Lymphocytosis + Thrombocytosis

Definitions

Key Differential Diagnoses

1. Chronic Lymphocytic Leukemia (CLL)

• The most important diagnosis to rule out when both are elevated
• CLL itself causes lymphocytosis; associated reactive thrombocytosis can occur early in disease
• However, in advanced CLL, thrombocytopenia (low platelets) is more typical due to marrow infiltration or autoimmune destruction
• Early-stage CLL with concurrent reactive thrombocytosis fits this pattern

2. Myeloproliferative Neoplasms (MPNs)

• Essential Thrombocythemia (ET): Marked thrombocytosis (often >600 × 10⁹/L); may have concurrent lymphocytosis
• Polycythemia Vera (PV): Can elevate all cell lines including lymphocytes and platelets
• Giant platelets on smear support MPN (Harrison's, p. 1736)
• Check JAK2 V617F mutation, calreticulin (CALR), MPL mutations

3. Reactive / Infectious Causes

• Viral infections: EBV (infectious mononucleosis), CMV, hepatitis viruses → lymphocytosis with reactive thrombocytosis during recovery phase
• Bacterial infections / chronic inflammation: Iron deficiency from chronic infection → reactive thrombocytosis; lymphocytosis from viral co-infection
• Post-splenectomy: Loss of splenic sequestration causes both lymphocytosis and thrombocytosis simultaneously — a classic post-splenectomy CBC pattern
• Pertussis (whooping cough): Marked lymphocytosis (WBC can reach 50–100 × 10⁹/L); reactive thrombocytosis possible

4. Lymphoma (especially Splenic Marginal Zone or Mantle Cell)

• Circulating lymphoma cells → lymphocytosis
• Reactive thrombocytosis in early/indolent disease

5. Hyposplenism / Functional Asplenia

• Sickle cell disease, celiac disease, autoimmune conditions
• Both lymphocytosis and thrombocytosis are characteristic findings

6. Stress / Exercise / Catecholamine Surge

• Transient demargination of lymphocytes and platelets from vascular walls
• Resolves quickly; not a true pathological increase

Diagnostic Approach

Lymphocytosis + Thrombocytosis
          │
          ├── Review peripheral blood smear
          │       ├── Smudge cells → CLL
          │       ├── Atypical lymphocytes → viral (EBV/CMV)
          │       ├── Giant platelets → MPN
          │       └── Blast forms → leukemia/lymphoma
          │
          ├── Clinical history
          │       ├── Age >60, indolent → CLL workup (flow cytometry)
          │       ├── Fever, pharyngitis, splenomegaly → EBV/CMV
          │       ├── Prior splenectomy → post-splenectomy state
          │       └── B symptoms (night sweats, weight loss) → lymphoma
          │
          └── Labs
                  ├── Flow cytometry (CD5/CD19/CD23 for CLL)
                  ├── JAK2 mutation, BCR-ABL
                  ├── LDH, uric acid, β2-microglobulin
                  ├── Monospot / EBV/CMV serology
                  └── Bone marrow biopsy if MPN/CLL confirmed

Summary Table