What is the initial workup for a patient presenting with lymphocytosis?

Initial Workup for Lymphocytosis

Begin with a complete blood count with differential and peripheral blood smear to determine the absolute lymphocyte count, identify atypical lymphocytes, and assess for clonality—this immediately distinguishes reactive from malignant causes. 1, 2

Immediate Laboratory Assessment

Core Blood Tests

• Complete Blood Count (CBC) with differential and platelets to quantify absolute lymphocyte count, evaluate for cytopenias suggesting bone marrow involvement, and identify abnormal lymphocyte populations 1, 2
• Peripheral blood smear to assess lymphocyte morphology, identify atypical lymphocytes (>10% suggests infectious mononucleosis), and detect large granular lymphocytes 3, 4
• Comprehensive metabolic panel including liver function tests, renal function, and electrolytes to evaluate organ involvement 1
• Lactate dehydrogenase (LDH) as an essential prognostic marker and indicator of cell turnover or lymphoma 1

Infectious Workup (If Acute Presentation)

• Heterophile antibody test (Monospot) if clinical features suggest infectious mononucleosis (fever, pharyngitis, fatigue), though sensitivity is only 87% and false negatives occur in the first week of illness 3
• EBV-specific serology (VCA IgM/IgG and EBNA) if heterophile test is negative but clinical suspicion remains high—VCA IgM present with EBNA absent indicates acute infection 3
• CMV, HIV, toxoplasma, and viral hepatitis testing when both heterophile and EBV serologies are negative to identify mononucleosis-like illnesses 3

Flow Cytometry for Clonality Assessment

Flow cytometry is essential and should be performed early to determine lymphoma subtype and establish clonality, distinguishing benign reactive lymphocytosis from chronic lymphocytic leukemia (CLL) or other lymphoproliferative disorders. 1, 2

• Pan-B cell markers (CD19, CD20, CD22) and pan-T cell markers (CD3, CD5) must be included in the initial immunophenotyping panel 1
• Clonal B-cell population with CD5+/CD19+/CD20+ phenotype establishes CLL diagnosis without requiring bone marrow biopsy 5, 1
• T-cell subset analysis identifies T-cell lymphocytosis, large granular lymphocyte disorders, or natural killer cell proliferations 4

Additional Testing Based on Initial Findings

If Malignancy Suspected (Persistent Lymphocytosis >3 Months)

• β2-microglobulin for prognostic stratification in CLL and follicular lymphoma 1
• Hepatitis B surface antigen, core antibody, and surface antibody before initiating anti-CD20 therapy due to reactivation risk 1
• Hepatitis C testing particularly for splenic marginal zone lymphoma or high-risk patients 5, 1
• Uric acid as part of tumor lysis syndrome panel before treatment initiation 1

If Bone Marrow Evaluation Needed

• Bone marrow biopsy with aspirate is essential for staging most lymphomas (involvement occurs in 39% of low-grade, 36% of intermediate-grade, 18% of high-grade lymphomas) but can be omitted in CLL when clonal lymphocytosis is confirmed by flow cytometry 5, 1

Critical Diagnostic Pitfalls to Avoid

• Do not rely solely on heterophile testing in children <10 years or during the first week of illness—proceed directly to EBV-specific serology if clinical suspicion is high 3
• Timing matters for infectious causes: repeat heterophile testing at 7-10 days if initially negative with classic clinical features, or proceed directly to EBV serology 3
• Chronic lymphocytosis (>6 months) with counts 4-15 × 10⁹/L requires flow cytometry to distinguish early CLL from benign clonal T-cell or NK-cell proliferations 4
• Ferritin levels >10,000 ng/mL in the context of fever, cytopenia, and organomegaly should prompt consideration of hemophagocytic lymphohistiocytosis, which requires fundamentally different treatment than sepsis 6

Algorithmic Approach

Acute lymphocytosis (<3 months):

1. CBC with differential and smear → atypical lymphocytes present → infectious workup (heterophile, EBV serology, CMV, HIV) 3
2. If no atypical lymphocytes and no infectious symptoms → flow cytometry to assess clonality 1, 2

Chronic lymphocytosis (>3 months):

1. Flow cytometry mandatory to establish clonality 1, 2
2. If clonal B-cell population → CLL workup (LDH, β2-microglobulin, hepatitis B/C, comprehensive metabolic panel) 1, 2
3. If clonal T-cell or NK-cell population → hematology referral for further characterization 4
4. If polyclonal → investigate for autoimmune disease, chronic infection, or medication effect 2