A 68-year-old Indian man presents with fatigue and lymphadenopathy. Peripheral blood shows 8,000/μL lymphocytes with 'smudge cells' and CD5+, CD19+, CD23+ B-cells. Which single feature most reliably distinguishes this CLL from small lymphocytic lymphoma (SLL)?

Question

Accepted Answer

B. Absolute lymphocyte count ≥5,000/μL in peripheral blood. ## CLL vs SLL — The Diagnostic Boundary

### Definition and Distinction

**Key Point:** CLL and SLL are the same disease (CD5+ B-cell lymphoproliferative disorder) but differ by ONE criterion: the absolute lymphocyte count in peripheral blood. CLL is defined by ≥5,000/μL circulating lymphocytes; SLL is the same disease with <5,000/μL lymphocytes and predominantly nodal/tissue involvement.

### Comparison Table: CLL vs SLL

| Feature | CLL | SLL |
|---------|-----|-----|
| **Peripheral blood lymphocytes** | **≥5,000/μL** | **<5,000/μL** |
| Primary site of disease | Peripheral blood and bone marrow | Lymph nodes and tissues |
| CD5+, CD19+, CD23+ phenotype | Yes | Yes |
| Smudge cells on smear | Present | May be absent or rare |
| Lymphadenopathy | Common | Common |
| Bone marrow involvement | Common | Common |
| Prognosis | Same | Same |
| Treatment | Same | Same |

### Why Peripheral Blood Count Is the Discriminator

1. **WHO/IWCLL criterion:** The diagnostic threshold of 5,000/μL absolute lymphocytes is the defining boundary between CLL and SLL.
2. **Immunophenotype is identical:** Both express CD5, CD19, CD23, and have the same genetic abnormalities (del 13q, trisomy 12, del 11q, del 17p).
3. **Tissue involvement is non-specific:** Both can have lymphadenopathy, splenomegaly, and bone marrow involvement.
4. **Clinical behavior is the same:** Prognosis and treatment are identical; the distinction is anatomic, not biologic.

**High-Yield:** The IWCLL (International Workshop on CLL) diagnostic criteria explicitly state: CLL requires ≥5,000/μL B-lymphocytes in peripheral blood for ≥3 months. Below this threshold with nodal disease = SLL. This is a **definitional distinction**, not a biological one.

**Clinical Pearl:** A patient with CD5+ B-cell lymphoproliferative disorder and 3,000/μL lymphocytes but massive lymphadenopathy is SLL, not CLL. The absolute count in blood is the gatekeeper.

[cite:Harrison 21e Ch 297]

Answer

A. Presence of lymphadenopathy on imaging

Answer

C. Bone marrow involvement on biopsy

Answer

D. CD5 and CD23 co-expression on B-cells

A 68-year-old Indian man presents with fatigue and lymphadenopathy. Peripheral blood shows 8,000/μL lymphocytes with 'smudge cells' and CD5+, CD19+, CD23+ B-cells. Which single feature most reliably distinguishes this CLL from small lymphocytic lymphoma (SLL)?

Explanation

CLL vs SLL — The Diagnostic Boundary

Definition and Distinction

Comparison Table: CLL vs SLL

Why Peripheral Blood Count Is the Discriminator

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Feature	CLL	SLL
Peripheral blood lymphocytes	≥5,000/μL	<5,000/μL
Primary site of disease	Peripheral blood and bone marrow	Lymph nodes and tissues
CD5+, CD19+, CD23+ phenotype	Yes	Yes
Smudge cells on smear	Present	May be absent or rare
Lymphadenopathy	Common	Common
Bone marrow involvement	Common	Common
Prognosis	Same	Same
Treatment	Same	Same