Classic Peripheral Blood Film Findings in Chronic Myeloid Leukemia
The peripheral blood film in chronic phase CML classically shows marked leukocytosis with left-shifted granulopoiesis at all stages of maturation (myeloblasts through mature neutrophils), accompanied by basophilia, eosinophilia, and thrombocytosis. 1
Characteristic White Blood Cell Findings
The hallmark feature is excessive left-shifted granulopoiesis detectable at all stages of maturation, meaning you will see the entire spectrum of myeloid cells from blasts to mature neutrophils on a single smear. 1 This creates a characteristic "myelocyte bulge" where myelocytes and metamyelocytes are prominently increased. 1
- Myelocytes and metamyelocytes are typically abundant (often 20-40% of white cells), creating the distinctive left shift that differentiates CML from reactive leukocytosis. 2
- Blast cells are present but remain <10% in chronic phase, distinguishing it from accelerated phase (10-19% blasts) or blast crisis (≥20% blasts). 1
- Promyelocytes are visible but usually comprise only 1-5% of cells. 2
Basophilia and Eosinophilia
Basophilia is a typical and diagnostically important feature of chronic phase CML, often comprising 3-10% or more of the differential count. 1 The absence of basophils should raise suspicion for atypical CML or other diagnoses, though rare CML cases can present without basophilia. 2
- Eosinophilia is also commonly present, contributing to the overall granulocytic expansion. 1
- Basophil percentage >20% indicates accelerated phase rather than chronic phase. 1
Platelet Findings
Thrombocytosis (elevated platelet count) is a typical feature of chronic phase CML at presentation, often with counts exceeding 450-600 × 10⁹/L. 1 In some cases, marked thrombocytosis may mimic essential thrombocythemia clinically, making Philadelphia chromosome testing mandatory. 3
Red Blood Cell Findings
- Nucleated red blood cells may be present on the peripheral smear, reflecting the myeloproliferative nature of the disease. 4
- Anemia is variable; some patients present with normal hemoglobin while others have mild to moderate anemia. 4
Critical Diagnostic Pitfall
Do not diagnose CML based solely on peripheral blood morphology—the Philadelphia chromosome (t(9;22)) or BCR-ABL1 fusion gene must be demonstrated to confirm the diagnosis. 1, 5 Approximately 5% of cases lack a visible Philadelphia chromosome on conventional cytogenetics and require FISH or RT-PCR for BCR-ABL1 detection. 1
Atypical CML (BCR-ABL1 negative) lacks basophilia and shows more pronounced dysplasia, distinguishing it from classic Philadelphia-positive CML. 1
Distinguishing CML from Leukemoid Reaction
The combination of left shift extending to myelocytes/promyelocytes, basophilia, eosinophilia, and thrombocytosis strongly favors CML over a reactive leukocytosis. 1 Reactive processes rarely produce significant basophilia or the full spectrum of myeloid maturation seen in CML. 6