Laboratory Differentiation Between CML and CLL
The critical diagnostic test that differentiates CML from CLL is the detection of the BCR::ABL1 fusion gene (Philadelphia chromosome) by cytogenetics and/or RT-PCR—present in CML but absent in CLL—combined with immunophenotyping that reveals distinct cell lineage markers (myeloid vs. lymphoid). 1, 2
Diagnostic Algorithm
Initial Testing for Both Conditions
Complete Blood Count (CBC) with Differential:
- Both show elevated white blood cell counts, but cell types differ
- CML: Left shift with immature myeloid cells (myelocytes, metamyelocytes, basophilia)
- CLL: Mature-appearing small lymphocytes with smudge cells
Definitive Tests for CML 1, 2
1. Cytogenetic Analysis (Bone Marrow Preferred):
- G-banded chromosome analysis detects t(9;22)(q34;q11.2) in 90-95% of CML cases
- Identifies additional chromosomal abnormalities indicating disease progression (extra der(22), trisomy 8, i(17q), trisomy 19, complex karyotype, 3q26.2 abnormalities)
- FISH for BCR::ABL1 if conventional cytogenetics fails or for variant/cryptic translocations (5-10% of cases)
2. Molecular Testing:
- Quantitative RT-PCR for BCR::ABL1 fusion on peripheral blood or bone marrow
- Establishes baseline for monitoring treatment response
- Identifies specific transcript type (e90a2, e13a2, e14a2)
Critical Point: Both cytogenetics AND RT-PCR should be performed at diagnosis per NCCN guidelines, as some cases have cryptic rearrangements undetectable by one method alone 2
Definitive Tests for CLL 3, 4
1. Immunophenotyping (Flow Cytometry):
- Diagnostic signature: CD5+/CD19+/CD23+ B-cells with dim surface immunoglobulin and CD20
- Monoclonal light chain restriction (kappa or lambda)
- Scoring system using multiple markers confirms diagnosis
- CD38 and ZAP-70 provide prognostic information
2. Morphology:
- Peripheral blood smear shows mature small lymphocytes with clumped chromatin
- Smudge cells (Gumprecht shadows) characteristic but not specific
3. Additional Testing When Needed:
- FISH panel for prognostic markers: del(17p), del(11q), trisomy 12, del(13q)
- Bone marrow biopsy if atypical features present
- IgVH mutational status for prognosis
Key Differentiating Features
| Test | CML | CLL |
|---|---|---|
| BCR::ABL1 | Positive (diagnostic) | Negative (required for exclusion) |
| Cell Lineage | Myeloid | Lymphoid (B-cell) |
| Immunophenotype | Not applicable | CD5+/CD19+/CD23+ |
| Philadelphia Chromosome | Present (90-95%) | Absent |
| Bone Marrow | Hypercellular, myeloid predominance | Lymphocytic infiltration |
Common Pitfalls to Avoid
For CML Diagnosis:
- Don't rely solely on RT-PCR—some commercial kits miss rare transcripts; discrepant results (negative RT-PCR with positive cytogenetics) warrant further investigation 1
- Don't skip bone marrow evaluation—peripheral blood alone may miss accelerated or blast phase disease 2
- Variant translocations (5-10% of cases) require FISH if cytogenetics appear normal
For CLL Diagnosis:
- Immunophenotyping is the only method that can definitively establish CLL diagnosis—morphology alone is insufficient 3
- In atypical cases, complement with histology and molecular genetics to exclude other B-cell lymphoproliferative disorders (mantle cell lymphoma, marginal zone lymphoma, hairy cell leukemia)
- CD38 and ZAP-70 are prognostic, not diagnostic markers
Exclusion Requirement
The exclusion of BCR::ABL1 fusion is mandatory for diagnosing other myeloproliferative neoplasms (including differentiating them from CML) and is required before establishing a CLL diagnosis to rule out concurrent CML. 2 While extremely rare, concurrent CML and CLL can occur and requires molecular characterization 5
Specimen Requirements
- CML: Bone marrow preferred for complete staging; peripheral blood acceptable for RT-PCR
- CLL: Peripheral blood sufficient for immunophenotyping and molecular studies; bone marrow only if atypical features present