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 mainly an exclusion diagnosis. Endogenous megakaryocyte colony (EMC) formation have been largely evaluated to identify specific criteria for ET, but results are impeded by the lack of medium standardization. We evaluated megakaryocyte (MK) colony formation in a serum-free collagen-based medium, without cytokine and in the presence of various concentrations of thrombopoietin (TPO). Thirty-six bone marrows from patients diagnosed with ET (n = 11), polycythemia vera (PV; n = 12), reactive thrombocytosis (RT; n = 6) and healthy donors (n = 7) were assessed. We demonstrate that 11 out 11 of the ET patients 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 number of TPO stimulated CFU-MK were significantly higher in cultures of cells from patients with ET than in patients with RT. In summary, we have described a standardized serum-free, collagen-based assay that allows differential diagnosis of ET and RT, according to endogenous CFU-MK formation and sensitivity to TPO.