A 52-year-old man with newly diagnosed chronic myelomonocytic leukemia (CMML) undergoes bone marrow examination. Which of the following findings is NOT consistent with CMML?

Explanation

## Definition and Diagnostic Criteria of CMML **Key Point:** CMML is a myelodysplastic/myeloproliferative neoplasm (MDS/MPN) characterized by monocytosis and dysplasia. It occupies a unique position between myelodysplastic syndrome and myeloproliferative neoplasm. ## Diagnostic Criteria for CMML **High-Yield:** CMML requires: 1. **Monocytosis ≥1000/μL** in peripheral blood 2. Monocytes **≥10% of white blood cells** 3. **Dysplasia** in one or more myeloid lineages 4. **<20% blasts** in peripheral blood and bone marrow 5. Absence of Philadelphia chromosome and BCR-ABL1 ## Blast Percentage: Critical Distinction **Warning:** The presence of **≥20% blasts** in bone marrow or peripheral blood **excludes CMML** and indicates: - **Acute myeloid leukemia (AML)** if blasts ≥20% - **CMML-2** (accelerated phase) if blasts 10–19% - **CMML-1** (stable phase) if blasts <10% Auer rods (Auer bodies) are abnormal cytoplasmic inclusions composed of fused azurophilic granules. While they can be seen in CMML, their presence does **NOT** define CMML. However, **≥20% blasts with Auer rods** would indicate **AML**, not CMML. ## Why the Distractors Are Correct | Feature | Status | Explanation | |---------|--------|-------------| | Monocytosis ≥1000/μL, ≥10% of WBC | ✓ Correct | Mandatory diagnostic criterion for CMML | | Dysplasia in myeloid lineages | ✓ Correct | Myelodysplastic features are required for CMML diagnosis | | Auer rods + >20% blasts | ✗ **INCORRECT** | ≥20% blasts indicates AML, not CMML | | Absence of Philadelphia chromosome | ✓ Correct | CMML is BCR-ABL1 negative; presence excludes CMML | ## CMML vs. AML: Blast Threshold ```mermaid flowchart TD A[Monocytosis + Dysplasia]:::outcome --> B{Blast percentage?}:::decision B -->|<10%| C[CMML-1]:::outcome B -->|10-19%| D[CMML-2]:::outcome B -->|≥20%| E[Acute Myeloid Leukemia]:::urgent ``` **Clinical Pearl:** The blast threshold of 20% is the critical dividing line between CMML (a chronic disorder) and AML (an acute disorder). This distinction has major implications for prognosis and treatment. [cite:Robbins 10e Ch 13]