The determination of spontaneous megakaryocyte colony formation is an unequivocal test for discrimination between essential thrombocythaemia and reactive thrombocytosis.

Spontaneous colony formation from bone marrow megakaryocyte progenitors (BMsCFU-Mk) was studied in 24 patients with essential thrombocythaemia (ET), 20 patients with reactive thrombocytosis (RT), 20 patients with polycthaemia rubra vera with thrombocytosis (PRVtr), 16 patients with chronic myeloid leukaemia with thrombocytosis (CMLtr) and 18 normal control subjects (C). The culture medium which was used in the methylcellulose assay in vitro contained 30% of plasma from a single patient with hereditary haemochromatosis. Remarkable BMsCFU-Mk growth was recorded in all patients with ET but in none with RT or in C. BMs-CFU-Mk were present in 11/20 patients with PRVtr and 7/16 patients with CMLtr. Spontaneous bone marrow erythroid progenitors (BMsBFU-E) were also determined in these patients. BMsBFU-E were found in 21/24 patients with ET and none in the patients with RT and C. All patients with PRVtr and one patient with CMLtr showed BMsBFU-E. We conclude that our implementation of the in vitro methylcellulose assay allows the BMsCFU-Mk to be used as an unequivocal test for discrimination between ET and RT which has not been shown in previously published studies. In addition, we present evidence that in 10 patients BMsCFU-Mk and/or BMsBFU-E growth in the test persisted after long-lasting haematological remission.