Circulating levels of osteonectin in normal subjects and patients with thrombocytopenia

In order to clarify the relative contributions of activated platelets and other sources to circulating osteonectin, the amounts of osteonectin present in serum and in plasma prepared without significant platelet activation were compared to platelet count in normal subjects and patients with various degrees of thrombocytopenia. Serum osteonectin showed a logarithmic positive correlation with platelet count (r = 0.87, P less than 0.0001, n = 52). Osteonectin concentration was significantly lower in plasma than in matched sera in all subjects sampled, and not significantly different in plasma of thrombocytopenic and normal subjects. These results confirm the major contribution of platelets to serum osteonectin. The positive zero intercept of the plot osteonectin vs. platelet count (19 +/- 2.45 ng/ml, mean +/- SD, P less than 0.0001, n = 52) and the presence of significant amounts of osteonectin in plasma, document the existence of a basic level of circulating osteonectin, independent of platelet activation, and to which the relative contribution of bone remains to be assessed.