How Does CML Progress from Chronic to Blast Phase?

Most people with Philadelphia-positive chronic myeloid leukemia (CML) are diagnosed in the chronic phase, where the disease is usually well-controlled with daily targeted therapy (TKIs). While many patients worry about the disease advancing to the blast phase (historically preceded by an “accelerated phase”), this progression has specific medical definitions. A single rising polymerase chain reaction (PCR) test result does not mean you have entered the blast phase ^[1][2]. However, if your doctor finds a high percentage of leukemia cells in a single bone marrow or blood sample, or tumors outside the marrow, that can immediately confirm disease progression ^[3][4]. Your care team monitors your blood routinely to catch warning signs early so they can adjust your treatment.

Glossary of Terms to Know:

• PCR (Polymerase Chain Reaction): A highly sensitive blood test that measures BCR::ABL1 transcripts (the leukemia signal), usually reported on an International Scale (IS) ^[1].
• Blasts: Immature, dysfunctional white blood cells ^[3].
• Extramedullary: Outside of the bone marrow and blood (such as in lymph nodes or the spleen) ^[4].
• Cytogenetics: The study of chromosomes and genetic abnormalities inside your cells ^[5].

Why a PCR Increase Does Not Equal Blast Phase

Routine monitoring for CML involves checking your blood using a PCR test to measure the BCR::ABL1 transcript level. It is entirely normal to feel anxious if this number goes up, but a molecular response loss is not synonymous with blast-phase transformation ^[1].

A rising PCR means the molecular burden of the disease has changed. Small variations can happen simply due to laboratory differences ^[6]. More significant rises can be caused by missed TKI doses, taking new medications or supplements that interfere with drug absorption, or the leukemia developing a resistance mutation ^[1][7].

What to do if your PCR rises:

• Do not stop or change your TKI independently.
• Review your medication adherence and write down any new drugs or supplements you have taken.
• Contact your hematology team. They may simply repeat the test, check for resistance mutations (often done via a simple blood draw), or, if clinical signs warrant it, order a bone marrow biopsy to evaluate for progression ^[2][7].

How Doctors Diagnose Advanced CML

If your care team suspects that your CML is progressing, they evaluate specific clinical criteria, not just your PCR trend.

The Role of Blasts

The primary hallmark of blast-phase CML is a high percentage of blasts in your blood or bone marrow. In the chronic phase, blasts make up fewer than 10% of these cells ^[3].

Different expert organizations use different cutoff numbers to define the blast phase. The World Health Organization (WHO) defines the blast phase as having 20% or more blasts, while the European LeukemiaNet (ELN) and MD Anderson historically use a 30% cutoff ^[3][8]. Note that the 2022 WHO classification removed the intermediate “accelerated phase” entirely, grouping CML into just two phases (chronic and blast), though the International Consensus Classification (ICC) still retains a three-phase model ^[9][8]. Ask your hematologist which classification system they use.

Extramedullary Disease

Progression is also diagnosed if leukemia cells form tumors outside the bone marrow, such as in the lymph nodes, skin, or central nervous system. This extramedullary blast proliferation automatically classifies the disease as blast phase, even if the blast percentage in the bone marrow remains low ^[4][10].

Contextual Warning Signs: Blood Counts, Clonal Evolution, and Mutations

Your doctor will also watch for contextual clues that suggest the disease is resisting treatment or becoming more aggressive. These findings raise concern, though they do not definitively diagnose blast phase on their own:

• Unexplained Blood Count Changes: Persistent anemia, very high white blood cell counts, or abnormal platelet numbers can be warning signs. However, these are nonspecific—they can also be caused by TKI side effects, bleeding, nutritional issues, or infections ^[3][11].
• Clonal Evolution: If your leukemia cells acquire new chromosome abnormalities over time (such as an extra Philadelphia chromosome, “+8”, or “i(17q)”), this is a high-risk warning sign that the disease is changing ^[5][12].
• Kinase-Domain Mutations: Finding a BCR::ABL1 mutation means the leukemia has developed a mechanism to resist your current TKI. This does not mean you are in the blast phase; it means your doctor needs to review which alternative TKI is most appropriate for your specific mutation, health history, and cardiovascular risk ^[7][13].

Symptoms: Routine Evaluation vs. Emergency Care

Advanced CML can cause noticeable physical symptoms, but many of these overlap with TKI side effects or minor infections ^[3][14]. Do not panic if you feel tired, but do know when to act.

Seek IMMEDIATE emergency care if you experience:

• Fever of 100.4°F (38°C) or higher, or shaking chills (especially if you have been told your white blood cell count is low).
• Severe or uncontrolled bleeding.
• Sudden, severe pain in your upper-left abdomen or left shoulder (which can indicate a spleen emergency).
• Severe shortness of breath or sudden confusion.

Contact your care team promptly (do not wait for your next appointment) if you develop:

• Unexplained drenching night sweats or unintentional weight loss ^[15][16].
• New, persistent bone pain ^[17].
• A feeling of fullness or mild-to-moderate aching in the left side of your abdomen (caused by an enlarged spleen) ^[17].
• New, unexplained bruising or mild bleeding ^[18].

The Role of Regular Monitoring

Because treating advanced CML is much more difficult than treating it in the chronic phase, proactive monitoring is essential. Guidelines typically recommend checking your PCR levels every 3 months until you achieve a stable “major molecular response” ^[19]. Once your disease is deeply and stably controlled, your doctor may space these tests out slightly. By standardizing these tests on the International Scale and reviewing them alongside your physical symptoms and routine blood counts, your team can catch potential issues and adjust your care before the disease has a chance to progress ^[1].