THROMBOPOIETIN MEASUREMENT IN THROMBOCYTOSIS - DYSREGULATION AND LACKOF FEEDBACK INHIBITION IN ESSENTIAL THROMBOCYTHEMIA

Essential thrombocythaemia (ET), a myeloproliferative disorder (MPD) m anifested by excessive platelet production, lacks a specific diagnosti c test to facilitate differentiation from other thrombocytoses. We stu died thrombopoietin (TPO) levels in 41 patients with thrombocytosis: 2 5 ET patients, eight with other MPD, and eight with reactive thrombocy tosis. Mean age and platelet counts for these groups were comparable. TPO levels for 96 healthy individuals provided a reference range for n ormal. The majority of ET patients (19/25 or 76%) had normal TPO level s. No patient with ET had a TPO level below 75 pg/ml, compared with 57 % of healthy donors and 8/16 (50%) patients with other thrombocytoses (P < 0.05). TPO levels in ET are not appropriately down-regulated, as occurs with cytokines relevant to other MPD. In thrombocytosis, a TPO level <75 pg/ml indicates that ET is unlikely.