Endogenous megakaryocytic colony formation and thrombopoietin sensitivity of megakaryocytic progenitor cells are useful to distinguish between essential thrombocythemia and reactive thrombocytosis

Diagnosis of essential thrombocythemia (ET) is controversial and remains ma inly an exclusion diagnosis. Endogenous megakaryocyte colony (EMC) formatio n have been largely evaluated to identify specific criteria for ET, but res ults are impeded by the lack of medium standardization. We evaluated megaka ryocyte (MK) colony formation in a serum-free collagen-based medium, withou t cytokine and in the presence of various concentrations of thrombopoietin (TPO). Thirty-six bone marrows from patients diagnosed with ET (n = 11), po lycythemia vera (PV; n = 12), reactive thrombocytosis (RT; n = 6) and healt hy donors (n = 7) were assessed. We demonstrate that 11 out 11 of the ET pa tients had spontaneous megakaryocyte colony-forming unit (CFU-MK) formation , in contrast to none of the RT patients and healthy donors. MK progenitors from ET patients remained responsive to TPO, because exogenous addition of TPO significantly increased cloning efficiency. Moreover, at low doses of TPO (0.5 ng/ml and 5 ng/ml), the number of positive cultures and mean numbe r of TPO stimulated CFU-MK were significantly higher in cultures of cells f rom patients with ET than in patients with RT. In summary, we have describe d a standardized serum-free, collagen-based assay that allows differential diagnosis of ET and RT, according to endogenous CFU-MK formation and sensit ivity to TPO.