Which morphologic and cytochemical feature best distinguishes chronic myelomonocytic leukemia (CMML) from chronic myeloid leukemia (CML)?

Explanation

## CMML vs CML — Morphologic and Cytogenetic Distinction ### Core Diagnostic Difference **Key Point:** CMML is a myelodysplastic/myeloproliferative neoplasm (MDS/MPN) characterized by **monocytosis ≥1,000/μL** with dysplastic features and **absence of Philadelphia chromosome**. CML is a pure myeloproliferative neoplasm driven by BCR-ABL1. This distinction is critical because CMML has worse prognosis and different treatment approach. ### Comparison Table: CMML vs CML | Feature | CMML | CML | |---------|------|-----| | **Monocyte count** | **≥1,000/μL (absolute)** | **<1,000/μL (usually)** | | **Philadelphia chromosome** | **Absent** | **Present (>95%)** | | **BCR-ABL1** | Absent | Present | | Dysplastic features | Present (hypogranular neutrophils, abnormal monocytes) | Absent or minimal | | Blast percentage | 0–19% (defines CMML-0, -1, -2) | <5% in chronic phase | | Myeloid maturation | Abnormal (dysplasia) | Normal left shift | | Median survival | 12–24 months | >5 years (with TKI) | | Treatment | Hypomethylating agents, allogeneic HSCT | Tyrosine kinase inhibitors | | Transformation risk | High (to AML) | Lower in chronic phase | ### Why Monocytosis + Dysplasia + Absence of Philadelphia Chromosome Is Diagnostic 1. **Monocytosis ≥1,000/μL is a hallmark:** CML rarely has absolute monocytosis; CMML requires it by definition. 2. **Dysplastic changes are key:** CMML shows abnormal monocyte morphology (pseudo-Pelger nuclei, hypogranularity), abnormal neutrophils, and left shift with dysplasia. CML shows orderly left shift without dysplasia. 3. **Philadelphia chromosome absence:** This single finding excludes CML and points toward CMML or other MDS/MPN entities. 4. **WHO/ICC classification:** CMML is now classified as MDS/MPN overlap, reflecting its hybrid myelodysplastic and myeloproliferative features. **High-Yield:** CMML is defined by the combination of monocytosis + dysplasia + absence of BCR-ABL1. Any patient with monocytosis >1,000/μL and no Philadelphia chromosome should raise suspicion for CMML. **Clinical Pearl:** CMML patients often present with higher blast burden at diagnosis (median 5–10% blasts) compared to CML in chronic phase (<5% blasts). This contributes to worse prognosis and higher risk of transformation to AML. **Mnemonic:** **CMML = Monocytosis + Myelodysplasia + Missing Philadelphia** (absent BCR-ABL1). [cite:Robbins 10e Ch 13]