Definition

A thrombocytosis is defined as a platelet count above the upper limit of the reference range. By convention, the normal range of blood platelet count in humans is considered to be 150 to 400 × 109/l. It is likely that this wide variation includes healthy individuals who have modest degrees of reactive thrombocytosis, such as young women with iron deficiency secondary to excessive menstrual blood loss. Consequently, although this is not well documented, the upper limit of the normal range in truly healthy people may be less than 400 × 109/l. Platelet values in the upper normal range should be interpreted in the clinical context of the individual patient who should be followed for eventual clear diagnostic evaluation.

Classification and differential diagnosis

Based on the mechanisms inducing the increased platelet production, two major types of thrombocytosis, i.e. platelet count 400 × 109/l can be found: reactive and clonal. Reactive thrombocytosis (RT) is the commonest cause of an elevated platelet count and includes an heterogeneous group of disorders characterized by increased megakaryopoiesis produced by an intrinsecally normal megakaryocyte compartment. This is thought to be due to an increased cytokine production secondary to several pathological events (Table 41.1).

On the other hand, essential thrombocythemia (ET) and the thrombocythemic states found in the myeloproliferative disorders (MPDs) are usually characterized by a clonal thrombocytosis. In this situation, a single hematopoietic precursor or stem cell and its progeny gain a growth advantage over their normal counterparts, so that the proliferation of the latter is suppressed.