Wang JC, et al. - Given the most reliable criteria being the presence of driver mutations, namely JAK2, CALR, or MPL gene mutations in making a definite diagnosis of essential thrombocytosis (ET) from reactive thrombocytosis (RT), researchers sought to identify other clonal marker gene mutations to make differential diagnosis in the absence of these driver mutations, so-called triple-negative ET. They performed Insulin-Like Receptor Growth Factor-1 Receptor (IGF-1R) quantification by flow cytometry in mononuclear cells (MNC) from peripheral blood, in 33 patients with ET (untreated or off treatment with hydroxyurea), 28 patients with RT, and 16 normal volunteer controls. ET patients vs RT patients or controls displayed significantly elevated IGF-1R levels. Findings support adding quantification of IGF-1R in blood MNC by flow cytometry as valuable in differentiating ET from RT.