What is a major criterion for the 2008 WHO diagnostic criteria for essential thrombocythemia?

The major criterion for the 2008 WHO diagnostic criteria for essential thrombocythemia is that the platelet count must be greater than 450 × 10⁹/L. This threshold is established to define the condition, as essential thrombocythemia is characterized by an overproduction of platelets due to the abnormal proliferation of megakaryocytes in the bone marrow.

A platelet count exceeding this value acts as a significant indicator of the disease, highlighting its role in diagnostic assessment. In the context of essential thrombocythemia, the elevated platelet count can lead to various complications, including thrombosis and hemorrhage, thereby justifying its inclusion as a critical diagnostic measure.

While other criteria, such as megakaryocyte morphology and the presence of specific genetic mutations (like JAK2(V617F)), contribute to the diagnosis and classification of the disease, the platelet count remains a fundamental component that is essential for establishing the presence of essential thrombocythemia according to the WHO criteria.