Chronic myeloid leukemia (CML) is one of those Step 1 “if you see it, you should immediately think of it” diagnoses: an older adult with massive leukocytosis, splenomegaly, and a telltale BCR-ABL fusion that turns a normal hematopoietic stem cell into a tyrosine-kinase–driven myeloid factory. The good news? CML is extremely testable because its genetics, smear findings, and treatment are all high-yield and tightly linked.
Where CML Fits (Big Picture)
CML is a myeloproliferative neoplasm characterized by clonal proliferation of myeloid cells (especially granulocytes) with increased mature and immature forms in the peripheral blood.
Step framing:
- Acute leukemia = blasts dominate, rapid course
- Chronic leukemia = more mature cells, often indolent early
- CML specifically = myeloid lineage, BCR-ABL, tyrosine kinase, low LAP
First Aid cross-reference (typical placement): Heme/Onc → Leukemias → Chronic Myelogenous Leukemia (CML); Translocations; Tyrosine kinase inhibitors.
Definition (What It Is)
Chronic myeloid leukemia (CML) is a myeloproliferative disorder driven by a reciprocal translocation t(9;22) creating the Philadelphia chromosome, which produces the BCR-ABL fusion protein—a constitutively active tyrosine kinase.
Key definition line to memorize:
- CML = BCR-ABL tyrosine kinase–driven granulocytic proliferation
Pathophysiology (Why It Happens)
The Core Genetic Event: Philadelphia Chromosome
- t(9;22) → ABL proto-oncogene (chr 9) translocates to BCR (chr 22)
- Results in BCR-ABL fusion protein with constitutively active tyrosine kinase activity
- Drives proliferation and decreased apoptosis in myeloid precursors
High-yield association:
- Philadelphia chromosome is classic for CML
- Can also be seen in a subset of ALL (especially adult ALL), but the “classic” Step pairing is CML.
What the Tyrosine Kinase Does (Step-friendly)
BCR-ABL signaling promotes:
- Increased cell proliferation (growth signaling)
- Reduced apoptosis
- Altered adhesion of cells to bone marrow stroma → “spillover” into blood
Clinical Presentation (How It Shows Up)
Typical Patient
- Often middle-aged to older adult
- May be found incidentally on CBC
Symptoms and Signs
Common themes: hypermetabolism + splenic enlargement
- Fatigue, malaise (anemia, inflammatory burden)
- Weight loss, night sweats, low-grade fever (B symptoms can occur)
- Early satiety / LUQ fullness due to splenomegaly
- Sometimes symptoms of hyperviscosity if counts are extremely high (less emphasized than in Waldenström, but still plausible)
Classic Exam Clue
- Massive splenomegaly from extramedullary hematopoiesis and sequestration
Diagnosis (How You Nail It on a Question)
CBC and Differential
- Marked leukocytosis (often very high)
- Left shift with myelocytes, metamyelocytes, increased basophils and eosinophils
- Platelets may be increased (thrombocytosis can occur)
Peripheral Smear (What They Want You to Visualize)
- Lots of granulocytic forms at different maturation stages (a “myeloid spectrum”)
- Basophilia is a particularly helpful clue for CML
Leukocyte Alkaline Phosphatase (LAP) Score
This is a classic board-style discriminator:
| Condition | LAP score | Why it matters |
|---|---|---|
| CML | Low | Neoplastic granulocytes have decreased functional activity |
| Leukemoid reaction | High | Reactive neutrophils ramp up LAP |
High-yield pairing:
- CML vs leukemoid reaction: LAP low vs high
Confirmatory Testing (Gold Standard Concept)
- Detect BCR-ABL
- PCR (very sensitive; also used for monitoring)
- FISH for t(9;22)
- Karyotype showing Philadelphia chromosome
Step 1 takeaway: If they ask “most specific test” → BCR-ABL detection.
Disease Course: Chronic Phase → Blast Crisis
CML typically progresses through:
- Chronic phase: often indolent, responds well to TKIs
- Accelerated phase
- Blast crisis: resembles acute leukemia (either AML or ALL phenotype)
Blast crisis clues:
- Increasing blasts
- Worsening anemia/thrombocytopenia
- Recurrent infections/bleeding
- Treatment resistance
High-yield detail: Blast crisis can look like AML or ALL, which is a favorite “curveball” concept.
Treatment (Where the Genetics Becomes Therapy)
First-Line: Tyrosine Kinase Inhibitors (TKIs)
Because CML is driven by BCR-ABL tyrosine kinase, treatment targets that enzyme.
Core drugs:
- Imatinib
- Other TKIs: dasatinib, nilotinib (often show up as alternatives)
Mechanism you should be able to say out loud:
- TKIs inhibit BCR-ABL tyrosine kinase → reduce proliferative signaling
What If Resistant / Advanced Disease?
- Switch to a different TKI (depending on mutation profile)
- Allogeneic stem cell transplant can be curative in select cases (more often emphasized for refractory disease or blast crisis in exam contexts)
High-Yield Associations & Board Triggers
“If you see this, think CML”
- t(9;22) (Philadelphia chromosome)
- BCR-ABL fusion gene
- Constitutively active tyrosine kinase
- Low LAP
- Basophilia
- Massive splenomegaly
- Responds to imatinib (or other TKIs)
CML vs Leukemoid Reaction (Classic Differential)
| Feature | CML | Leukemoid reaction |
|---|---|---|
| Trigger | Neoplasm | Severe infection/stress/inflammation |
| WBC | Very high | High |
| LAP | Low | High |
| Basophils | ↑ | Usually not prominent |
| BCR-ABL | Positive | Negative |
| Splenomegaly | Common | Usually absent/mild |
CML vs CLL (Quick sanity check)
- CML: myeloid, granulocytes, t(9;22), low LAP
- CLL: lymphocytes, smudge cells, hypogammaglobulinemia, autoimmune hemolytic anemia association
First Aid–Style Rapid Review (What to Memorize)
One-liner:
CML = t(9;22) → BCR-ABL → constitutive tyrosine kinase → granulocytic proliferation; low LAP; treat with imatinib.
Must-know bullets:
- Philadelphia chromosome: t(9;22)
- BCR-ABL fusion protein = tyrosine kinase
- Low LAP (vs leukemoid reaction high LAP)
- Basophilia + splenomegaly
- Can progress to blast crisis
- Imatinib (± dasatinib/nilotinib) targets the kinase
Common Exam Pitfalls (Avoidable Misses)
- Mixing up LAP:
- CML = low LAP
- Leukemoid reaction = high LAP
- Forgetting basophilia: it’s a strong hint toward CML.
- Calling it acute leukemia just because the WBC is huge: CML usually has mature and immature granulocytes, not predominantly blasts (until blast crisis).
- Missing the therapeutic logic: BCR-ABL is the driver → TKI is the answer.
Mini Practice Vignette (Think Like NBME)
A 55-year-old has fatigue and early satiety. Exam shows splenomegaly. CBC: WBC 120,000 with myelocytes, metamyelocytes, and basophilia. LAP score is low.
Most likely diagnosis: CML
Most appropriate targeted therapy: Imatinib (BCR-ABL TKI)
Confirmatory test: PCR for BCR-ABL or FISH for t(9;22)