High Platelet Count (Thrombocytosis): Causes, Types, and Management

Thrombocytosis = platelet count >450,000/µL (some labs >400,000). Classified as:

1. Reactive (Secondary) Thrombocytosis — 80–90%

Response to underlying condition; platelets functional; low thrombotic risk.

• Infection, inflammation (RA, IBD), tissue damage (surgery, trauma), iron deficiency, hemolysis, splenectomy, malignancy (solid tumors), medications (epinephrine, vincristine), rebound after chemo/alcohol cessation.

2. Primary (Clonal) Thrombocytosis — 10–20%

Myeloproliferative neoplasms (MPNs); higher thrombotic/hemorrhagic risk.

• Essential Thrombocythemia (ET): JAK2, CALR, MPL mutations; isolated ↑ platelets.
• Polycythemia Vera (PV): JAK2 V617F; ↑ RBC, WBC, platelets.
• Primary Myelofibrosis (PMF): Fibrosis, cytopenias later.
• Chronic Myeloid Leukemia (CML): BCR-ABL1; ↑ all lineages.
• MDS/MPN Overlap.

Symptoms

• Vasomotor: erythromelalgia (burning hands/feet), headache, dizziness, visual changes.
• Thrombosis: arterial (stroke, MI), venous (DVT, PE), microvascular.
• Bleeding: if >1,000,000 (acquired von Willebrand syndrome).

Workup

CBC, smear, iron studies, CRP, JAK2, CALR, MPL, BCR-ABL, abdominal US (spleen), bone marrow biopsy if MPN suspected.

Treatment

• Reactive: treat underlying cause.
• ET/PV: risk-stratified (age, thrombosis history, JAK2, CV risk factors) → low-dose aspirin ± cytoreduction (hydroxyurea, interferon, anagrelide, ruxolitinib).

See hematologist for primary thrombocytosis.