Classically, CML was considered triphasic based on the World Health Organization classification.
- Chronic phase (CP)
- Accelerated phase (AP)
- Blast (crisis) phase (BP)
The disease began in CP and, in some cases, progressed to AP before BP based on worsening clinical symptoms, adverse changes in laboratory findings, additional cytogenetic/molecular abnormalities, or a poorer response to therapy (compared to CP).
In 2022, updates to the World Health Organization classification of myeloproliferative neoplasms removed AP, as tyrosine kinase inhibitor (TKI) therapy has decreased the incidence of progression to advanced phase disease, therefore making AP classification less relevant. Emphasis is now placed on high-risk features associated with CP progression and resistance to TKI therapy (see Table 1).
Table 1. Criteria for Progression to Blast Phase.| Must meet any one or more of the following criteria: |
| ≥20% myeloid blasts in the blood or bone marrow |
| Presence of an extramedullary proliferation of blasts |
| Presence of increased lymphoblasts in peripheral blood or bone marrow |
