A Comparison of Platelet Size, Platelet Count, and Platelet 35S Incorporation as Assays for Thrombopoietin

Average platelet size, platelet count, and 35S-incorporation into platelets were compared as methods for the measurement of thrombopoietin-stimulated thrombopoiesis. In mice injected with rabbit anti-mouse platelet serum (RAMPS) average platelet size was shown to be increased as mice were recovering from thrombocytopenia. Also, 35S-measurements on platelets of these mice showed significant increases in cpm/average platelet 2-4 days after RAMPS treatment. Significant increases in 35S-incorporation into the total circulating mass of platelets were found on days 3-4. In normal mice or mice in rebound-thrombocytosis injected with thrombopoietin, platelet size remained unchanged, whereas the platelet cound and 35S-incorporation into platelets were shown to be significantly increased. Moreover, a dose-response experiment in mice pretreated with RAMPS showed a slight increase in platelet count as the dose of TSF was increased, but platelet sizes were unaltered. The % 35S-incorporation into platelets showed a significant linear dose-response, i.e. as the dose of thrombopoietin was increased, as increase in % 35S-incorporation into platelets was observed. These data indicated that of the three indirect measurements of thrombopoietin, the % 35S-incorporation into mouse platelets was the most sensitive, followed by platelet counting; the least sensitive measurement of thrombopoiesis was change in platelet size.