Serum interleukin-6 levels in patients with thrombocytosis.

Interleukin-6 (IL-6) has been shown to increase platelet counts in several animal models and to enhance megakaryocytopoiesis in vitro. In order to investigate the possible relationship between IL-6 and thrombocytosis, serum IL-6 levels in patients with platelet counts > or = 6 x 10(5)/microliters were measured using an IL-6-responsive bioassay. A cohort of healthy volunteers with normal platelet counts was used to establish a control mean serum IL-6 level [2.19 U/ml +/- 1.08 SD (range 0-5.5)]. Patients with primary thrombocytosis had a mean serum IL-6 level not significantly different from controls. In comparison, serum IL-6 levels of patients with reactive thrombocytosis were significantly greater than controls (38.3 U/ml +/- 94.6; range 0-933; P < 0.001). Although no significant correlation was observed between the degree of serum IL-6 elevation and the height of the platelet count in any individual, elevated serum IL-6 was highly correlated with reactive thrombocytosis.