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?

Question

Accepted Answer

C. Presence of Auer rods and >20% blasts in bone marrow. ## 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]

Answer

A. Dysplasia in one or more myeloid lineages (myelodysplastic features)

Answer

B. Absence of Philadelphia chromosome and BCR-ABL1 fusion gene

Answer

D. Monocytosis ≥1000/μL in peripheral blood with monocytes comprising ≥10% of white blood cells

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

Diagnostic Criteria for CMML

Blast Percentage: Critical Distinction

Why the Distractors Are Correct

CMML vs. AML: Blast Threshold

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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