Thrombocytosis
The term for having high platelets is thrombocytosis, which is generally defined as a platelet count above 450 × 10⁹/L. 1, 2
Definition and Classification
Thrombocytosis refers to an elevated platelet count, with the threshold established at >450 × 10⁹/L following the 2008 World Health Organization classification. 2 This represents a standardized definition that has simplified the diagnostic approach for evaluating elevated platelet counts.
Severity Grading
The severity of thrombocytosis can be stratified as follows: 3
- Mild: 500,000-700,000/μL
- Moderate: 700,000-900,000/μL
- Severe: >900,000/μL
- Extreme: >1,000/μL
Primary vs. Secondary Thrombocytosis
Primary Thrombocytosis (12-12.5% of cases)
Primary thrombocytosis represents a clonal myeloproliferative disorder characterized by the presence of molecular markers such as JAK2V617F, CALR, or MPL mutations. 1, 2 This category includes: 1
- Essential thrombocythemia (ET) - accounts for 45% of primary thrombocytosis cases
- Polycythemia vera - presents with elevated platelets alongside increased red cell mass
- Primary myelofibrosis
- Chronic myeloid leukemia
Primary thrombocytosis carries significantly higher thrombotic risk, with both arterial and venous thromboembolic complications occurring more frequently than in secondary causes. 2, 4
Secondary (Reactive) Thrombocytosis (83-88% of cases)
Secondary thrombocytosis is far more common and occurs as a reactive process to various underlying conditions. 2, 4 The major causes include: 2, 4
- Tissue damage/injury (32-42% of secondary cases)
- Infection (17-24%)
- Chronic inflammatory disorders (10-12%) - including connective tissue diseases and inflammatory bowel disease 1
- Iron deficiency anemia (11%) - frequently overlooked but common 1, 2
- Malignancy (13%) - solid tumors and lymphoproliferative disorders 1, 2
- Post-splenectomy or functional hyposplenism 1, 5
- Hemolytic anemia and post-hemorrhagic states 1, 5
Secondary thrombocytosis rarely causes thromboembolic complications unless additional risk factors are present, and venous events are more common than arterial. 4
Clinical Significance
The median platelet count and incidence of thrombosis are significantly higher in primary versus secondary thrombocytosis. 2 In primary thrombocytosis, platelet counts are typically >1,000/μL, whereas secondary thrombocytosis is usually mild (72-86% of cases). 3, 4
In extreme thrombocytosis (>1,000/μL), acquired von Willebrand disease can develop due to abnormal adsorption of large von Willebrand factor multimers to clonal platelets, paradoxically increasing bleeding risk. 6, 5
Important Caveats
Pseudothrombocytosis must be excluded - laboratory artifacts can falsely elevate platelet counts, particularly in conditions with increased hematocrit where standard anticoagulant amounts with reduced plasma volume affect measurements. 1, 5
Drug-induced thrombocytosis can occur with vinca alkaloids (through thrombocyte-stimulating properties), iron supplementation (transient elevation), and epinephrine (platelet demargination). 7