Platelet c‐mpl expression is dysregulated in patients with essential thrombocythaemia but this is not of diagnostic value

Essential thrombocythaemia (ET) can be difficult to discriminate from an occult case of reactive thrombocytosis (RT). Since thrombopoietin (TPO) is the primary regulator of thrombopoiesis, we have investigated whether levels of TPO and/or its receptor, c‐mpl, are of value in the differential diagnosis of ET. Plasma TPO levels in patients with ET, RT and other myeloproliferative disorders (MPDs) did not differ significantly from normal controls. However, surface c‐mpl expression was significantly reduced in platelets from 18 ET patients, 0–65.5% of controls (P