## 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]
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