Chronic myeloid leukemia (CML) is a slow-growing blood cancer diagnosed through a combination of blood tests, bone marrow analysis, and genetic testing that identifies a specific chromosome abnormality. Up to half of all people with CML have no symptoms when they’re diagnosed. Instead, the disease is often caught on a routine blood test that shows an unusually high white blood cell count.
How CML Is Usually Discovered
Because CML progresses slowly in its early stage, many people feel perfectly fine when it’s first detected. A standard complete blood count (CBC) drawn during an annual physical or for an unrelated reason reveals the first clue: a white blood cell count that’s significantly higher than normal. The blood sample also tends to show an unusual mix of immature white blood cells that wouldn’t normally appear in circulation, along with elevated levels of certain cell types called basophils and eosinophils.
When symptoms do exist, they’re often vague. Fatigue, unexplained weight loss, night sweats, and a feeling of fullness or discomfort on the left side of the abdomen are the most common complaints. That abdominal discomfort comes from an enlarged spleen, which is the most common physical finding in CML. More than 50% of patients have a noticeably enlarged spleen at the time of discovery, and its size tends to track with how high the white blood cell count has climbed.
The Blood Tests That Raise Suspicion
An abnormal CBC is the starting point, but it’s not enough on its own. Several other conditions, including severe infections, can push white blood cell counts high enough to mimic CML. Doctors call this a “leukemoid reaction,” and telling the two apart matters enormously because one is a temporary response to illness and the other is cancer.
A peripheral blood smear, where a technician examines a blood sample under a microscope, helps sort this out. In CML, the smear shows increased numbers of immature basophils and eosinophils. A leukemoid reaction, by contrast, shows mainly mature immune cells with a spike in immature versions of a different cell type called neutrophils. Additional markers like a leukocyte alkaline phosphatase (LAP) score can further separate the two. In CML, this score is characteristically low, while infections and other reactive conditions drive it high.
Confirming CML With Genetic Testing
The definitive test for CML looks for a genetic abnormality called the Philadelphia chromosome. This forms when pieces of chromosome 9 and chromosome 22 break off and swap places. The result is an abnormally short chromosome 22 that carries a new, fused gene. That fused gene produces a protein that tells white blood cells to multiply uncontrollably. Finding this chromosome confirms the diagnosis.
Three main lab techniques can detect it:
- Conventional cytogenetics (karyotyping): A lab grows cells from a bone marrow sample and photographs their chromosomes to visually identify the swap. This test also reveals whether any additional chromosome changes are present, which can affect prognosis.
- FISH (fluorescence in situ hybridization): This uses fluorescent probes that bind directly to the swapped gene segments, lighting them up under a special microscope. It’s a quick method with a sensitivity of about 97.6% and can be run on blood or bone marrow samples.
- PCR (polymerase chain reaction): The most sensitive option, PCR detects tiny amounts of the fusion gene’s genetic material. It can pick up one abnormal cell among hundreds of thousands of normal ones, making it especially useful for tracking the disease after treatment begins.
Most people will have at least two of these tests during the diagnostic workup. Cytogenetics and FISH are typically used to establish the initial diagnosis, while PCR becomes the primary tool for monitoring over time.
What a Bone Marrow Biopsy Shows
A bone marrow biopsy is a standard part of the diagnostic process. During this procedure, a needle is inserted into the back of the hip bone to withdraw a small core of marrow tissue and a liquid sample. It’s done under local anesthesia and takes about 15 to 30 minutes. Most people describe a deep pressure or aching sensation that lasts a few seconds during the actual sample collection.
The marrow sample gives doctors two critical pieces of information. First, it provides cells for the genetic tests described above. Second, it reveals the percentage of blast cells, which are very immature white blood cells. This percentage determines which phase of CML you’re in, and the phase directly shapes treatment decisions and outlook.
The Three Phases of CML
CML isn’t staged the way most cancers are. Instead, it’s classified into three phases based primarily on how many blast cells are present in the blood or bone marrow.
- Chronic phase: Less than 10% blasts in the blood or bone marrow. This is the earliest and most treatable phase, and it’s where the vast majority of people are diagnosed. The disease is stable and responds well to targeted therapy. Many people in this phase live near-normal lifespans with treatment.
- Accelerated phase: Blast cells make up 10% to 20% of the blood or bone marrow. Other warning signs include a very low platelet count (unrelated to treatment) or a rapidly enlarging spleen. The disease is becoming harder to control and may need more aggressive treatment.
- Blast phase (blast crisis): At least 20% blasts in the blood or bone marrow. At this point, CML behaves more like an aggressive acute leukemia. A very large spleen often signals this transformation. This phase requires immediate, intensive treatment.
Because the chronic phase can last years before progressing, early detection through routine blood work gives most people a significant head start on treatment.
What Happens After Diagnosis
Once CML is confirmed and the phase is established, doctors use risk scoring systems that factor in your age, spleen size, platelet count, and the percentages of certain cell types in your blood. These scores help predict how well the disease will respond to targeted therapy and guide the choice of initial treatment.
For people diagnosed in the chronic phase, treatment typically begins with a daily oral medication that specifically blocks the protein produced by the Philadelphia chromosome. This class of drug has transformed CML from a disease with a poor prognosis into one that most people manage as a chronic condition, similar to how you might take daily medication for high blood pressure or diabetes. Response is tracked with regular PCR blood tests, usually every three months in the first year, to measure how quickly the abnormal gene signal decreases.
The speed and depth of that molecular response in the first 3 to 12 months of treatment is one of the strongest predictors of long-term outcome. If the initial medication isn’t reducing the fusion gene signal fast enough, alternative options in the same drug class are available.