Guideline Summary: Diagnosis and Evaluation of Lymphocytosis

Initial Diagnostic Work‑up

• Peripheral blood immunophenotyping by flow cytometry is the primary tool for diagnosing lymphocytosis and usually eliminates the need for lymph‑node or bone‑marrow biopsy. 1
• A peripheral‑blood smear should be examined to assess lymphocyte morphology and to detect pro‑lymphocytes, complementing flow‑cytometry findings. 1
• The minimum flow‑cytometry panel must include pan‑B markers (CD19, CD20, CD23), pan‑T markers (CD3, CD4, CD8), CD5, and surface light‑chain restriction (kappa / lambda) to determine clonality. 1

Immunophenotypic Features of Chronic Lymphocytic Leukemia (CLL)

• CLL cells characteristically co‑express CD19, CD20, CD23 and CD5 while lacking other pan‑T antigens. 1, 2
• Expression of CD20, surface immunoglobulins, and CD79b is markedly weaker in CLL B‑cells than in normal B‑cells. 1
• Each malignant clone shows restriction to either kappa or lambda light chains, or may be negative for both. 1

Diagnostic Criteria for CLL

• An absolute lymphocytosis greater than 5 × 10⁹/L of mature‑appearing lymphocytes in peripheral blood is required. 2
• The characteristic immunophenotype is CD5⁺, CD19⁺, weak CD20⁺, CD23⁺ with light‑chain restriction. 3
• Pro‑lymphocytes may be present but must not exceed 55 % of the lymphocyte population; higher percentages indicate prolymphocytic leukemia. 1, 3

Monoclonal B‑Cell Lymphocytosis (MBL)

• MBL is defined by a monoclonal B‑cell count below 5 × 10⁹/L in the absence of lymphadenopathy, organomegaly, cytopenias, or clinical symptoms. 1
• Approximately 1–2 % of MBL cases progress to CLL each year. 1
• Patients should be counseled that MBL does not constitute leukemia or lymphoma. 1

Key Differential Diagnoses

Indications for Bone‑Marrow Biopsy

• Not required for the majority of lymphocytosis cases. 1
• Considered when flow‑cytometry is inconclusive for CLL diagnosis. 1
• Recommended before initiating therapy in the context of clinical trials. 3, 4

Indications for Lymph‑Node (Excisional) Biopsy

• Generally unnecessary unless flow‑cytometry is non‑diagnostic, confirmation of small‑lymphocytic lymphoma (SLL) is needed in patients with <5 × 10⁹/L monoclonal B‑cells plus adenopathy or splenomegaly, or when tissue is required for research purposes. 1, 5

Pre‑Treatment Evaluations (when therapy is indicated)

• Extended FISH panel to detect del(17p), del(11q) or trisomy 12, given their therapeutic relevance. 3, 4
• IGHV mutation status analysis to better estimate expected duration of response. 3, 4
• TP53 mutation testing (if del(17p) is absent) because it carries the strongest prognostic and predictive significance. 4
• Computed‑tomography imaging may be employed to assess tumor burden in symptomatic patients or before venetoclax therapy, but is not routinely required for asymptomatic individuals. 3, 4

Lymphocytosis Evaluation and Diagnosis

Initial Evaluation

• The National Comprehensive Cancer Network recommends a complete blood count (CBC) with differential to confirm absolute lymphocyte count >4000/μL 6
• Peripheral blood smear examination is used to assess lymphocyte morphology (mature vs. atypical/abnormal cells) 7, 8
• Flow cytometry immunophenotyping of peripheral blood should include a minimum panel with pan-B markers (CD19, CD20, CD23), pan-T markers (CD3, CD4, CD8), CD5, and surface immunoglobulin light chain restriction (kappa/lambda) to assess clonality 8, 9
• A comprehensive metabolic panel and lactate dehydrogenase (LDH) level should be obtained 9, 10
• Hepatitis B testing (HBsAg and HBcAb) and hepatitis C testing in high-risk patients should be performed 9, 10

Additional Testing Based on Initial Results

• If a monoclonal B-cell population is detected, quantitative immunoglobulins (IgG, IgA, IgM) and beta-2 microglobulin should be measured 7, 10
• A CT scan of chest/abdomen/pelvis with contrast may be considered if treatment is being considered or if there are concerning symptoms 7, 9
• Bone marrow biopsy with aspirate may be necessary if treatment is being considered, for evaluating early-stage disease, or for assessing cytopenias 7, 9, 10
• If a monoclonal T-cell population is detected, HTLV-I serology, T-cell receptor gene rearrangement studies, CT scan of chest/abdomen/pelvis, and bone marrow aspirate and biopsy may be considered 11

Special Considerations

• For CLL diagnosis, absolute lymphocyte count must exceed 5,000/μL with specific immunophenotypic features (CD5+, CD19+, CD20+ dim, CD23+, with light chain restriction) 8, 6
• Monoclonal B-cell lymphocytosis (MBL) is diagnosed when monoclonal B-cells are <5,000/μL without other features of lymphoproliferative disorder 7
• Hemophagocytic lymphohistiocytosis (HLH) should be considered if lymphocytosis is accompanied by fever, splenomegaly, cytopenias, hyperferritinemia, and elevated triglycerides 12

When to Consider Bone Marrow Evaluation

• Bone marrow evaluation should be considered when treatment is being considered for a confirmed lymphoproliferative disorder 7, 9
• It may also be necessary to evaluate unexplained cytopenias in the setting of lymphocytosis 7
• Bone marrow evaluation is not routinely needed for CLL diagnosis if peripheral blood flow cytometry confirms the diagnosis 8

Pitfalls to Avoid

• Immunophenotyping is essential for diagnosis of CLL and should not be relied solely on absolute lymphocyte count 8, 6
• Up to 55% prolymphocytes can still be consistent with CLL diagnosis; >55% suggests prolymphocytic leukemia 8
• Avoid overinterpreting isolated lymphocytosis without clinical context 6