Essential Thrombocytosis

Essential thrombocytopenia is a myeloproliferative  neoplasm characterized by excessive, clonal platelet production with a tendency for thrombosis and haemorrhage.

Diagnostic Criteria (WHO 2016)

Diagnosis of ET requires meeting all 4 major criteria or the first 3 major criteria and the minor criterion

Major criteria

• Platelet count ≥450 × 109/L 
• Bone marrow biopsy showing proliferation mainly of the megakaryocyte lineage with increased numbers of enlarged, mature megakaryocytes with hyperlobulated nuclei. No significant increase or left shift in neutrophil granulopoiesis or erythropoiesis and very rarely minor (grade 1) increase in reticulin fibers. 
• Not meeting WHO criteria for BCR-ABL1+ CML, PV, PMF, myelodysplastic syndromes, or other myeloid neoplasms 
• Presence of JAK2, CALR, or MPL mutation 

Minor criterion

• Presence of a clonal marker or absence of evidence for reactive thrombocytosis 

Blood Film Features:

• Marked thrombocytosis
• Platelet anisocytosis with varying appearance from small to large platelets

Other features to look for:

• Hydroxyurea treatment: macrocytes, stomatocytes, hypersegmented neutrophils on a background of leukoerythroblastosis
• Post-ET Myelofibrinosis with a leucoerythroblastic picture with tear-drop red blood cells, circulating by Lester red bleed cells

Bone Marrow Features:

• Increased megakaryopoiesis 
• Loose clustering of megakaryocytes
• Large  megakaryocytes with abundant mature cytoplasm, and deeply lobulated and hyperlobulated nuclei

Other Resources:

• ASH image bank: Essential thrombocytosis
• BSH 2014: Modification of Diagnostic Criteria for Essential Thrombocythaemia
• Blood 2016: How I treat essential thrombocytosis