Subtypes and the Diagnostic Work-up

When you look at a diagnostic work-up for CMML, you are looking at a complex biological puzzle. Doctors use two main “labels” to describe your specific version of the disease. These labels help predict how the disease might behave and which treatments may be most effective.

The Two Ways to Classify CMML

Your diagnosis is classified along two different axes: how the cells act (Phenotype) and how many immature cells are present (Blast Percentage).

1. Classification by Phenotype: MD-CMML vs. MP-CMML

This classification is based on your total white blood cell (WBC) count. The cutoff used by both the World Health Organization (WHO) and the International Consensus Classification (ICC) is 13 x 10⁹/L ^[1][2].

• Myelodysplastic (MD-CMML): Your WBC count is less than 13 x 10⁹/L. This subtype often focuses on “ineffective” blood production, where the main challenge is low blood counts (cytopenias) like anemia ^[1].
• Myeloproliferative (MP-CMML): Your WBC count is 13 x 10⁹/L or higher. This subtype is characterized by the overproduction of cells. It often carries a more aggressive course, a higher risk of the disease changing into acute leukemia, and physical symptoms like an enlarged spleen (splenomegaly) ^[1][3].

2. Classification by Blasts: CMML-1 vs. CMML-2

This group is defined by the number of “blasts” in your blood and bone marrow. Blasts are immature, “baby” cells that haven’t yet grown into functional white blood cells. In CMML, doctors also count promonocytes—a specific type of immature monocyte—as “blast equivalents” ^[4][5].

If the blast count reaches 20% or higher in either the blood or the marrow, the disease is generally considered to have transformed into Acute Myeloid Leukemia (AML) ^[6]. However, the final classification depends on complete genetic and morphological findings, as some genetically defined AMLs may use different criteria.

Your Diagnostic Work-up “Completeness Checklist”

A comprehensive work-up for CMML should include several key pieces of information (which may appear across multiple reports) to ensure an accurate diagnosis and risk assessment.

• Absolute Monocyte Count: Must be ≥0.5 x 10⁹/L and make up ≥10% of your total white blood cells ^[1].
• Blast & Promonocyte Percentage: The report should state the percentage in both the blood and the marrow ^[4].
• Cytogenetics (Karyotype): A look at the structure of your chromosomes. About 30% of patients have abnormalities like Trisomy 8 or Loss of Y ^[7].
• Molecular Panel (NGS): This uses Next-Generation Sequencing to find specific mutations. Over 90% of patients have mutations in genes like TET2, SRSF2, or ASXL1 ^[8][9].
• Bone Marrow Morphology: A description of how the cells look under a microscope, checking for “dysplasia” (abnormal shapes) ^[10].

How Doctors Rule Out “Mimics”

Monocytosis (high monocytes) can be caused by many things other than cancer. Your doctor must distinguish CMML from:

• Reactive Monocytosis: High monocytes caused by a temporary infection or inflammation ^[11].
• Chronic Myeloid Leukemia (CML): A different blood cancer defined by a specific genetic swap called BCR::ABL1. A “negative” test for this gene is required ^[12].
• Other Neoplasms: Doctors will also rule out rearrangements involving PDGFRA, PDGFRB, FGFR1, or PCM1::JAK2, and differentiate CMML from diseases like Primary Myelofibrosis (PMF) by looking at cell shapes and fibrosis in the marrow ^[13].

The Role of Genetic Mutations

The mutations found in your NGS panel (such as ASXL1 or RAS-pathway mutations like NRAS) are not just for diagnosis; they are factored into validated prognostic models ^[14][3]. However, it is important to know that a single mutation alone does not determine your future. The effect of any mutation depends on the specific variant (e.g., a “truncating” ASXL1 mutation vs. another type), co-occurring mutations (such as TET2 without ASXL1, which some models treat favorably ^[15]), your blood counts, and your overall health. These findings are used together by your doctor to guide treatment decisions.