The shortened chromosome 22 is called the Philadelphia chromosome. It carries a new fused gene, BCR-ABL1, which makes a tyrosine kinase enzyme that is permanently switched on. That enzyme pushes cells to divide and makes them resist normal cell death.
Risk Factors
In most people, no cause is found. High-dose ionizing radiation is one of the few established risk factors. CML is more common in middle-aged and older adults and slightly more common in men. It is not inherited and is not passed on to children.
Symptoms and Phases of CML
CML often turns up by chance on a routine blood test. When symptoms do occur, they tend to develop gradually:
- Fatigue and reduced exercise tolerance from anemia
- Weight loss, night sweats, or low-grade fevers
- Fullness or discomfort under the left ribs, or feeling full quickly, from an enlarged spleen (splenomegaly)
- Less commonly, bruising, bleeding, or gout-like joint pain
Doctors describe CML in phases based on how many immature cells (blasts) are present and how the disease is behaving.
| Phase | Key features | What it means |
|---|---|---|
| Chronic phase | Raised white count, few blasts, often mild or no symptoms | Where most patients are diagnosed; responds very well to oral treatment |
| Accelerated / advanced features | Rising blasts or basophils, new chromosome changes, treatment resistance | Disease becoming less stable; treatment usually changed or intensified |
| Blast phase | Blasts at acute leukemia levels (traditionally at least 20%) | Behaves like acute leukemia; needs intensive therapy and often transplant |
Newer classifications put less weight on a separate accelerated phase and focus on high-risk features instead, but the three-phase idea still helps patients understand where they stand.
Diagnosing Chronic Myeloid Leukemia: The Tests
The workup follows a logical order, from a simple blood count to precise molecular testing. For a broader view of how these investigations compare across leukemias, see our guide to leukemia diagnosis tests.
Complete Blood Count and Blood Film
The complete blood count (CBC) typically shows a raised white cell count, sometimes very high. The blood film shows granulocytes at every stage of maturity, with many myelocytes and neutrophils. An increase in basophils is a classic clue. Platelets are often high (thrombocytosis), though they can be low in advanced disease, and mild anemia is common.
Bone Marrow Aspirate and Biopsy
A marrow test shows a packed, hypercellular marrow full of granulocyte precursors. It is important at diagnosis because it gives an accurate blast count for staging and allows full chromosome analysis, which can pick up additional changes beyond the Philadelphia chromosome.
Cytogenetics and FISH
Karyotyping looks at the chromosomes directly and identifies t(9;22). Fluorescence in situ hybridization (FISH) uses glowing probes that light up where BCR and ABL1 have fused. FISH can be done on blood and is a quick way to confirm the diagnosis.
Quantitative PCR
Quantitative reverse-transcription PCR measures the amount of BCR-ABL1 message in the blood. Results are reported on the International Scale (IS) so labs can be compared. The baseline level and transcript type at diagnosis matter because PCR becomes the main way to track the response to treatment.
How Diagnosis Shapes Treatment
Once CML is confirmed, a prognostic score such as Sokal, Hasford, or ELTS is calculated from age, spleen size, and blood counts. The score helps predict outcome and can influence the choice of first drug.
Chronic-phase CML is treated with oral tyrosine kinase inhibitors (TKIs). Imatinib was the first, followed by dasatinib, nilotinib, and bosutinib. Asciminib binds a different site on the BCR-ABL1 protein and is an option after other TKIs, or where certain resistance mutations are present. Allogeneic stem cell transplant is reserved for advanced disease or failure of several TKIs. Our article on the diagnosis and management of chronic myelogenous leukemia covers treatment in more depth.
Monitoring Response
PCR is repeated regularly, often every three months at first. Widely used milestones are a BCR-ABL1 level of 10% or less (IS) at three months, 1% or less at six months, and 0.1% or less, called major molecular response, by twelve months. Missing a milestone prompts a check on adherence and testing for resistance mutations in the kinase domain.
Some patients with a deep, stable response after several years of therapy may try stopping the TKI under close PCR monitoring. This is called treatment-free remission. It should only be attempted under specialist supervision.
When to See a Doctor
See your doctor if you have persistent tiredness, unexplained weight loss or night sweats, or a feeling of fullness or pain under your left ribs. If a routine blood test shows a high white cell count, ask for a clear explanation and follow-up. Early chronic-phase CML is highly treatable, and good leukemia diagnosis and management starts with that first abnormal result. More background on hematology and our leukemia guide can help you prepare questions for your appointment.
Frequently Asked Questions
Can CML be diagnosed from a blood test alone?
A CBC and blood film can strongly suggest CML, and FISH or PCR on blood can confirm BCR-ABL1. A bone marrow test is still recommended at diagnosis to count blasts accurately and check for additional chromosome changes that affect staging.
Is the Philadelphia chromosome inherited?
No. It is acquired in a single blood stem cell during a person’s lifetime. It is found only in the leukemia cells, not in other body cells, and cannot be passed to children.
How is CML different from acute leukemia?
In chronic-phase CML the cells still mature, so the disease progresses slowly and is controlled with tablets. Acute leukemias are dominated by immature blasts, develop quickly, and usually need intensive chemotherapy. Untreated CML can eventually transform into a blast phase that behaves like acute leukemia.
How often will I need PCR tests?
Usually about every three months during the first year or two, then less often once a stable major molecular response is reached. Testing becomes more frequent again if treatment is stopped or changed.
