Which feature best distinguishes chronic myeloid leukemia (CML) from chronic lymphocytic leukemia (CLL)?

Question

Accepted Answer

C. Presence of Philadelphia chromosome t(9;22). ## Distinguishing CML from CLL

### Philadelphia Chromosome — The Gold Standard Discriminator

**Key Point:** The Philadelphia chromosome t(9;22) resulting in the BCR-ABL1 fusion gene is pathognomonic for CML and is present in >95% of cases. This is the single most reliable feature that separates CML from CLL.

### Comparison Table: CML vs CLL

| Feature | CML | CLL |
|---------|-----|-----|
| **Philadelphia chromosome t(9;22)** | **Present (>95%)** | **Absent** |
| Cell lineage | Myeloid | Lymphoid (B-cell) |
| Peripheral blood | Myeloid left shift, basophilia, eosinophilia | Mature lymphocytes, smudge cells |
| Splenomegaly | Very common (90%) | Common (50–60%) |
| Leukocyte count | Often >50,000/μL | Variable, often modest |
| Median age at diagnosis | 50–60 years | >70 years |
| BCR-ABL1 transcript | BCR-ABL p210 (most common) | Absent |

### Why Philadelphia Chromosome Is Superior

1. **Cytogenetic hallmark:** Present in nearly all CML cases; virtually never in CLL.
2. **Molecular target:** Drives pathogenesis via constitutive tyrosine kinase activity.
3. **Diagnostic criterion:** WHO classification mandates BCR-ABL1 positivity for CML diagnosis.
4. **Therapeutic relevance:** Tyrosine kinase inhibitors (imatinib, dasatinib, nilotinib) are standard of care for CML but not CLL.

**Clinical Pearl:** A patient with myeloid leukocytosis and Philadelphia chromosome positivity is CML until proven otherwise. Conversely, a lymphoid leukocytosis with Philadelphia chromosome would be extremely rare and would represent a CML in lymphoid blast crisis or a rare BCR-ABL1+ lymphoma.

**High-Yield:** FISH or PCR for BCR-ABL1 is now the standard diagnostic method; karyotyping alone may miss variant translocations. Always order BCR-ABL1 testing in any chronic leukemia with myeloid predominance.

[cite:Robbins 10e Ch 13]

Answer

A. Elevated leukocyte count (>50,000/μL)

Answer

B. Lymphocytosis on peripheral blood smear

Answer

D. Splenomegaly on clinical examination

Which feature best distinguishes chronic myeloid leukemia (CML) from chronic lymphocytic leukemia (CLL)?

Explanation

Distinguishing CML from CLL

Philadelphia Chromosome — The Gold Standard Discriminator

Comparison Table: CML vs CLL

Why Philadelphia Chromosome Is Superior

Practice similar questions

Join our NEET PG community

Feature	CML	CLL
Philadelphia chromosome t(9;22)	Present (>95%)	Absent
Cell lineage	Myeloid	Lymphoid (B-cell)
Peripheral blood	Myeloid left shift, basophilia, eosinophilia	Mature lymphocytes, smudge cells
Splenomegaly	Very common (90%)	Common (50–60%)
Leukocyte count	Often >50,000/μL	Variable, often modest
Median age at diagnosis	50–60 years	>70 years
BCR-ABL1 transcript	BCR-ABL p210 (most common)	Absent