The correlation between platelet count and bleeding time is nonlinear. The bleeding time prolongs more prominently as platelet count decreases toward 10000 mu l(-1); however, it becomes stable near 100000 mu l(-1). Clinical observations suggest that platelet functional capacity may also play a role in bleeding complications during thrombocytopenia. However, no routine method has been described for evaluation of platelet function during thrombocytopenia. To test if platelet functional capacity is affected by the cell count, as suggested by the platelet count-bleeding time correlation. To evaluate lumiaggregometry as a possible tool for studying platelet functions and prediction of bleeding in thrombocytopenic patients. Collagen-induced ATP release was studied in different dilutions of 22 healthy platelet-rich plasmas. The relationship between platelet count and ATP release was formulized. ATP release was also tested in 24 thrombocytopenic (10000-50000 mu l(-1)) patients, and the results were compared with expected levels derived from the formulae. ATP release increased in a cubic fashion as platelet count elevated. ATP secretion values were within normal limits or increased in patients with idiopathic thrombocytopenic purpura. However, some patients with megakaryocyte deficiency had a secretion defect. ATP secretion was decreased in four out of seven patients with bleeding symptoms compared with no persons without bleeding (P = 0.003). Platelet functional capacity is affected by the cell count. Lumiaggregometry is potentially useful in evaluating platelet functions during thrombocytopenia. Blood Coagul Fibrinolysis 19:633-637 (C) 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins.