Summary. Platelet kinetics were studied in 18 patients with idiopathic thrombocytopenic purpura (ITP) before splenectomy, 1 month and 8 months after splenectomy. A gamma function was used for the calculation of platelet mean life span (MLS).

Before splenectomy platelet MLS was greatly reduced and platelet production was 2.8 times normal. The peripheral platelet count and platelet recovery were significantly related to platelet MLS. One month after splenectomy platelet MLS increased appreciably while platelet production decreased but continued to be significantly above normal. These changes together with the removal of the splenic platelet pool resulted in a peripheral platelet count significantly higher than normal and a total platelet mass not differing from normal. Eight months after splenectomy platelet MLS increased slightly but was not significantly different from that found 1 month after splenectomy. Platelet production, however, decreased significantly and became normal. This resulted in a normal peripheral platelet count and a decreased total platelet mass which was significantly lower than in non-splenectomized control subjects. The results suggest that platelet production is regulated by a feed-back mechanism that is governed by the peripheral platelet count. The high platelet count encountered 1 month after splenectomy is due to a sluggishness of this regulatory mechanism.